[GitHub] [kafka] jeffkbkim commented on a diff in pull request #13637: KAFKA-14462; [10/N] Add TargetAssignmentBuilder

["jeffkbkim (via GitHub)" <gi...@apache.org>]

jeffkbkim commented on code in PR #13637:
URL: https://github.com/apache/kafka/pull/13637#discussion_r1178516453


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilder.java:
##########
@@ -0,0 +1,327 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.Record;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.MemberAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignorException;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+
+/**
+ * Build a new Target Assignment based on the provided parameters. As a result,
+ * it yields the records that must be persisted to the log and the new member
+ * assignments as a map.
+ *
+ * Records are only created for members which have a new target assignment. If
+ * their assignment did not change, no new record is needed.
+ *
+ * When a member is deleted, it is assumed that its target assignment record
+ * is deleted as part of the member deletion process. In other words, this class
+ * does not yield a tombstone for remove members.
+ */
+public class TargetAssignmentBuilder {
+    /**
+     * The assignment result returned by {{@link TargetAssignmentBuilder#build()}}.
+     */
+    public static class TargetAssignmentResult {
+        /**
+         * The records that must be applied to the __consumer_offsets
+         * topics to persist the new target assignment.
+         */
+        private final List<Record> records;
+
+        /**
+         * The new target assignment for all members.
+         */
+        private final Map<String, Assignment> assignments;
+
+        TargetAssignmentResult(
+            List<org.apache.kafka.coordinator.group.Record> records,
+            Map<String, Assignment> assignments
+        ) {
+            Objects.requireNonNull(records);
+            Objects.requireNonNull(assignments);
+            this.records = records;
+            this.assignments = assignments;
+        }
+
+        /**
+         * @return The records.
+         */
+        public List<Record> records() {
+            return records;
+        }
+
+        /**
+         * @return The assignment.

Review Comment:
   nit: assignments



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilder.java:
##########
@@ -0,0 +1,327 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.Record;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.MemberAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignorException;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+
+/**
+ * Build a new Target Assignment based on the provided parameters. As a result,
+ * it yields the records that must be persisted to the log and the new member
+ * assignments as a map.
+ *
+ * Records are only created for members which have a new target assignment. If
+ * their assignment did not change, no new record is needed.
+ *
+ * When a member is deleted, it is assumed that its target assignment record
+ * is deleted as part of the member deletion process. In other words, this class
+ * does not yield a tombstone for remove members.
+ */
+public class TargetAssignmentBuilder {
+    /**
+     * The assignment result returned by {{@link TargetAssignmentBuilder#build()}}.
+     */
+    public static class TargetAssignmentResult {
+        /**
+         * The records that must be applied to the __consumer_offsets
+         * topics to persist the new target assignment.
+         */
+        private final List<Record> records;
+
+        /**
+         * The new target assignment for all members.
+         */
+        private final Map<String, Assignment> assignments;
+
+        TargetAssignmentResult(
+            List<org.apache.kafka.coordinator.group.Record> records,
+            Map<String, Assignment> assignments
+        ) {
+            Objects.requireNonNull(records);
+            Objects.requireNonNull(assignments);
+            this.records = records;
+            this.assignments = assignments;
+        }
+
+        /**
+         * @return The records.
+         */
+        public List<Record> records() {
+            return records;
+        }
+
+        /**
+         * @return The assignment.
+         */
+        public Map<String, Assignment> assignments() {
+            return assignments;
+        }
+    }
+
+    /**
+     * The group id.
+     */
+    private final String groupId;
+
+    /**
+     * The group epoch.
+     */
+    private final int groupEpoch;
+
+    /**
+     * The partition assignor to compute the assignment.
+     */
+    private final PartitionAssignor assignor;
+
+    /**
+     * The members in the group.
+     */
+    private Map<String, ConsumerGroupMember> members = Collections.emptyMap();
+
+    /**
+     * The subscription metadata.
+     */
+    private Map<String, TopicMetadata> subscriptionMetadata = Collections.emptyMap();
+
+    /**
+     * The current target assignment.
+     */
+    private Map<String, Assignment> assignments = Collections.emptyMap();
+
+    /**
+     * The members which have been updated or deleted. Deleted members
+     * are signaled by a null value.
+     */
+    private Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();
+
+    /**
+     * Constructs the object.
+     *
+     * @param groupId       The group id.
+     * @param groupEpoch    The group epoch to compute a target assignment for.
+     * @param assignor      The assignor to use to compute the target assignment.
+     */
+    public TargetAssignmentBuilder(
+        String groupId,
+        int groupEpoch,
+        PartitionAssignor assignor
+    ) {
+        this.groupId = Objects.requireNonNull(groupId);
+        this.groupEpoch = groupEpoch;
+        this.assignor = Objects.requireNonNull(assignor);
+    }
+
+    /**
+     * Adds all the current members.
+     *
+     * @param members   The current members in the consumer groups.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withMembers(
+        Map<String, ConsumerGroupMember> members
+    ) {
+        this.members = members;
+        return this;
+    }
+
+    /**
+     * Adds the subscription metadata to use.
+     *
+     * @param subscriptionMetadata  The subscription metadata.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withSubscriptionMetadata(
+        Map<String, TopicMetadata> subscriptionMetadata
+    ) {
+        this.subscriptionMetadata = subscriptionMetadata;
+        return this;
+    }
+
+    /**
+     * Adds the current target assignments.
+     *
+     * @param assignments   The current assignments.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withTargetAssignments(
+        Map<String, Assignment> assignments
+    ) {
+        this.assignments = assignments;
+        return this;
+    }
+
+    /**
+     * Updates a member. This is useful when the updated member is
+     * not yet materialized in memory.
+     *
+     * @param memberId      The member id.
+     * @param updatedMember The updated member.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withUpdatedMember(
+        String memberId,
+        ConsumerGroupMember updatedMember
+    ) {
+        this.updatedMembers.put(memberId, updatedMember);
+        return this;
+    }
+
+    /**
+     * Removes a member. This is useful when the removed member
+     * is not yet materialized in memory.
+     *
+     * @param memberId The member id.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withRemoveMembers(
+        String memberId
+    ) {
+        return withUpdatedMember(memberId, null);
+    }
+
+    /**
+     * Builds the new target assignment.
+     *
+     * @return A TargetAssignmentResult which contains the records to update
+     *         the current target assignment.
+     * @throws PartitionAssignorException if the assignment can not be computed.
+     */
+    public TargetAssignmentResult build() throws PartitionAssignorException {
+        Map<String, AssignmentMemberSpec> memberSpecs = new HashMap<>();
+
+        // Prepare the member spec for all members.
+        members.forEach((memberId, member) -> addMemberSpec(
+            memberSpecs,
+            member,
+            assignments.getOrDefault(memberId, Assignment.EMPTY)
+        ));
+
+        // Update the member spec if updated or deleted members.
+        updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+            if (updatedMemberOrNull == null) {
+                memberSpecs.remove(memberId);
+            } else {
+                addMemberSpec(
+                    memberSpecs,
+                    updatedMemberOrNull,
+                    assignments.getOrDefault(memberId, Assignment.EMPTY)
+                );
+            }
+        });
+
+        // Prepare the topic metadata.
+        Map<Uuid, AssignmentTopicMetadata> topics = new HashMap<>();
+        subscriptionMetadata.forEach((topicName, topicMetadata) ->
+            topics.put(topicMetadata.id(), new AssignmentTopicMetadata(topicMetadata.numPartitions()))
+        );
+
+        // Compute the assignment.
+        GroupAssignment newGroupAssignment = assignor.assign(new AssignmentSpec(
+            Collections.unmodifiableMap(memberSpecs),
+            Collections.unmodifiableMap(topics)
+        ));
+
+        // Compute delta from previous to new assignment and create the
+        // relevant records.
+        List<Record> records = new ArrayList<>();
+        Map<String, Assignment> newTargetAssignment = new HashMap<>();
+
+        memberSpecs.keySet().forEach(memberId -> {
+            Assignment oldMemberAssignment = assignments.get(memberId);
+            Assignment newMemberAssignment = newMemberAssignment(newGroupAssignment, memberId);
+
+            newTargetAssignment.put(memberId, newMemberAssignment);
+
+            if (oldMemberAssignment == null) {
+                // If the member had no assignment, we always create a record for him.
+                records.add(newTargetAssignmentRecord(
+                    groupId,
+                    memberId,
+                    newMemberAssignment.partitions()
+                ));
+            } else {
+                // If the member had an assignment, we only create a record if the
+                // new assignment is different.
+                if (!newMemberAssignment.equals(oldMemberAssignment)) {
+                    records.add(newTargetAssignmentRecord(
+                        groupId,
+                        memberId,
+                        newMemberAssignment.partitions()
+                    ));
+                }
+            }
+        });
+
+        // Bump the assignment epoch.

Review Comment:
   to confirm, the epoch will already be bumped when creating the Builder object right?



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilderTest.java:
##########
@@ -0,0 +1,679 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkAssignment;
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkTopicAssignment;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class TargetAssignmentBuilderTest {
+
+    public static class TargetAssignmentBuilderTestContext {
+        private final String groupId;
+        private final int groupEpoch;
+        private final PartitionAssignor assignor = mock(PartitionAssignor.class);
+        private final Map<String, ConsumerGroupMember> members = new HashMap<>();
+        private final Map<String, TopicMetadata> subscriptionMetadata = new HashMap<>();
+        private final Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();
+        private final Map<String, Assignment> targetAssignments = new HashMap<>();
+        private final Map<String, org.apache.kafka.coordinator.group.assignor.MemberAssignment> assignments = new HashMap<>();
+
+        public TargetAssignmentBuilderTestContext(
+            String groupId,
+            int groupEpoch
+        ) {
+            this.groupId = groupId;
+            this.groupEpoch = groupEpoch;
+        }
+
+        public TargetAssignmentBuilderTestContext addGroupMember(
+            String memberId,
+            List<String> subscriptions,
+            Map<Uuid, Set<Integer>> targetPartitions
+        ) {
+            members.put(memberId, new ConsumerGroupMember.Builder(memberId)
+                .setSubscribedTopicNames(subscriptions)
+                .setRebalanceTimeoutMs(5000)
+                .build());
+
+            targetAssignments.put(memberId, new Assignment(
+                (byte) 0,
+                targetPartitions,
+                VersionedMetadata.EMPTY
+            ));
+
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext addTopicMetadata(
+            Uuid topicId,
+            String topicName,
+            int numPartitions
+        ) {
+            subscriptionMetadata.put(topicName, new TopicMetadata(
+                topicId,
+                topicName,
+                numPartitions
+            ));
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions
+        ) {
+            return updateMemberSubscription(
+                memberId,
+                subscriptions,
+                Optional.empty(),
+                Optional.empty()
+            );
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions,
+            Optional<String> instanceId,
+            Optional<String> rackId
+        ) {
+            ConsumerGroupMember existingMember = members.get(memberId);
+            ConsumerGroupMember.Builder builder;
+            if (existingMember != null) {
+                builder = new ConsumerGroupMember.Builder(existingMember);
+            } else {
+                builder = new ConsumerGroupMember.Builder(memberId);
+            }
+            updatedMembers.put(memberId, builder
+                .setSubscribedTopicNames(subscriptions)
+                .maybeUpdateInstanceId(instanceId)
+                .maybeUpdateRackId(rackId)
+                .build());
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext removeMemberSubscription(
+            String memberId
+        ) {
+            this.updatedMembers.put(memberId, null);
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext prepareMemberAssignment(
+            String memberId,
+            Map<Uuid, Set<Integer>> assignment
+        ) {
+            assignments.put(memberId, new org.apache.kafka.coordinator.group.assignor.MemberAssignment(assignment));
+            return this;
+        }
+
+        private AssignmentMemberSpec assignmentMemberSpec(
+            ConsumerGroupMember member,
+            Assignment targetAssignment
+        ) {
+            Set<Uuid> subscribedTopics = new HashSet<>();
+            member.subscribedTopicNames().forEach(topicName -> {
+                TopicMetadata topicMetadata = subscriptionMetadata.get(topicName);
+                if (topicMetadata != null) {
+                    subscribedTopics.add(topicMetadata.id());
+                }
+            });
+
+            return new AssignmentMemberSpec(
+                Optional.ofNullable(member.instanceId()),
+                Optional.ofNullable(member.rackId()),
+                subscribedTopics,
+                targetAssignment.partitions()
+            );
+        }
+
+        public TargetAssignmentBuilder.TargetAssignmentResult build() {
+            // Prepare expected member specs.
+            Map<String, AssignmentMemberSpec> memberSpecs = new HashMap<>();
+            members.forEach((memberId, member) -> {
+                memberSpecs.put(memberId, assignmentMemberSpec(
+                    member,
+                    targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                ));
+            });
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull == null) {
+                    memberSpecs.remove(memberId);
+                } else {
+                    memberSpecs.put(memberId, assignmentMemberSpec(
+                        updatedMemberOrNull,
+                        targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                    ));
+                }
+            });
+
+            Map<Uuid, AssignmentTopicMetadata> topicMetadata = new HashMap<>();
+            subscriptionMetadata.forEach((topicName, metadata) -> {
+                topicMetadata.put(metadata.id(), new AssignmentTopicMetadata(metadata.numPartitions()));
+            });
+
+            AssignmentSpec assignmentSpec = new AssignmentSpec(
+                memberSpecs,
+                topicMetadata
+            );
+
+            // We use `any` here to always return an assignment but use `verify` later on
+            // to ensure that the input was correct.
+            when(assignor.assign(any())).thenReturn(new GroupAssignment(assignments));
+
+            // Create and populate the assignment builder.
+            TargetAssignmentBuilder builder = new TargetAssignmentBuilder(groupId, groupEpoch, assignor)
+                .withMembers(members)
+                .withSubscriptionMetadata(subscriptionMetadata)
+                .withTargetAssignments(targetAssignments);
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull != null) {
+                    builder.withUpdatedMember(memberId, updatedMemberOrNull);
+                } else {
+                    builder.withRemoveMembers(memberId);
+                }
+            });
+
+            TargetAssignmentBuilder.TargetAssignmentResult result = builder.build();
+
+            verify(assignor, times(1)).assign(assignmentSpec);
+
+            return result;
+        }
+    }
+
+    @Test
+    public void testEmpty() {
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+        assertEquals(Collections.emptyMap(), result.assignments());
+    }
+
+    @Test
+    public void testAssignmentIsNotChanged() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertEquals(Collections.singletonList(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        )), result.records());
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        )));
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testAssignmentSwapped() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertUnorderedList(Arrays.asList(
+            newTargetAssignmentRecord("my-group", "member-1", mkAssignment(
+                mkTopicAssignment(fooTopicId, 4, 5, 6),
+                mkTopicAssignment(barTopicId, 4, 5, 6)
+            )),
+            newTargetAssignmentRecord("my-group", "member-2", mkAssignment(
+                mkTopicAssignment(fooTopicId, 1, 2, 3),
+                mkTopicAssignment(barTopicId, 1, 2, 3)
+            ))
+        ), result.records().subList(0, 2));
+
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        )));
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testNewMember() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        context.updateMemberSubscription("member-3", Arrays.asList("foo", "bar", "zar"));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        ));
+
+        context.prepareMemberAssignment("member-3", mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertUnorderedList(Arrays.asList(
+            newTargetAssignmentRecord("my-group", "member-1", mkAssignment(
+                mkTopicAssignment(fooTopicId, 1, 2),
+                mkTopicAssignment(barTopicId, 1, 2)
+            )),
+            newTargetAssignmentRecord("my-group", "member-2", mkAssignment(
+                mkTopicAssignment(fooTopicId, 3, 4),
+                mkTopicAssignment(barTopicId, 3, 4)
+            )),
+            newTargetAssignmentRecord("my-group", "member-3", mkAssignment(
+                mkTopicAssignment(fooTopicId, 5, 6),
+                mkTopicAssignment(barTopicId, 5, 6)
+            ))
+        ), result.records().subList(0, result.records().size() - 1));
+
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        )));
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        )));
+        expectedAssignment.put("member-3", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testUpdateMember() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 3, 4)
+        ));
+
+        context.addGroupMember("member-3", Arrays.asList("bar", "zar"), mkAssignment(
+            mkTopicAssignment(barTopicId, 5, 6)
+        ));
+
+        context.updateMemberSubscription(
+            "member-3",
+            Arrays.asList("foo", "bar", "zar"),
+            Optional.of("instance-id-3"),
+            Optional.of("rack-0")
+        );
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        ));
+
+        context.prepareMemberAssignment("member-3", mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertUnorderedList(Arrays.asList(
+            newTargetAssignmentRecord("my-group", "member-1", mkAssignment(
+                mkTopicAssignment(fooTopicId, 1, 2),
+                mkTopicAssignment(barTopicId, 1, 2)
+            )),
+            newTargetAssignmentRecord("my-group", "member-2", mkAssignment(
+                mkTopicAssignment(fooTopicId, 3, 4),
+                mkTopicAssignment(barTopicId, 3, 4)
+            )),
+            newTargetAssignmentRecord("my-group", "member-3", mkAssignment(
+                mkTopicAssignment(fooTopicId, 5, 6),
+                mkTopicAssignment(barTopicId, 5, 6)
+            ))
+        ), result.records().subList(0, result.records().size() - 1));
+
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        )));
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        )));
+        expectedAssignment.put("member-3", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testPartialAssignmentUpdate() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        ));
+
+        context.addGroupMember("member-3", Arrays.asList("bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        ));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4, 5),
+            mkTopicAssignment(barTopicId, 3, 4, 5)
+        ));
+
+        context.prepareMemberAssignment("member-3", mkAssignment(
+            mkTopicAssignment(fooTopicId, 6),
+            mkTopicAssignment(barTopicId, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        // Member 1 has not record because its assignment did not change.
+        assertUnorderedList(Arrays.asList(
+            newTargetAssignmentRecord("my-group", "member-2", mkAssignment(
+                mkTopicAssignment(fooTopicId, 3, 4, 5),
+                mkTopicAssignment(barTopicId, 3, 4, 5)
+            )),
+            newTargetAssignmentRecord("my-group", "member-3", mkAssignment(
+                mkTopicAssignment(fooTopicId, 6),
+                mkTopicAssignment(barTopicId, 6)
+            ))
+        ), result.records().subList(0, result.records().size() - 1));
+
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        )));
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4, 5),
+            mkTopicAssignment(barTopicId, 3, 4, 5)
+        )));
+        expectedAssignment.put("member-3", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 6),
+            mkTopicAssignment(barTopicId, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testDeleteMember() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        ));
+
+        context.addGroupMember("member-3", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        ));
+
+        context.removeMemberSubscription("member-3");
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertUnorderedList(Arrays.asList(
+            newTargetAssignmentRecord("my-group", "member-1", mkAssignment(
+                mkTopicAssignment(fooTopicId, 1, 2, 3),
+                mkTopicAssignment(barTopicId, 1, 2, 3)
+            )),
+            newTargetAssignmentRecord("my-group", "member-2", mkAssignment(
+                mkTopicAssignment(fooTopicId, 4, 5, 6),
+                mkTopicAssignment(barTopicId, 4, 5, 6)
+            ))
+        ), result.records().subList(0, result.records().size() - 1));
+
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        )));
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    public static <T> void assertUnorderedList(
+        List<T> expected,
+        List<T> actual
+    ) {
+        assertEquals(new HashSet<>(expected), new HashSet<>(actual));

Review Comment:
   wondering if we can assert the size before and after converting it into a set



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilder.java:
##########
@@ -0,0 +1,327 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.Record;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.MemberAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignorException;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+
+/**
+ * Build a new Target Assignment based on the provided parameters. As a result,
+ * it yields the records that must be persisted to the log and the new member
+ * assignments as a map.
+ *
+ * Records are only created for members which have a new target assignment. If
+ * their assignment did not change, no new record is needed.
+ *
+ * When a member is deleted, it is assumed that its target assignment record
+ * is deleted as part of the member deletion process. In other words, this class
+ * does not yield a tombstone for remove members.
+ */
+public class TargetAssignmentBuilder {
+    /**
+     * The assignment result returned by {{@link TargetAssignmentBuilder#build()}}.
+     */
+    public static class TargetAssignmentResult {
+        /**
+         * The records that must be applied to the __consumer_offsets
+         * topics to persist the new target assignment.
+         */
+        private final List<Record> records;
+
+        /**
+         * The new target assignment for all members.
+         */
+        private final Map<String, Assignment> assignments;
+
+        TargetAssignmentResult(
+            List<org.apache.kafka.coordinator.group.Record> records,
+            Map<String, Assignment> assignments
+        ) {
+            Objects.requireNonNull(records);
+            Objects.requireNonNull(assignments);
+            this.records = records;
+            this.assignments = assignments;
+        }
+
+        /**
+         * @return The records.
+         */
+        public List<Record> records() {
+            return records;
+        }
+
+        /**
+         * @return The assignment.
+         */
+        public Map<String, Assignment> assignments() {
+            return assignments;
+        }
+    }
+
+    /**
+     * The group id.
+     */
+    private final String groupId;
+
+    /**
+     * The group epoch.
+     */
+    private final int groupEpoch;
+
+    /**
+     * The partition assignor to compute the assignment.
+     */
+    private final PartitionAssignor assignor;
+
+    /**
+     * The members in the group.
+     */
+    private Map<String, ConsumerGroupMember> members = Collections.emptyMap();
+
+    /**
+     * The subscription metadata.
+     */
+    private Map<String, TopicMetadata> subscriptionMetadata = Collections.emptyMap();
+
+    /**
+     * The current target assignment.
+     */
+    private Map<String, Assignment> assignments = Collections.emptyMap();
+
+    /**
+     * The members which have been updated or deleted. Deleted members
+     * are signaled by a null value.
+     */
+    private Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();
+
+    /**
+     * Constructs the object.
+     *
+     * @param groupId       The group id.
+     * @param groupEpoch    The group epoch to compute a target assignment for.
+     * @param assignor      The assignor to use to compute the target assignment.
+     */
+    public TargetAssignmentBuilder(
+        String groupId,
+        int groupEpoch,
+        PartitionAssignor assignor
+    ) {
+        this.groupId = Objects.requireNonNull(groupId);
+        this.groupEpoch = groupEpoch;
+        this.assignor = Objects.requireNonNull(assignor);
+    }
+
+    /**
+     * Adds all the current members.
+     *
+     * @param members   The current members in the consumer groups.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withMembers(
+        Map<String, ConsumerGroupMember> members
+    ) {
+        this.members = members;
+        return this;
+    }
+
+    /**
+     * Adds the subscription metadata to use.
+     *
+     * @param subscriptionMetadata  The subscription metadata.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withSubscriptionMetadata(
+        Map<String, TopicMetadata> subscriptionMetadata
+    ) {
+        this.subscriptionMetadata = subscriptionMetadata;
+        return this;
+    }
+
+    /**
+     * Adds the current target assignments.
+     *
+     * @param assignments   The current assignments.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withTargetAssignments(
+        Map<String, Assignment> assignments
+    ) {
+        this.assignments = assignments;
+        return this;
+    }
+
+    /**
+     * Updates a member. This is useful when the updated member is
+     * not yet materialized in memory.
+     *
+     * @param memberId      The member id.
+     * @param updatedMember The updated member.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withUpdatedMember(
+        String memberId,
+        ConsumerGroupMember updatedMember
+    ) {
+        this.updatedMembers.put(memberId, updatedMember);
+        return this;
+    }
+
+    /**
+     * Removes a member. This is useful when the removed member
+     * is not yet materialized in memory.
+     *
+     * @param memberId The member id.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withRemoveMembers(

Review Comment:
   should this be `withRemovedMember`?



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilder.java:
##########
@@ -0,0 +1,327 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.Record;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.MemberAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignorException;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+
+/**
+ * Build a new Target Assignment based on the provided parameters. As a result,
+ * it yields the records that must be persisted to the log and the new member
+ * assignments as a map.
+ *
+ * Records are only created for members which have a new target assignment. If
+ * their assignment did not change, no new record is needed.
+ *
+ * When a member is deleted, it is assumed that its target assignment record
+ * is deleted as part of the member deletion process. In other words, this class
+ * does not yield a tombstone for remove members.
+ */
+public class TargetAssignmentBuilder {
+    /**
+     * The assignment result returned by {{@link TargetAssignmentBuilder#build()}}.
+     */
+    public static class TargetAssignmentResult {
+        /**
+         * The records that must be applied to the __consumer_offsets
+         * topics to persist the new target assignment.
+         */
+        private final List<Record> records;
+
+        /**
+         * The new target assignment for all members.
+         */
+        private final Map<String, Assignment> assignments;
+
+        TargetAssignmentResult(
+            List<org.apache.kafka.coordinator.group.Record> records,
+            Map<String, Assignment> assignments
+        ) {
+            Objects.requireNonNull(records);
+            Objects.requireNonNull(assignments);
+            this.records = records;
+            this.assignments = assignments;
+        }
+
+        /**
+         * @return The records.
+         */
+        public List<Record> records() {
+            return records;
+        }
+
+        /**
+         * @return The assignment.
+         */
+        public Map<String, Assignment> assignments() {
+            return assignments;
+        }
+    }
+
+    /**
+     * The group id.
+     */
+    private final String groupId;
+
+    /**
+     * The group epoch.
+     */
+    private final int groupEpoch;
+
+    /**
+     * The partition assignor to compute the assignment.
+     */
+    private final PartitionAssignor assignor;
+
+    /**
+     * The members in the group.
+     */
+    private Map<String, ConsumerGroupMember> members = Collections.emptyMap();
+
+    /**
+     * The subscription metadata.
+     */
+    private Map<String, TopicMetadata> subscriptionMetadata = Collections.emptyMap();
+
+    /**
+     * The current target assignment.
+     */
+    private Map<String, Assignment> assignments = Collections.emptyMap();
+
+    /**
+     * The members which have been updated or deleted. Deleted members
+     * are signaled by a null value.
+     */
+    private Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();
+
+    /**
+     * Constructs the object.
+     *
+     * @param groupId       The group id.
+     * @param groupEpoch    The group epoch to compute a target assignment for.
+     * @param assignor      The assignor to use to compute the target assignment.
+     */
+    public TargetAssignmentBuilder(
+        String groupId,
+        int groupEpoch,
+        PartitionAssignor assignor
+    ) {
+        this.groupId = Objects.requireNonNull(groupId);
+        this.groupEpoch = groupEpoch;
+        this.assignor = Objects.requireNonNull(assignor);
+    }
+
+    /**
+     * Adds all the current members.
+     *
+     * @param members   The current members in the consumer groups.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withMembers(
+        Map<String, ConsumerGroupMember> members
+    ) {
+        this.members = members;
+        return this;
+    }
+
+    /**
+     * Adds the subscription metadata to use.
+     *
+     * @param subscriptionMetadata  The subscription metadata.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withSubscriptionMetadata(
+        Map<String, TopicMetadata> subscriptionMetadata
+    ) {
+        this.subscriptionMetadata = subscriptionMetadata;
+        return this;
+    }
+
+    /**
+     * Adds the current target assignments.
+     *
+     * @param assignments   The current assignments.

Review Comment:
   i think this is confusing because we have both current and target assignment objects



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilderTest.java:
##########
@@ -0,0 +1,679 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkAssignment;
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkTopicAssignment;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class TargetAssignmentBuilderTest {
+
+    public static class TargetAssignmentBuilderTestContext {
+        private final String groupId;
+        private final int groupEpoch;
+        private final PartitionAssignor assignor = mock(PartitionAssignor.class);
+        private final Map<String, ConsumerGroupMember> members = new HashMap<>();
+        private final Map<String, TopicMetadata> subscriptionMetadata = new HashMap<>();
+        private final Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();
+        private final Map<String, Assignment> targetAssignments = new HashMap<>();
+        private final Map<String, org.apache.kafka.coordinator.group.assignor.MemberAssignment> assignments = new HashMap<>();
+
+        public TargetAssignmentBuilderTestContext(
+            String groupId,
+            int groupEpoch
+        ) {
+            this.groupId = groupId;
+            this.groupEpoch = groupEpoch;
+        }
+
+        public TargetAssignmentBuilderTestContext addGroupMember(
+            String memberId,
+            List<String> subscriptions,
+            Map<Uuid, Set<Integer>> targetPartitions
+        ) {
+            members.put(memberId, new ConsumerGroupMember.Builder(memberId)
+                .setSubscribedTopicNames(subscriptions)
+                .setRebalanceTimeoutMs(5000)
+                .build());
+
+            targetAssignments.put(memberId, new Assignment(
+                (byte) 0,
+                targetPartitions,
+                VersionedMetadata.EMPTY
+            ));
+
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext addTopicMetadata(
+            Uuid topicId,
+            String topicName,
+            int numPartitions
+        ) {
+            subscriptionMetadata.put(topicName, new TopicMetadata(
+                topicId,
+                topicName,
+                numPartitions
+            ));
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions
+        ) {
+            return updateMemberSubscription(
+                memberId,
+                subscriptions,
+                Optional.empty(),
+                Optional.empty()
+            );
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions,
+            Optional<String> instanceId,
+            Optional<String> rackId
+        ) {
+            ConsumerGroupMember existingMember = members.get(memberId);
+            ConsumerGroupMember.Builder builder;
+            if (existingMember != null) {
+                builder = new ConsumerGroupMember.Builder(existingMember);
+            } else {
+                builder = new ConsumerGroupMember.Builder(memberId);
+            }
+            updatedMembers.put(memberId, builder
+                .setSubscribedTopicNames(subscriptions)
+                .maybeUpdateInstanceId(instanceId)
+                .maybeUpdateRackId(rackId)
+                .build());
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext removeMemberSubscription(
+            String memberId
+        ) {
+            this.updatedMembers.put(memberId, null);
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext prepareMemberAssignment(
+            String memberId,
+            Map<Uuid, Set<Integer>> assignment
+        ) {
+            assignments.put(memberId, new org.apache.kafka.coordinator.group.assignor.MemberAssignment(assignment));
+            return this;
+        }
+
+        private AssignmentMemberSpec assignmentMemberSpec(
+            ConsumerGroupMember member,
+            Assignment targetAssignment
+        ) {
+            Set<Uuid> subscribedTopics = new HashSet<>();
+            member.subscribedTopicNames().forEach(topicName -> {
+                TopicMetadata topicMetadata = subscriptionMetadata.get(topicName);
+                if (topicMetadata != null) {
+                    subscribedTopics.add(topicMetadata.id());
+                }
+            });
+
+            return new AssignmentMemberSpec(
+                Optional.ofNullable(member.instanceId()),
+                Optional.ofNullable(member.rackId()),
+                subscribedTopics,
+                targetAssignment.partitions()
+            );
+        }
+
+        public TargetAssignmentBuilder.TargetAssignmentResult build() {
+            // Prepare expected member specs.
+            Map<String, AssignmentMemberSpec> memberSpecs = new HashMap<>();
+            members.forEach((memberId, member) -> {
+                memberSpecs.put(memberId, assignmentMemberSpec(
+                    member,
+                    targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                ));
+            });
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull == null) {
+                    memberSpecs.remove(memberId);
+                } else {
+                    memberSpecs.put(memberId, assignmentMemberSpec(
+                        updatedMemberOrNull,
+                        targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                    ));
+                }
+            });
+
+            Map<Uuid, AssignmentTopicMetadata> topicMetadata = new HashMap<>();
+            subscriptionMetadata.forEach((topicName, metadata) -> {
+                topicMetadata.put(metadata.id(), new AssignmentTopicMetadata(metadata.numPartitions()));
+            });
+
+            AssignmentSpec assignmentSpec = new AssignmentSpec(
+                memberSpecs,
+                topicMetadata
+            );
+
+            // We use `any` here to always return an assignment but use `verify` later on
+            // to ensure that the input was correct.
+            when(assignor.assign(any())).thenReturn(new GroupAssignment(assignments));
+
+            // Create and populate the assignment builder.
+            TargetAssignmentBuilder builder = new TargetAssignmentBuilder(groupId, groupEpoch, assignor)
+                .withMembers(members)
+                .withSubscriptionMetadata(subscriptionMetadata)
+                .withTargetAssignments(targetAssignments);
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull != null) {
+                    builder.withUpdatedMember(memberId, updatedMemberOrNull);
+                } else {
+                    builder.withRemoveMembers(memberId);
+                }
+            });
+
+            TargetAssignmentBuilder.TargetAssignmentResult result = builder.build();
+
+            verify(assignor, times(1)).assign(assignmentSpec);
+
+            return result;
+        }
+    }
+
+    @Test
+    public void testEmpty() {
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+        assertEquals(Collections.emptyMap(), result.assignments());
+    }
+
+    @Test
+    public void testAssignmentIsNotChanged() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertEquals(Collections.singletonList(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        )), result.records());
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        )));
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testAssignmentSwapped() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertUnorderedList(Arrays.asList(
+            newTargetAssignmentRecord("my-group", "member-1", mkAssignment(
+                mkTopicAssignment(fooTopicId, 4, 5, 6),
+                mkTopicAssignment(barTopicId, 4, 5, 6)
+            )),
+            newTargetAssignmentRecord("my-group", "member-2", mkAssignment(
+                mkTopicAssignment(fooTopicId, 1, 2, 3),
+                mkTopicAssignment(barTopicId, 1, 2, 3)
+            ))
+        ), result.records().subList(0, 2));
+
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));

Review Comment:
   nit: if we're using constants in L336, can we just use `2` here too?



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilder.java:
##########
@@ -0,0 +1,327 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.Record;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.MemberAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignorException;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+
+/**
+ * Build a new Target Assignment based on the provided parameters. As a result,
+ * it yields the records that must be persisted to the log and the new member
+ * assignments as a map.
+ *
+ * Records are only created for members which have a new target assignment. If
+ * their assignment did not change, no new record is needed.
+ *
+ * When a member is deleted, it is assumed that its target assignment record
+ * is deleted as part of the member deletion process. In other words, this class
+ * does not yield a tombstone for remove members.
+ */
+public class TargetAssignmentBuilder {
+    /**
+     * The assignment result returned by {{@link TargetAssignmentBuilder#build()}}.
+     */
+    public static class TargetAssignmentResult {
+        /**
+         * The records that must be applied to the __consumer_offsets
+         * topics to persist the new target assignment.
+         */
+        private final List<Record> records;
+
+        /**
+         * The new target assignment for all members.
+         */
+        private final Map<String, Assignment> assignments;
+
+        TargetAssignmentResult(
+            List<org.apache.kafka.coordinator.group.Record> records,
+            Map<String, Assignment> assignments
+        ) {
+            Objects.requireNonNull(records);
+            Objects.requireNonNull(assignments);
+            this.records = records;
+            this.assignments = assignments;
+        }
+
+        /**
+         * @return The records.
+         */
+        public List<Record> records() {
+            return records;
+        }
+
+        /**
+         * @return The assignment.
+         */
+        public Map<String, Assignment> assignments() {
+            return assignments;
+        }
+    }
+
+    /**
+     * The group id.
+     */
+    private final String groupId;
+
+    /**
+     * The group epoch.
+     */
+    private final int groupEpoch;
+
+    /**
+     * The partition assignor to compute the assignment.
+     */
+    private final PartitionAssignor assignor;
+
+    /**
+     * The members in the group.
+     */
+    private Map<String, ConsumerGroupMember> members = Collections.emptyMap();
+
+    /**
+     * The subscription metadata.
+     */
+    private Map<String, TopicMetadata> subscriptionMetadata = Collections.emptyMap();
+
+    /**
+     * The current target assignment.
+     */
+    private Map<String, Assignment> assignments = Collections.emptyMap();
+
+    /**
+     * The members which have been updated or deleted. Deleted members
+     * are signaled by a null value.
+     */
+    private Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();
+
+    /**
+     * Constructs the object.
+     *
+     * @param groupId       The group id.
+     * @param groupEpoch    The group epoch to compute a target assignment for.
+     * @param assignor      The assignor to use to compute the target assignment.
+     */
+    public TargetAssignmentBuilder(
+        String groupId,
+        int groupEpoch,
+        PartitionAssignor assignor
+    ) {
+        this.groupId = Objects.requireNonNull(groupId);
+        this.groupEpoch = groupEpoch;
+        this.assignor = Objects.requireNonNull(assignor);
+    }
+
+    /**
+     * Adds all the current members.
+     *
+     * @param members   The current members in the consumer groups.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withMembers(
+        Map<String, ConsumerGroupMember> members
+    ) {
+        this.members = members;
+        return this;
+    }
+
+    /**
+     * Adds the subscription metadata to use.
+     *
+     * @param subscriptionMetadata  The subscription metadata.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withSubscriptionMetadata(
+        Map<String, TopicMetadata> subscriptionMetadata
+    ) {
+        this.subscriptionMetadata = subscriptionMetadata;
+        return this;
+    }
+
+    /**
+     * Adds the current target assignments.
+     *
+     * @param assignments   The current assignments.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withTargetAssignments(
+        Map<String, Assignment> assignments
+    ) {
+        this.assignments = assignments;
+        return this;
+    }
+
+    /**
+     * Updates a member. This is useful when the updated member is
+     * not yet materialized in memory.
+     *
+     * @param memberId      The member id.
+     * @param updatedMember The updated member.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withUpdatedMember(
+        String memberId,
+        ConsumerGroupMember updatedMember
+    ) {
+        this.updatedMembers.put(memberId, updatedMember);
+        return this;
+    }
+
+    /**
+     * Removes a member. This is useful when the removed member
+     * is not yet materialized in memory.
+     *
+     * @param memberId The member id.
+     * @return This object.
+     */
+    public TargetAssignmentBuilder withRemoveMembers(
+        String memberId
+    ) {
+        return withUpdatedMember(memberId, null);
+    }
+
+    /**
+     * Builds the new target assignment.
+     *
+     * @return A TargetAssignmentResult which contains the records to update
+     *         the current target assignment.

Review Comment:
   nit: existing



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilder.java:
##########
@@ -0,0 +1,327 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.Record;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.MemberAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignorException;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+
+/**
+ * Build a new Target Assignment based on the provided parameters. As a result,
+ * it yields the records that must be persisted to the log and the new member
+ * assignments as a map.
+ *
+ * Records are only created for members which have a new target assignment. If
+ * their assignment did not change, no new record is needed.
+ *
+ * When a member is deleted, it is assumed that its target assignment record
+ * is deleted as part of the member deletion process. In other words, this class
+ * does not yield a tombstone for remove members.
+ */
+public class TargetAssignmentBuilder {
+    /**
+     * The assignment result returned by {{@link TargetAssignmentBuilder#build()}}.
+     */
+    public static class TargetAssignmentResult {
+        /**
+         * The records that must be applied to the __consumer_offsets
+         * topics to persist the new target assignment.
+         */
+        private final List<Record> records;
+
+        /**
+         * The new target assignment for all members.
+         */
+        private final Map<String, Assignment> assignments;
+
+        TargetAssignmentResult(
+            List<org.apache.kafka.coordinator.group.Record> records,
+            Map<String, Assignment> assignments
+        ) {
+            Objects.requireNonNull(records);
+            Objects.requireNonNull(assignments);
+            this.records = records;
+            this.assignments = assignments;
+        }
+
+        /**
+         * @return The records.
+         */
+        public List<Record> records() {
+            return records;
+        }
+
+        /**
+         * @return The assignment.
+         */
+        public Map<String, Assignment> assignments() {
+            return assignments;
+        }
+    }
+
+    /**
+     * The group id.
+     */
+    private final String groupId;
+
+    /**
+     * The group epoch.
+     */
+    private final int groupEpoch;
+
+    /**
+     * The partition assignor to compute the assignment.
+     */
+    private final PartitionAssignor assignor;
+
+    /**
+     * The members in the group.
+     */
+    private Map<String, ConsumerGroupMember> members = Collections.emptyMap();
+
+    /**
+     * The subscription metadata.
+     */
+    private Map<String, TopicMetadata> subscriptionMetadata = Collections.emptyMap();
+
+    /**
+     * The current target assignment.
+     */
+    private Map<String, Assignment> assignments = Collections.emptyMap();
+
+    /**
+     * The members which have been updated or deleted. Deleted members
+     * are signaled by a null value.
+     */
+    private Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();
+
+    /**
+     * Constructs the object.
+     *
+     * @param groupId       The group id.
+     * @param groupEpoch    The group epoch to compute a target assignment for.
+     * @param assignor      The assignor to use to compute the target assignment.
+     */
+    public TargetAssignmentBuilder(
+        String groupId,
+        int groupEpoch,
+        PartitionAssignor assignor
+    ) {
+        this.groupId = Objects.requireNonNull(groupId);
+        this.groupEpoch = groupEpoch;
+        this.assignor = Objects.requireNonNull(assignor);
+    }
+
+    /**
+     * Adds all the current members.
+     *
+     * @param members   The current members in the consumer groups.

Review Comment:
   nit: consumer group



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilder.java:
##########
@@ -0,0 +1,327 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.Record;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.MemberAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignorException;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+
+/**
+ * Build a new Target Assignment based on the provided parameters. As a result,
+ * it yields the records that must be persisted to the log and the new member
+ * assignments as a map.
+ *
+ * Records are only created for members which have a new target assignment. If
+ * their assignment did not change, no new record is needed.
+ *
+ * When a member is deleted, it is assumed that its target assignment record
+ * is deleted as part of the member deletion process. In other words, this class
+ * does not yield a tombstone for remove members.
+ */
+public class TargetAssignmentBuilder {
+    /**
+     * The assignment result returned by {{@link TargetAssignmentBuilder#build()}}.
+     */
+    public static class TargetAssignmentResult {
+        /**
+         * The records that must be applied to the __consumer_offsets
+         * topics to persist the new target assignment.
+         */
+        private final List<Record> records;
+
+        /**
+         * The new target assignment for all members.
+         */
+        private final Map<String, Assignment> assignments;
+
+        TargetAssignmentResult(
+            List<org.apache.kafka.coordinator.group.Record> records,
+            Map<String, Assignment> assignments
+        ) {
+            Objects.requireNonNull(records);
+            Objects.requireNonNull(assignments);
+            this.records = records;
+            this.assignments = assignments;
+        }
+
+        /**
+         * @return The records.
+         */
+        public List<Record> records() {
+            return records;
+        }
+
+        /**
+         * @return The assignment.
+         */
+        public Map<String, Assignment> assignments() {
+            return assignments;
+        }
+    }
+
+    /**
+     * The group id.
+     */
+    private final String groupId;
+
+    /**
+     * The group epoch.
+     */
+    private final int groupEpoch;
+
+    /**
+     * The partition assignor to compute the assignment.
+     */
+    private final PartitionAssignor assignor;
+
+    /**
+     * The members in the group.
+     */
+    private Map<String, ConsumerGroupMember> members = Collections.emptyMap();
+
+    /**
+     * The subscription metadata.
+     */
+    private Map<String, TopicMetadata> subscriptionMetadata = Collections.emptyMap();
+
+    /**
+     * The current target assignment.
+     */
+    private Map<String, Assignment> assignments = Collections.emptyMap();
+
+    /**
+     * The members which have been updated or deleted. Deleted members
+     * are signaled by a null value.
+     */
+    private Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();

Review Comment:
   can this be final?



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilderTest.java:
##########
@@ -0,0 +1,679 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkAssignment;
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkTopicAssignment;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class TargetAssignmentBuilderTest {
+
+    public static class TargetAssignmentBuilderTestContext {
+        private final String groupId;
+        private final int groupEpoch;
+        private final PartitionAssignor assignor = mock(PartitionAssignor.class);
+        private final Map<String, ConsumerGroupMember> members = new HashMap<>();
+        private final Map<String, TopicMetadata> subscriptionMetadata = new HashMap<>();
+        private final Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();
+        private final Map<String, Assignment> targetAssignments = new HashMap<>();
+        private final Map<String, org.apache.kafka.coordinator.group.assignor.MemberAssignment> assignments = new HashMap<>();
+
+        public TargetAssignmentBuilderTestContext(
+            String groupId,
+            int groupEpoch
+        ) {
+            this.groupId = groupId;
+            this.groupEpoch = groupEpoch;
+        }
+
+        public TargetAssignmentBuilderTestContext addGroupMember(
+            String memberId,
+            List<String> subscriptions,
+            Map<Uuid, Set<Integer>> targetPartitions
+        ) {
+            members.put(memberId, new ConsumerGroupMember.Builder(memberId)
+                .setSubscribedTopicNames(subscriptions)
+                .setRebalanceTimeoutMs(5000)
+                .build());
+
+            targetAssignments.put(memberId, new Assignment(
+                (byte) 0,
+                targetPartitions,
+                VersionedMetadata.EMPTY
+            ));
+
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext addTopicMetadata(
+            Uuid topicId,
+            String topicName,
+            int numPartitions
+        ) {
+            subscriptionMetadata.put(topicName, new TopicMetadata(
+                topicId,
+                topicName,
+                numPartitions
+            ));
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions
+        ) {
+            return updateMemberSubscription(
+                memberId,
+                subscriptions,
+                Optional.empty(),
+                Optional.empty()
+            );
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions,
+            Optional<String> instanceId,
+            Optional<String> rackId
+        ) {
+            ConsumerGroupMember existingMember = members.get(memberId);
+            ConsumerGroupMember.Builder builder;
+            if (existingMember != null) {
+                builder = new ConsumerGroupMember.Builder(existingMember);
+            } else {
+                builder = new ConsumerGroupMember.Builder(memberId);
+            }
+            updatedMembers.put(memberId, builder
+                .setSubscribedTopicNames(subscriptions)
+                .maybeUpdateInstanceId(instanceId)
+                .maybeUpdateRackId(rackId)
+                .build());
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext removeMemberSubscription(
+            String memberId
+        ) {
+            this.updatedMembers.put(memberId, null);
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext prepareMemberAssignment(
+            String memberId,
+            Map<Uuid, Set<Integer>> assignment
+        ) {
+            assignments.put(memberId, new org.apache.kafka.coordinator.group.assignor.MemberAssignment(assignment));
+            return this;
+        }
+
+        private AssignmentMemberSpec assignmentMemberSpec(
+            ConsumerGroupMember member,
+            Assignment targetAssignment
+        ) {
+            Set<Uuid> subscribedTopics = new HashSet<>();
+            member.subscribedTopicNames().forEach(topicName -> {
+                TopicMetadata topicMetadata = subscriptionMetadata.get(topicName);
+                if (topicMetadata != null) {
+                    subscribedTopics.add(topicMetadata.id());
+                }
+            });
+
+            return new AssignmentMemberSpec(
+                Optional.ofNullable(member.instanceId()),
+                Optional.ofNullable(member.rackId()),
+                subscribedTopics,
+                targetAssignment.partitions()
+            );
+        }
+
+        public TargetAssignmentBuilder.TargetAssignmentResult build() {
+            // Prepare expected member specs.
+            Map<String, AssignmentMemberSpec> memberSpecs = new HashMap<>();
+            members.forEach((memberId, member) -> {
+                memberSpecs.put(memberId, assignmentMemberSpec(
+                    member,
+                    targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                ));
+            });
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull == null) {
+                    memberSpecs.remove(memberId);
+                } else {
+                    memberSpecs.put(memberId, assignmentMemberSpec(
+                        updatedMemberOrNull,
+                        targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                    ));
+                }
+            });
+
+            Map<Uuid, AssignmentTopicMetadata> topicMetadata = new HashMap<>();
+            subscriptionMetadata.forEach((topicName, metadata) -> {
+                topicMetadata.put(metadata.id(), new AssignmentTopicMetadata(metadata.numPartitions()));
+            });
+
+            AssignmentSpec assignmentSpec = new AssignmentSpec(
+                memberSpecs,
+                topicMetadata
+            );
+
+            // We use `any` here to always return an assignment but use `verify` later on
+            // to ensure that the input was correct.
+            when(assignor.assign(any())).thenReturn(new GroupAssignment(assignments));
+
+            // Create and populate the assignment builder.
+            TargetAssignmentBuilder builder = new TargetAssignmentBuilder(groupId, groupEpoch, assignor)
+                .withMembers(members)
+                .withSubscriptionMetadata(subscriptionMetadata)
+                .withTargetAssignments(targetAssignments);
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull != null) {
+                    builder.withUpdatedMember(memberId, updatedMemberOrNull);
+                } else {
+                    builder.withRemoveMembers(memberId);
+                }
+            });
+
+            TargetAssignmentBuilder.TargetAssignmentResult result = builder.build();
+
+            verify(assignor, times(1)).assign(assignmentSpec);
+
+            return result;
+        }
+    }
+
+    @Test
+    public void testEmpty() {
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));

Review Comment:
   is the size non-deterministic? isn't the size 1 since we don't expect any TargetAssignmentMember records?



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilderTest.java:
##########
@@ -0,0 +1,679 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkAssignment;
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkTopicAssignment;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class TargetAssignmentBuilderTest {
+
+    public static class TargetAssignmentBuilderTestContext {
+        private final String groupId;
+        private final int groupEpoch;
+        private final PartitionAssignor assignor = mock(PartitionAssignor.class);
+        private final Map<String, ConsumerGroupMember> members = new HashMap<>();
+        private final Map<String, TopicMetadata> subscriptionMetadata = new HashMap<>();
+        private final Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();
+        private final Map<String, Assignment> targetAssignments = new HashMap<>();
+        private final Map<String, org.apache.kafka.coordinator.group.assignor.MemberAssignment> assignments = new HashMap<>();
+
+        public TargetAssignmentBuilderTestContext(
+            String groupId,
+            int groupEpoch
+        ) {
+            this.groupId = groupId;
+            this.groupEpoch = groupEpoch;
+        }
+
+        public TargetAssignmentBuilderTestContext addGroupMember(
+            String memberId,
+            List<String> subscriptions,
+            Map<Uuid, Set<Integer>> targetPartitions
+        ) {
+            members.put(memberId, new ConsumerGroupMember.Builder(memberId)
+                .setSubscribedTopicNames(subscriptions)
+                .setRebalanceTimeoutMs(5000)
+                .build());
+
+            targetAssignments.put(memberId, new Assignment(
+                (byte) 0,
+                targetPartitions,
+                VersionedMetadata.EMPTY
+            ));
+
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext addTopicMetadata(
+            Uuid topicId,
+            String topicName,
+            int numPartitions
+        ) {
+            subscriptionMetadata.put(topicName, new TopicMetadata(
+                topicId,
+                topicName,
+                numPartitions
+            ));
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions
+        ) {
+            return updateMemberSubscription(
+                memberId,
+                subscriptions,
+                Optional.empty(),
+                Optional.empty()
+            );
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions,
+            Optional<String> instanceId,
+            Optional<String> rackId
+        ) {
+            ConsumerGroupMember existingMember = members.get(memberId);
+            ConsumerGroupMember.Builder builder;
+            if (existingMember != null) {
+                builder = new ConsumerGroupMember.Builder(existingMember);
+            } else {
+                builder = new ConsumerGroupMember.Builder(memberId);
+            }
+            updatedMembers.put(memberId, builder
+                .setSubscribedTopicNames(subscriptions)
+                .maybeUpdateInstanceId(instanceId)
+                .maybeUpdateRackId(rackId)
+                .build());
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext removeMemberSubscription(
+            String memberId
+        ) {
+            this.updatedMembers.put(memberId, null);
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext prepareMemberAssignment(
+            String memberId,
+            Map<Uuid, Set<Integer>> assignment
+        ) {
+            assignments.put(memberId, new org.apache.kafka.coordinator.group.assignor.MemberAssignment(assignment));
+            return this;
+        }
+
+        private AssignmentMemberSpec assignmentMemberSpec(
+            ConsumerGroupMember member,
+            Assignment targetAssignment
+        ) {
+            Set<Uuid> subscribedTopics = new HashSet<>();
+            member.subscribedTopicNames().forEach(topicName -> {
+                TopicMetadata topicMetadata = subscriptionMetadata.get(topicName);
+                if (topicMetadata != null) {
+                    subscribedTopics.add(topicMetadata.id());
+                }
+            });
+
+            return new AssignmentMemberSpec(
+                Optional.ofNullable(member.instanceId()),
+                Optional.ofNullable(member.rackId()),
+                subscribedTopics,
+                targetAssignment.partitions()
+            );
+        }
+
+        public TargetAssignmentBuilder.TargetAssignmentResult build() {
+            // Prepare expected member specs.
+            Map<String, AssignmentMemberSpec> memberSpecs = new HashMap<>();
+            members.forEach((memberId, member) -> {
+                memberSpecs.put(memberId, assignmentMemberSpec(
+                    member,
+                    targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                ));
+            });
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull == null) {
+                    memberSpecs.remove(memberId);
+                } else {
+                    memberSpecs.put(memberId, assignmentMemberSpec(
+                        updatedMemberOrNull,
+                        targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                    ));
+                }
+            });
+
+            Map<Uuid, AssignmentTopicMetadata> topicMetadata = new HashMap<>();
+            subscriptionMetadata.forEach((topicName, metadata) -> {
+                topicMetadata.put(metadata.id(), new AssignmentTopicMetadata(metadata.numPartitions()));
+            });
+
+            AssignmentSpec assignmentSpec = new AssignmentSpec(
+                memberSpecs,
+                topicMetadata
+            );
+
+            // We use `any` here to always return an assignment but use `verify` later on
+            // to ensure that the input was correct.
+            when(assignor.assign(any())).thenReturn(new GroupAssignment(assignments));
+
+            // Create and populate the assignment builder.
+            TargetAssignmentBuilder builder = new TargetAssignmentBuilder(groupId, groupEpoch, assignor)
+                .withMembers(members)
+                .withSubscriptionMetadata(subscriptionMetadata)
+                .withTargetAssignments(targetAssignments);
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull != null) {
+                    builder.withUpdatedMember(memberId, updatedMemberOrNull);
+                } else {
+                    builder.withRemoveMembers(memberId);
+                }
+            });
+
+            TargetAssignmentBuilder.TargetAssignmentResult result = builder.build();
+
+            verify(assignor, times(1)).assign(assignmentSpec);
+
+            return result;
+        }
+    }
+
+    @Test
+    public void testEmpty() {
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+        assertEquals(Collections.emptyMap(), result.assignments());
+    }
+
+    @Test
+    public void testAssignmentIsNotChanged() {

Review Comment:
   nit: HasNot



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilderTest.java:
##########
@@ -0,0 +1,679 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkAssignment;
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkTopicAssignment;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class TargetAssignmentBuilderTest {
+
+    public static class TargetAssignmentBuilderTestContext {
+        private final String groupId;
+        private final int groupEpoch;
+        private final PartitionAssignor assignor = mock(PartitionAssignor.class);
+        private final Map<String, ConsumerGroupMember> members = new HashMap<>();
+        private final Map<String, TopicMetadata> subscriptionMetadata = new HashMap<>();
+        private final Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();
+        private final Map<String, Assignment> targetAssignments = new HashMap<>();
+        private final Map<String, org.apache.kafka.coordinator.group.assignor.MemberAssignment> assignments = new HashMap<>();
+
+        public TargetAssignmentBuilderTestContext(
+            String groupId,
+            int groupEpoch
+        ) {
+            this.groupId = groupId;
+            this.groupEpoch = groupEpoch;
+        }
+
+        public TargetAssignmentBuilderTestContext addGroupMember(
+            String memberId,
+            List<String> subscriptions,
+            Map<Uuid, Set<Integer>> targetPartitions
+        ) {
+            members.put(memberId, new ConsumerGroupMember.Builder(memberId)
+                .setSubscribedTopicNames(subscriptions)
+                .setRebalanceTimeoutMs(5000)
+                .build());
+
+            targetAssignments.put(memberId, new Assignment(
+                (byte) 0,
+                targetPartitions,
+                VersionedMetadata.EMPTY
+            ));
+
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext addTopicMetadata(
+            Uuid topicId,
+            String topicName,
+            int numPartitions
+        ) {
+            subscriptionMetadata.put(topicName, new TopicMetadata(
+                topicId,
+                topicName,
+                numPartitions
+            ));
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions
+        ) {
+            return updateMemberSubscription(
+                memberId,
+                subscriptions,
+                Optional.empty(),
+                Optional.empty()
+            );
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions,
+            Optional<String> instanceId,
+            Optional<String> rackId
+        ) {
+            ConsumerGroupMember existingMember = members.get(memberId);
+            ConsumerGroupMember.Builder builder;
+            if (existingMember != null) {
+                builder = new ConsumerGroupMember.Builder(existingMember);
+            } else {
+                builder = new ConsumerGroupMember.Builder(memberId);
+            }
+            updatedMembers.put(memberId, builder
+                .setSubscribedTopicNames(subscriptions)
+                .maybeUpdateInstanceId(instanceId)
+                .maybeUpdateRackId(rackId)
+                .build());
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext removeMemberSubscription(
+            String memberId
+        ) {
+            this.updatedMembers.put(memberId, null);
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext prepareMemberAssignment(
+            String memberId,
+            Map<Uuid, Set<Integer>> assignment
+        ) {
+            assignments.put(memberId, new org.apache.kafka.coordinator.group.assignor.MemberAssignment(assignment));
+            return this;
+        }
+
+        private AssignmentMemberSpec assignmentMemberSpec(
+            ConsumerGroupMember member,
+            Assignment targetAssignment
+        ) {
+            Set<Uuid> subscribedTopics = new HashSet<>();
+            member.subscribedTopicNames().forEach(topicName -> {
+                TopicMetadata topicMetadata = subscriptionMetadata.get(topicName);
+                if (topicMetadata != null) {
+                    subscribedTopics.add(topicMetadata.id());
+                }
+            });
+
+            return new AssignmentMemberSpec(
+                Optional.ofNullable(member.instanceId()),
+                Optional.ofNullable(member.rackId()),
+                subscribedTopics,
+                targetAssignment.partitions()
+            );
+        }
+
+        public TargetAssignmentBuilder.TargetAssignmentResult build() {
+            // Prepare expected member specs.
+            Map<String, AssignmentMemberSpec> memberSpecs = new HashMap<>();
+            members.forEach((memberId, member) -> {
+                memberSpecs.put(memberId, assignmentMemberSpec(
+                    member,
+                    targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                ));
+            });
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull == null) {
+                    memberSpecs.remove(memberId);
+                } else {
+                    memberSpecs.put(memberId, assignmentMemberSpec(
+                        updatedMemberOrNull,
+                        targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                    ));
+                }
+            });
+
+            Map<Uuid, AssignmentTopicMetadata> topicMetadata = new HashMap<>();
+            subscriptionMetadata.forEach((topicName, metadata) -> {
+                topicMetadata.put(metadata.id(), new AssignmentTopicMetadata(metadata.numPartitions()));
+            });
+
+            AssignmentSpec assignmentSpec = new AssignmentSpec(
+                memberSpecs,
+                topicMetadata
+            );
+
+            // We use `any` here to always return an assignment but use `verify` later on
+            // to ensure that the input was correct.
+            when(assignor.assign(any())).thenReturn(new GroupAssignment(assignments));
+
+            // Create and populate the assignment builder.
+            TargetAssignmentBuilder builder = new TargetAssignmentBuilder(groupId, groupEpoch, assignor)
+                .withMembers(members)
+                .withSubscriptionMetadata(subscriptionMetadata)
+                .withTargetAssignments(targetAssignments);
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull != null) {
+                    builder.withUpdatedMember(memberId, updatedMemberOrNull);
+                } else {
+                    builder.withRemoveMembers(memberId);
+                }
+            });
+
+            TargetAssignmentBuilder.TargetAssignmentResult result = builder.build();
+
+            verify(assignor, times(1)).assign(assignmentSpec);
+
+            return result;
+        }
+    }
+
+    @Test
+    public void testEmpty() {
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+        assertEquals(Collections.emptyMap(), result.assignments());
+    }
+
+    @Test
+    public void testAssignmentIsNotChanged() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertEquals(Collections.singletonList(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        )), result.records());
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        )));
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testAssignmentSwapped() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertUnorderedList(Arrays.asList(
+            newTargetAssignmentRecord("my-group", "member-1", mkAssignment(
+                mkTopicAssignment(fooTopicId, 4, 5, 6),
+                mkTopicAssignment(barTopicId, 4, 5, 6)
+            )),
+            newTargetAssignmentRecord("my-group", "member-2", mkAssignment(
+                mkTopicAssignment(fooTopicId, 1, 2, 3),
+                mkTopicAssignment(barTopicId, 1, 2, 3)
+            ))
+        ), result.records().subList(0, 2));
+
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        )));
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testNewMember() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        context.updateMemberSubscription("member-3", Arrays.asList("foo", "bar", "zar"));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        ));
+
+        context.prepareMemberAssignment("member-3", mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertUnorderedList(Arrays.asList(
+            newTargetAssignmentRecord("my-group", "member-1", mkAssignment(
+                mkTopicAssignment(fooTopicId, 1, 2),
+                mkTopicAssignment(barTopicId, 1, 2)
+            )),
+            newTargetAssignmentRecord("my-group", "member-2", mkAssignment(
+                mkTopicAssignment(fooTopicId, 3, 4),
+                mkTopicAssignment(barTopicId, 3, 4)
+            )),
+            newTargetAssignmentRecord("my-group", "member-3", mkAssignment(
+                mkTopicAssignment(fooTopicId, 5, 6),
+                mkTopicAssignment(barTopicId, 5, 6)
+            ))
+        ), result.records().subList(0, result.records().size() - 1));

Review Comment:
   maybe we can use this for the above test too?



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/consumer/TargetAssignmentBuilderTest.java:
##########
@@ -0,0 +1,679 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.kafka.coordinator.group.consumer;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.coordinator.group.assignor.AssignmentMemberSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentSpec;
+import org.apache.kafka.coordinator.group.assignor.AssignmentTopicMetadata;
+import org.apache.kafka.coordinator.group.assignor.GroupAssignment;
+import org.apache.kafka.coordinator.group.assignor.PartitionAssignor;
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkAssignment;
+import static org.apache.kafka.coordinator.group.consumer.AssignmentTestUtil.mkTopicAssignment;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentEpochRecord;
+import static org.apache.kafka.coordinator.group.RecordHelpers.newTargetAssignmentRecord;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class TargetAssignmentBuilderTest {
+
+    public static class TargetAssignmentBuilderTestContext {
+        private final String groupId;
+        private final int groupEpoch;
+        private final PartitionAssignor assignor = mock(PartitionAssignor.class);
+        private final Map<String, ConsumerGroupMember> members = new HashMap<>();
+        private final Map<String, TopicMetadata> subscriptionMetadata = new HashMap<>();
+        private final Map<String, ConsumerGroupMember> updatedMembers = new HashMap<>();
+        private final Map<String, Assignment> targetAssignments = new HashMap<>();
+        private final Map<String, org.apache.kafka.coordinator.group.assignor.MemberAssignment> assignments = new HashMap<>();
+
+        public TargetAssignmentBuilderTestContext(
+            String groupId,
+            int groupEpoch
+        ) {
+            this.groupId = groupId;
+            this.groupEpoch = groupEpoch;
+        }
+
+        public TargetAssignmentBuilderTestContext addGroupMember(
+            String memberId,
+            List<String> subscriptions,
+            Map<Uuid, Set<Integer>> targetPartitions
+        ) {
+            members.put(memberId, new ConsumerGroupMember.Builder(memberId)
+                .setSubscribedTopicNames(subscriptions)
+                .setRebalanceTimeoutMs(5000)
+                .build());
+
+            targetAssignments.put(memberId, new Assignment(
+                (byte) 0,
+                targetPartitions,
+                VersionedMetadata.EMPTY
+            ));
+
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext addTopicMetadata(
+            Uuid topicId,
+            String topicName,
+            int numPartitions
+        ) {
+            subscriptionMetadata.put(topicName, new TopicMetadata(
+                topicId,
+                topicName,
+                numPartitions
+            ));
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions
+        ) {
+            return updateMemberSubscription(
+                memberId,
+                subscriptions,
+                Optional.empty(),
+                Optional.empty()
+            );
+        }
+
+        public TargetAssignmentBuilderTestContext updateMemberSubscription(
+            String memberId,
+            List<String> subscriptions,
+            Optional<String> instanceId,
+            Optional<String> rackId
+        ) {
+            ConsumerGroupMember existingMember = members.get(memberId);
+            ConsumerGroupMember.Builder builder;
+            if (existingMember != null) {
+                builder = new ConsumerGroupMember.Builder(existingMember);
+            } else {
+                builder = new ConsumerGroupMember.Builder(memberId);
+            }
+            updatedMembers.put(memberId, builder
+                .setSubscribedTopicNames(subscriptions)
+                .maybeUpdateInstanceId(instanceId)
+                .maybeUpdateRackId(rackId)
+                .build());
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext removeMemberSubscription(
+            String memberId
+        ) {
+            this.updatedMembers.put(memberId, null);
+            return this;
+        }
+
+        public TargetAssignmentBuilderTestContext prepareMemberAssignment(
+            String memberId,
+            Map<Uuid, Set<Integer>> assignment
+        ) {
+            assignments.put(memberId, new org.apache.kafka.coordinator.group.assignor.MemberAssignment(assignment));
+            return this;
+        }
+
+        private AssignmentMemberSpec assignmentMemberSpec(
+            ConsumerGroupMember member,
+            Assignment targetAssignment
+        ) {
+            Set<Uuid> subscribedTopics = new HashSet<>();
+            member.subscribedTopicNames().forEach(topicName -> {
+                TopicMetadata topicMetadata = subscriptionMetadata.get(topicName);
+                if (topicMetadata != null) {
+                    subscribedTopics.add(topicMetadata.id());
+                }
+            });
+
+            return new AssignmentMemberSpec(
+                Optional.ofNullable(member.instanceId()),
+                Optional.ofNullable(member.rackId()),
+                subscribedTopics,
+                targetAssignment.partitions()
+            );
+        }
+
+        public TargetAssignmentBuilder.TargetAssignmentResult build() {
+            // Prepare expected member specs.
+            Map<String, AssignmentMemberSpec> memberSpecs = new HashMap<>();
+            members.forEach((memberId, member) -> {
+                memberSpecs.put(memberId, assignmentMemberSpec(
+                    member,
+                    targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                ));
+            });
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull == null) {
+                    memberSpecs.remove(memberId);
+                } else {
+                    memberSpecs.put(memberId, assignmentMemberSpec(
+                        updatedMemberOrNull,
+                        targetAssignments.getOrDefault(memberId, Assignment.EMPTY)
+                    ));
+                }
+            });
+
+            Map<Uuid, AssignmentTopicMetadata> topicMetadata = new HashMap<>();
+            subscriptionMetadata.forEach((topicName, metadata) -> {
+                topicMetadata.put(metadata.id(), new AssignmentTopicMetadata(metadata.numPartitions()));
+            });
+
+            AssignmentSpec assignmentSpec = new AssignmentSpec(
+                memberSpecs,
+                topicMetadata
+            );
+
+            // We use `any` here to always return an assignment but use `verify` later on
+            // to ensure that the input was correct.
+            when(assignor.assign(any())).thenReturn(new GroupAssignment(assignments));
+
+            // Create and populate the assignment builder.
+            TargetAssignmentBuilder builder = new TargetAssignmentBuilder(groupId, groupEpoch, assignor)
+                .withMembers(members)
+                .withSubscriptionMetadata(subscriptionMetadata)
+                .withTargetAssignments(targetAssignments);
+
+            updatedMembers.forEach((memberId, updatedMemberOrNull) -> {
+                if (updatedMemberOrNull != null) {
+                    builder.withUpdatedMember(memberId, updatedMemberOrNull);
+                } else {
+                    builder.withRemoveMembers(memberId);
+                }
+            });
+
+            TargetAssignmentBuilder.TargetAssignmentResult result = builder.build();
+
+            verify(assignor, times(1)).assign(assignmentSpec);
+
+            return result;
+        }
+    }
+
+    @Test
+    public void testEmpty() {
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+        assertEquals(Collections.emptyMap(), result.assignments());
+    }
+
+    @Test
+    public void testAssignmentIsNotChanged() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertEquals(Collections.singletonList(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        )), result.records());
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        )));
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testAssignmentSwapped() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertUnorderedList(Arrays.asList(
+            newTargetAssignmentRecord("my-group", "member-1", mkAssignment(
+                mkTopicAssignment(fooTopicId, 4, 5, 6),
+                mkTopicAssignment(barTopicId, 4, 5, 6)
+            )),
+            newTargetAssignmentRecord("my-group", "member-2", mkAssignment(
+                mkTopicAssignment(fooTopicId, 1, 2, 3),
+                mkTopicAssignment(barTopicId, 1, 2, 3)
+            ))
+        ), result.records().subList(0, 2));
+
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        )));
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testNewMember() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2, 3)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 4, 5, 6)
+        ));
+
+        context.updateMemberSubscription("member-3", Arrays.asList("foo", "bar", "zar"));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        ));
+
+        context.prepareMemberAssignment("member-3", mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertUnorderedList(Arrays.asList(
+            newTargetAssignmentRecord("my-group", "member-1", mkAssignment(
+                mkTopicAssignment(fooTopicId, 1, 2),
+                mkTopicAssignment(barTopicId, 1, 2)
+            )),
+            newTargetAssignmentRecord("my-group", "member-2", mkAssignment(
+                mkTopicAssignment(fooTopicId, 3, 4),
+                mkTopicAssignment(barTopicId, 3, 4)
+            )),
+            newTargetAssignmentRecord("my-group", "member-3", mkAssignment(
+                mkTopicAssignment(fooTopicId, 5, 6),
+                mkTopicAssignment(barTopicId, 5, 6)
+            ))
+        ), result.records().subList(0, result.records().size() - 1));
+
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        )));
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        )));
+        expectedAssignment.put("member-3", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testUpdateMember() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2, 3),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 4, 5, 6),
+            mkTopicAssignment(barTopicId, 3, 4)
+        ));
+
+        context.addGroupMember("member-3", Arrays.asList("bar", "zar"), mkAssignment(
+            mkTopicAssignment(barTopicId, 5, 6)
+        ));
+
+        context.updateMemberSubscription(
+            "member-3",
+            Arrays.asList("foo", "bar", "zar"),
+            Optional.of("instance-id-3"),
+            Optional.of("rack-0")
+        );
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        ));
+
+        context.prepareMemberAssignment("member-3", mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        assertUnorderedList(Arrays.asList(
+            newTargetAssignmentRecord("my-group", "member-1", mkAssignment(
+                mkTopicAssignment(fooTopicId, 1, 2),
+                mkTopicAssignment(barTopicId, 1, 2)
+            )),
+            newTargetAssignmentRecord("my-group", "member-2", mkAssignment(
+                mkTopicAssignment(fooTopicId, 3, 4),
+                mkTopicAssignment(barTopicId, 3, 4)
+            )),
+            newTargetAssignmentRecord("my-group", "member-3", mkAssignment(
+                mkTopicAssignment(fooTopicId, 5, 6),
+                mkTopicAssignment(barTopicId, 5, 6)
+            ))
+        ), result.records().subList(0, result.records().size() - 1));
+
+        assertEquals(newTargetAssignmentEpochRecord(
+            "my-group",
+            20
+        ), result.records().get(result.records().size() - 1));
+
+        Map<String, Assignment> expectedAssignment = new HashMap<>();
+        expectedAssignment.put("member-1", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        )));
+        expectedAssignment.put("member-2", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        )));
+        expectedAssignment.put("member-3", new Assignment(mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        )));
+
+        assertEquals(expectedAssignment, result.assignments());
+    }
+
+    @Test
+    public void testPartialAssignmentUpdate() {
+        Uuid fooTopicId = Uuid.randomUuid();
+        Uuid barTopicId = Uuid.randomUuid();
+
+        TargetAssignmentBuilderTestContext context = new TargetAssignmentBuilderTestContext(
+            "my-group",
+            20
+        );
+
+        context.addTopicMetadata(fooTopicId, "foo", 6);
+        context.addTopicMetadata(barTopicId, "bar", 6);
+
+        context.addGroupMember("member-1", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.addGroupMember("member-2", Arrays.asList("foo", "bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4),
+            mkTopicAssignment(barTopicId, 3, 4)
+        ));
+
+        context.addGroupMember("member-3", Arrays.asList("bar", "zar"), mkAssignment(
+            mkTopicAssignment(fooTopicId, 5, 6),
+            mkTopicAssignment(barTopicId, 5, 6)
+        ));
+
+        context.prepareMemberAssignment("member-1", mkAssignment(
+            mkTopicAssignment(fooTopicId, 1, 2),
+            mkTopicAssignment(barTopicId, 1, 2)
+        ));
+
+        context.prepareMemberAssignment("member-2", mkAssignment(
+            mkTopicAssignment(fooTopicId, 3, 4, 5),
+            mkTopicAssignment(barTopicId, 3, 4, 5)
+        ));
+
+        context.prepareMemberAssignment("member-3", mkAssignment(
+            mkTopicAssignment(fooTopicId, 6),
+            mkTopicAssignment(barTopicId, 6)
+        ));
+
+        TargetAssignmentBuilder.TargetAssignmentResult result = context.build();
+
+        // Member 1 has not record because its assignment did not change.

Review Comment:
   nit: "has no record"



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