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

[GitBox <gi...@apache.org>]

lkokhreidze commented on a change in pull request #10851:
URL: https://github.com/apache/kafka/pull/10851#discussion_r701038718



##########
File path: streams/src/main/java/org/apache/kafka/streams/processor/internals/assignment/ClientTagAwareStandbyTaskAssignor.java
##########
@@ -0,0 +1,203 @@
+/*
+ * 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.Set;
+import java.util.UUID;
+
+import static org.apache.kafka.streams.processor.internals.assignment.StandbyTaskAssignmentUtils.computeTasksToRemainingStandbys;
+
+/**
+ * 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);
+
+    @Override
+    public boolean assign(final Map<UUID, ClientState> clients,
+                          final Set<TaskId> allTaskIds,
+                          final Set<TaskId> statefulTaskIds,
+                          final AssignorConfiguration.AssignmentConfigs configs) {
+        final int numStandbyReplicas = configs.numStandbyReplicas;
+        final Set<String> rackAwareAssignmentTags = new HashSet<>(configs.rackAwareAssignmentTags);
+        final Map<TaskId, UUID> statefulTasksWithClients = new HashMap<>();
+
+        statefulTaskIds.forEach(statefulTaskId -> clients.forEach((uuid, clientState) -> {
+            if (clientState.activeTasks().contains(statefulTaskId)) {
+                statefulTasksWithClients.put(statefulTaskId, uuid);
+            }
+        }));
+
+        final Map<TaskId, Integer> tasksToRemainingStandbys = computeTasksToRemainingStandbys(
+            numStandbyReplicas,
+            allTaskIds
+        );
+
+        final Map<String, Set<String>> tagKeyToTagValues = new HashMap<>();
+        final Map<String, Set<UUID>> tagValueToClients = new HashMap<>();
+
+        fillClientsTagStatistics(clients, tagValueToClients, tagKeyToTagValues);
+
+        statefulTasksWithClients.forEach((taskId, clientId) -> assignStandbyTasksForActiveTask(
+            numStandbyReplicas,
+            taskId,
+            clientId,
+            rackAwareAssignmentTags,
+            clients,
+            tasksToRemainingStandbys,
+            tagKeyToTagValues,
+            tagValueToClients
+        ));
+
+        return true;
+    }
+
+    @Override
+    public boolean isAllowedTaskMovement(final ClientState source, final ClientState destination) {
+        final Map<String, String> sourceClientTags = source.clientTags();
+        final Map<String, String> destinationClientTags = destination.clientTags();
+
+        for (final Entry<String, String> sourceClientTagEntry : sourceClientTags.entrySet()) {
+            if (!sourceClientTagEntry.getValue().equals(destinationClientTags.get(sourceClientTagEntry.getKey()))) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    private static void fillClientsTagStatistics(final Map<UUID, ClientState> clientStates,
+                                                 final Map<String, Set<UUID>> tagValueToClients,
+                                                 final Map<String, Set<String>> tagKeyToTagValues) {
+        for (final Entry<UUID, ClientState> clientStateEntry : clientStates.entrySet()) {
+            final UUID clientId = clientStateEntry.getKey();
+            final ClientState clientState = clientStateEntry.getValue();
+
+            clientState.clientTags().forEach((tagKey, tagValue) -> {
+                tagKeyToTagValues.computeIfAbsent(tagKey, ignored -> new HashSet<>()).add(tagValue);
+                tagValueToClients.computeIfAbsent(tagValue, ignored -> new HashSet<>()).add(clientId);
+            });
+        }
+    }
+
+    private static void assignStandbyTasksForActiveTask(final int numStandbyReplicas,
+                                                        final TaskId activeTaskId,
+                                                        final UUID activeTaskClient,
+                                                        final Set<String> rackAwareAssignmentTags,
+                                                        final Map<UUID, ClientState> clientStates,
+                                                        final Map<TaskId, Integer> tasksToRemainingStandbys,
+                                                        final Map<String, Set<String>> tagKeyToTagValues,
+                                                        final Map<String, Set<UUID>> tagValueToClients) {
+        final ConstrainedPrioritySet standbyTaskClientsByTaskLoad = new ConstrainedPrioritySet(
+            (client, t) -> !clientStates.get(client).hasAssignedTask(t),
+            client -> clientStates.get(client).assignedTaskLoad()
+        );
+
+        final Set<UUID> usedClients = new HashSet<>();
+
+        standbyTaskClientsByTaskLoad.offerAll(clientStates.keySet());
+
+        int numRemainingStandbys = tasksToRemainingStandbys.get(activeTaskId);
+
+        usedClients.add(activeTaskClient);
+
+        while (numRemainingStandbys > 0) {
+            final Set<UUID> clientsOnAlreadyUsedTagDimensions = findClientsOnUsedTagDimensions(
+                usedClients,
+                rackAwareAssignmentTags,
+                clientStates,
+                tagValueToClients,
+                tagKeyToTagValues
+            );
+
+            final UUID polledClient = standbyTaskClientsByTaskLoad.poll(
+                activeTaskId, uuid -> !clientsOnAlreadyUsedTagDimensions.contains(uuid)
+            );
+
+            if (polledClient == null) {
+                break;
+            }
+
+            final ClientState standbyTaskClient = clientStates.get(polledClient);
+
+            standbyTaskClient.assignStandby(activeTaskId);
+
+            usedClients.add(polledClient);
+
+            standbyTaskClientsByTaskLoad.offerAll(clientsOnAlreadyUsedTagDimensions);

Review comment:
       We need to add those clients back because we need to fallback to least loaded clients if we don't have any more valid clients on other dimensions.
   For instance, without this `shouldDistributeTasksOnLeastLoadedClientsWhenThereAreNoEnoughUniqueTagDimensions` test will fail.
   




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