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

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showuon commented on a change in pull request #10851:
URL: https://github.com/apache/kafka/pull/10851#discussion_r805156641



##########
File path: streams/src/main/java/org/apache/kafka/streams/processor/internals/assignment/DefaultStandbyTaskAssignor.java
##########
@@ -0,0 +1,67 @@
+/*
+ * 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.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.Map;
+import java.util.Set;
+import java.util.UUID;
+
+import static org.apache.kafka.streams.processor.internals.assignment.TaskAssignmentUtils.computeTasksToRemainingStandbys;
+import static org.apache.kafka.streams.processor.internals.assignment.TaskAssignmentUtils.pollClientAndMaybeAssignRemainingStandbyTasks;
+
+class DefaultStandbyTaskAssignor implements StandbyTaskAssignor {

Review comment:
       I know there was no any java doc for the default least load assignor. But do you think we can add some comments to it, just like in `ClientTagAwareStandbyTaskAssignor`? I believe not everyone knows default assignor algorithm is least loaded assignor.

##########
File path: streams/src/main/java/org/apache/kafka/streams/processor/internals/assignment/ClientTagAwareStandbyTaskAssignor.java
##########
@@ -0,0 +1,289 @@
+/*
+ * 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.Objects;
+import java.util.Set;
+import java.util.UUID;
+
+import static org.apache.kafka.streams.processor.internals.assignment.TaskAssignmentUtils.computeTasksToRemainingStandbys;
+import static org.apache.kafka.streams.processor.internals.assignment.TaskAssignmentUtils.pollClientAndMaybeAssignRemainingStandbyTasks;
+import static org.apache.kafka.streams.processor.internals.assignment.TaskAssignmentUtils.shouldBalanceLoad;
+
+/**
+ * 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's not enough capacity on the clients
+ * with 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,

Review comment:
       Could we add some java doc to this assign to briefly mention about the algorithm used in the assignor? Thanks.

##########
File path: streams/src/main/java/org/apache/kafka/streams/processor/internals/assignment/ClientState.java
##########
@@ -59,29 +60,31 @@
 
     private int capacity;
 
-    public ClientState() {
-        this(0);
+    public ClientState(final Map<String, String> clientTags) {
+        this(0, clientTags);
     }
 
-    ClientState(final int capacity) {
+    ClientState(final int capacity, final Map<String, String> clientTags) {

Review comment:
       I'm wondering could we keep the original constructor and pass empty map into the new one? So that we don't have to make changes to the old caller. That is:
   
   ```
   public ClientState() {
       this(0, Collections.emptyMap());
   }
   
   ClientState(final int capacity) {
       this(capacity, Collections.emptyMap());
   }
   ```
   WDYT?




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