[GitHub] [kafka] lkokhreidze commented on a change in pull request #10851: KAFKA-6718 / Rack aware standby task assignor

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lkokhreidze commented on a change in pull request #10851:
URL: https://github.com/apache/kafka/pull/10851#discussion_r693133897



##########
File path: streams/src/main/java/org/apache/kafka/streams/processor/internals/assignment/ClientTagAwareStandbyTaskAssignor.java
##########
@@ -0,0 +1,220 @@
+/*
+ * 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.streams.processor.internals.assignment;
+
+import org.apache.kafka.streams.processor.TaskId;
+import org.apache.kafka.streams.processor.internals.assignment.AssignorConfiguration.AssignmentConfigs;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Optional;
+import java.util.Set;
+import java.util.SortedMap;
+import java.util.UUID;
+import java.util.function.Function;
+
+import static java.util.stream.Collectors.toMap;
+
+/**
+ * Distributes standby tasks over different tag dimensions.
+ * Only tags specified via {@link AssignmentConfigs#rackAwareAssignmentTags} are taken into account.
+ * Standby task distribution is on a best-effort basis. For example, if there are not enough clients available
+ * on different tag dimensions compared to an active and corresponding standby task,
+ * in that case, the algorithm will fall back to distributing tasks on least-loaded clients.
+ */
+class ClientTagAwareStandbyTaskAssignor implements StandbyTaskAssignor {
+    private static final Logger log = LoggerFactory.getLogger(ClientTagAwareStandbyTaskAssignor.class);
+
+    private final AssignmentConfigs configs;
+
+    ClientTagAwareStandbyTaskAssignor(final AssignmentConfigs configs) {
+        this.configs = configs;
+    }
+
+    @Override
+    public void assignStandbyTasks(final SortedMap<UUID, ClientState> clientStates, final Set<TaskId> statefulTasks) {
+        final int numStandbyReplicas = configs.numStandbyReplicas;
+        final Set<String> rackAwareAssignmentTags = new HashSet<>(configs.rackAwareAssignmentTags);
+        final Map<TaskId, UUID> statefulTasksWithClients = new HashMap<>();
+
+        statefulTasks.forEach(statefulTaskId -> clientStates.forEach((uuid, clientState) -> {
+            if (clientState.activeTasks().contains(statefulTaskId)) {
+                statefulTasksWithClients.put(statefulTaskId, uuid);
+            }
+        }));
+
+        final StandbyTaskDistributor standbyTaskDistributor = new StandbyTaskDistributor(
+            numStandbyReplicas,
+            clientStates,
+            rackAwareAssignmentTags,
+            statefulTasksWithClients
+        );
+
+        statefulTasksWithClients.forEach(standbyTaskDistributor::assignStandbyTasksForActiveTask);
+    }
+
+    @Override
+    public boolean isValidTaskMovement(final TaskMovementAttempt taskMovementAttempt) {
+        final Map<String, String> sourceClientTags = taskMovementAttempt.sourceClient().clientTags();
+        final Map<String, String> destinationClientTags = taskMovementAttempt.destinationClient().clientTags();
+
+        for (final Entry<String, String> sourceClientTagEntry : sourceClientTags.entrySet()) {
+            if (!sourceClientTagEntry.getValue().equals(destinationClientTags.get(sourceClientTagEntry.getKey()))) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    private static final class StandbyTaskDistributor {
+        private final int numStandbyReplicas;
+        private final SortedMap<UUID, ClientState> clientStates;
+        private final Set<String> rackAwareAssignmentTags;
+        private final Map<String, Set<String>> tagKeyToTagValuesMapping;
+        private final Map<String, Set<UUID>> clientsPerTagValue;
+        private final ConstrainedPrioritySet standbyTaskClientsByTaskLoad;
+
+        private final Set<UUID> usedClients = new HashSet<>();
+        private final Map<TaskId, Integer> tasksToRemainingStandbys;
+
+        StandbyTaskDistributor(final int numStandbyReplicas,
+                               final SortedMap<UUID, ClientState> clientStates,
+                               final Set<String> rackAwareAssignmentTags,
+                               final Map<TaskId, UUID> statefulTasksWithClients) {
+            tagKeyToTagValuesMapping = new HashMap<>();
+            clientsPerTagValue = new HashMap<>();
+            standbyTaskClientsByTaskLoad = new ConstrainedPrioritySet(
+                (client, t) -> !clientStates.get(client).hasAssignedTask(t),
+                client -> clientStates.get(client).assignedTaskLoad()
+            );

Review comment:
       It's already part of the poll constraint of the priority queue. Example:
   
   ```
   final UUID polledClient = standbyTaskClientsByTaskLoad.poll(
       activeTaskId, uuid -> !clientsOnAlreadyUsedTagDimensions.contains(uuid)
   );
   ```
   I don't think it will be doable with constructor constraint, because we need to update constraint on each poll.




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