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

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

cadonna commented on a change in pull request #10851:
URL: https://github.com/apache/kafka/pull/10851#discussion_r720193502



##########
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<>();

Review comment:
       Let's assume you have two clients. `clientX` has tags `keyA:value1` and `keyB:value2` and `clientY` has tags `keyA:value2` and `keyB:value3`. Notice that `keyA` and `keyB` share `value2`. With your code, we will end up with a `tagValueToClients` map that looks like this:
   ```
   value1 -> {clientX}
   value2 -> {clientX, clientY}
   value3 -> {clientY}
   ```  
   Now, let's assume that an active task is assigned to `clientX`. It would be totally fine if we assign the standby task to `clientY` since each tag key of both clients do not share a value. However, your algorithm does not allow it, because on line 198 it also adds `clientY` to the clients that are not allowed to get the standby. The reason is that `tagValueToClients` only looks for clients that contain value `value2` and not for clients that contain it as a value for `keyA`.
   
   The following test fails because of this:
   ```
       @Test
       public void shouldTestBrunosTheory() {
           final Map<UUID, ClientState> clientStates = mkMap(
               mkEntry(UUID_1, createClientStateWithCapacity(2, mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_1)), TASK_0_0)),
               mkEntry(UUID_2, createClientStateWithCapacity(2, mkMap(mkEntry(ZONE_TAG, CLUSTER_1), mkEntry(CLUSTER_TAG, CLUSTER_3))))
           );
   
           final Set<TaskId> allActiveTasks = findAllActiveTasks(clientStates);
           final AssignmentConfigs assignmentConfigs = newAssignmentConfigs(1, ZONE_TAG, CLUSTER_TAG);
   
           new ClientTagAwareStandbyTaskAssignor().assign(clientStates, allActiveTasks, allActiveTasks, assignmentConfigs);
   
           assertTrue(
               standbyClientsHonorRackAwareness(
                   TASK_0_0,
                   clientStates,
                   asList(
                       mkSet(UUID_2)
                   )
               )
           );
       }
   ```




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