showuon commented on a change in pull request #10851:
URL: https://github.com/apache/kafka/pull/10851#discussion_r810487017



##########
File path: 
streams/src/test/java/org/apache/kafka/streams/processor/internals/assignment/ClientTagAwareStandbyTaskAssignorTest.java
##########
@@ -0,0 +1,479 @@
+/*
+ * 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.junit.Test;
+
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.UUID;
+import java.util.stream.Collectors;
+
+import static java.util.Arrays.asList;
+import static java.util.Collections.singletonList;
+import static org.apache.kafka.common.utils.Utils.mkEntry;
+import static org.apache.kafka.common.utils.Utils.mkMap;
+import static org.apache.kafka.common.utils.Utils.mkSet;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_0_0;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_0_1;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_0_2;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_1_0;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_1_1;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_1_2;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.uuidForInt;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class ClientTagAwareStandbyTaskAssignorTest {
+    private static final String ZONE_TAG = "zone";
+    private static final String CLUSTER_TAG = "cluster";
+
+    private static final String ZONE_1 = "zone1";
+    private static final String ZONE_2 = "zone2";
+    private static final String ZONE_3 = "zone3";
+
+    private static final String CLUSTER_1 = "cluster1";
+    private static final String CLUSTER_2 = "cluster2";
+    private static final String CLUSTER_3 = "cluster3";
+
+    private static final UUID UUID_1 = uuidForInt(1);
+    private static final UUID UUID_2 = uuidForInt(2);
+    private static final UUID UUID_3 = uuidForInt(3);
+    private static final UUID UUID_4 = uuidForInt(4);
+    private static final UUID UUID_5 = uuidForInt(5);
+    private static final UUID UUID_6 = uuidForInt(6);
+    private static final UUID UUID_7 = uuidForInt(7);
+    private static final UUID UUID_8 = uuidForInt(8);
+    private static final UUID UUID_9 = uuidForInt(9);
+
+    @Test
+    public void shouldPermitTaskMovementWhenClientTagsMatch() {
+        final ClientTagAwareStandbyTaskAssignor standbyTaskAssignor = new 
ClientTagAwareStandbyTaskAssignor();
+        final ClientState source = createClientStateWithCapacity(1, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_1)));
+        final ClientState destination = createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_1)));
+
+        assertTrue(standbyTaskAssignor.isAllowedTaskMovement(source, 
destination));
+    }
+
+    @Test
+    public void shouldDeclineTaskMovementWhenClientTagsDoNotMatch() {
+        final ClientTagAwareStandbyTaskAssignor standbyTaskAssignor = new 
ClientTagAwareStandbyTaskAssignor();
+        final ClientState source = createClientStateWithCapacity(1, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_1)));
+        final ClientState destination = createClientStateWithCapacity(1, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_1)));
+
+        assertFalse(standbyTaskAssignor.isAllowedTaskMovement(source, 
destination));
+    }
+
+    @Test
+    public void 
shouldDistributeStandbyTasksWhenActiveTasksAreLocatedOnSameZone() {
+        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, 
TASK_1_0)),
+            mkEntry(UUID_2, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_1)))),
+            mkEntry(UUID_3, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_1)))),
+
+            mkEntry(UUID_4, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_2)), TASK_0_1, 
TASK_1_1)),
+            mkEntry(UUID_5, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_2)))),
+            mkEntry(UUID_6, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_2)))),
+
+            mkEntry(UUID_7, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_3)), TASK_0_2, 
TASK_1_2)),
+            mkEntry(UUID_8, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_3)))),
+            mkEntry(UUID_9, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_3))))
+        );
+
+        final Set<TaskId> allActiveTasks = findAllActiveTasks(clientStates);
+        final AssignmentConfigs assignmentConfigs = newAssignmentConfigs(2, 
ZONE_TAG, CLUSTER_TAG);
+
+        new ClientTagAwareStandbyTaskAssignor().assign(clientStates, 
allActiveTasks, allActiveTasks, assignmentConfigs);
+
+        
assertTrue(clientStates.values().stream().allMatch(ClientState::reachedCapacity));
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_0,
+                clientStates,
+                asList(
+                    mkSet(UUID_9, UUID_5), mkSet(UUID_6, UUID_8)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_0,
+                clientStates,
+                asList(
+                    mkSet(UUID_9, UUID_5), mkSet(UUID_6, UUID_8)
+                )
+            )
+        );
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_1,
+                clientStates,
+                asList(
+                    mkSet(UUID_2, UUID_9), mkSet(UUID_3, UUID_8)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_1,
+                clientStates,
+                asList(
+                    mkSet(UUID_2, UUID_9), mkSet(UUID_3, UUID_8)
+                )
+            )
+        );
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_2,
+                clientStates,
+                asList(
+                    mkSet(UUID_5, UUID_3), mkSet(UUID_2, UUID_6)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_2,
+                clientStates,
+                asList(
+                    mkSet(UUID_5, UUID_3), mkSet(UUID_2, UUID_6)
+                )
+            )
+        );
+    }
+
+    @Test
+    public void 
shouldDistributeStandbyTasksWhenActiveTasksAreLocatedOnSameCluster() {
+        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, 
TASK_1_0)),
+            mkEntry(UUID_2, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_1)), TASK_0_1, 
TASK_1_1)),
+            mkEntry(UUID_3, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_1)), TASK_0_2, 
TASK_1_2)),
+
+            mkEntry(UUID_4, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_2)))),
+            mkEntry(UUID_5, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_2)))),
+            mkEntry(UUID_6, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_2)))),
+
+            mkEntry(UUID_7, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_3)))),
+            mkEntry(UUID_8, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_3)))),
+            mkEntry(UUID_9, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_3))))
+        );
+
+        final Set<TaskId> allActiveTasks = findAllActiveTasks(clientStates);
+        final AssignmentConfigs assignmentConfigs = newAssignmentConfigs(2, 
ZONE_TAG, CLUSTER_TAG);
+
+        new ClientTagAwareStandbyTaskAssignor().assign(clientStates, 
allActiveTasks, allActiveTasks, assignmentConfigs);
+
+        
assertTrue(clientStates.values().stream().allMatch(ClientState::reachedCapacity));
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_0,
+                clientStates,
+                asList(
+                    mkSet(UUID_9, UUID_5), mkSet(UUID_6, UUID_8)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_0,
+                clientStates,
+                asList(
+                    mkSet(UUID_9, UUID_5), mkSet(UUID_6, UUID_8)
+                )
+            )
+        );
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_1,
+                clientStates,
+                asList(
+                    mkSet(UUID_4, UUID_9), mkSet(UUID_6, UUID_7)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_1,
+                clientStates,
+                asList(
+                    mkSet(UUID_4, UUID_9), mkSet(UUID_6, UUID_7)
+                )
+            )
+        );
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_2,
+                clientStates,
+                asList(
+                    mkSet(UUID_5, UUID_7), mkSet(UUID_4, UUID_8)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_2,
+                clientStates,
+                asList(
+                    mkSet(UUID_5, UUID_7), mkSet(UUID_4, UUID_8)
+                )
+            )
+        );
+    }
+
+    @Test
+    public void shouldDoThePartialRackAwareness() {
+        final Map<UUID, ClientState> clientStates = mkMap(
+            mkEntry(UUID_1, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_1), mkEntry(ZONE_TAG, ZONE_1)), TASK_0_0)),
+            mkEntry(UUID_2, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_1), mkEntry(ZONE_TAG, ZONE_2)))),
+            mkEntry(UUID_3, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_1), mkEntry(ZONE_TAG, ZONE_3)))),
+
+            mkEntry(UUID_4, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_2), mkEntry(ZONE_TAG, ZONE_1)))),
+            mkEntry(UUID_5, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_2), mkEntry(ZONE_TAG, ZONE_2)))),
+            mkEntry(UUID_6, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_2), mkEntry(ZONE_TAG, ZONE_3)), TASK_1_0))
+        );
+
+        final Set<TaskId> allActiveTasks = findAllActiveTasks(clientStates);
+        final AssignmentConfigs assignmentConfigs = newAssignmentConfigs(2, 
CLUSTER_TAG, ZONE_TAG);
+
+        new ClientTagAwareStandbyTaskAssignor().assign(clientStates, 
allActiveTasks, allActiveTasks, assignmentConfigs);
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_0,
+                clientStates,
+                asList(
+                    mkSet(UUID_5, UUID_3),
+                    mkSet(UUID_5, UUID_6)

Review comment:
       After your explanation of partial rack awareness, I can understand the 
distribution will be `UUID_5, UUID_6`. But I don't know why it's possible with 
the result `UUID_5, UUID_3`? 
   I thought after 1st standby task assignment, we'll exclude `CLUSTER_1` and 
`ZONE_1` tags clients. So the remaining clients will be `UUID_5, UUID_6`. 
Therefore, the only possible results will be `UUID_5, UUID_6`. Is my 
understanding correct? Anything I missed?
   Thanks.

##########
File path: 
streams/src/test/java/org/apache/kafka/streams/processor/internals/assignment/ClientTagAwareStandbyTaskAssignorTest.java
##########
@@ -0,0 +1,479 @@
+/*
+ * 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.junit.Test;
+
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.UUID;
+import java.util.stream.Collectors;
+
+import static java.util.Arrays.asList;
+import static java.util.Collections.singletonList;
+import static org.apache.kafka.common.utils.Utils.mkEntry;
+import static org.apache.kafka.common.utils.Utils.mkMap;
+import static org.apache.kafka.common.utils.Utils.mkSet;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_0_0;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_0_1;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_0_2;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_1_0;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_1_1;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.TASK_1_2;
+import static 
org.apache.kafka.streams.processor.internals.assignment.AssignmentTestUtils.uuidForInt;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class ClientTagAwareStandbyTaskAssignorTest {
+    private static final String ZONE_TAG = "zone";
+    private static final String CLUSTER_TAG = "cluster";
+
+    private static final String ZONE_1 = "zone1";
+    private static final String ZONE_2 = "zone2";
+    private static final String ZONE_3 = "zone3";
+
+    private static final String CLUSTER_1 = "cluster1";
+    private static final String CLUSTER_2 = "cluster2";
+    private static final String CLUSTER_3 = "cluster3";
+
+    private static final UUID UUID_1 = uuidForInt(1);
+    private static final UUID UUID_2 = uuidForInt(2);
+    private static final UUID UUID_3 = uuidForInt(3);
+    private static final UUID UUID_4 = uuidForInt(4);
+    private static final UUID UUID_5 = uuidForInt(5);
+    private static final UUID UUID_6 = uuidForInt(6);
+    private static final UUID UUID_7 = uuidForInt(7);
+    private static final UUID UUID_8 = uuidForInt(8);
+    private static final UUID UUID_9 = uuidForInt(9);
+
+    @Test
+    public void shouldPermitTaskMovementWhenClientTagsMatch() {
+        final ClientTagAwareStandbyTaskAssignor standbyTaskAssignor = new 
ClientTagAwareStandbyTaskAssignor();
+        final ClientState source = createClientStateWithCapacity(1, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_1)));
+        final ClientState destination = createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_1)));
+
+        assertTrue(standbyTaskAssignor.isAllowedTaskMovement(source, 
destination));
+    }
+
+    @Test
+    public void shouldDeclineTaskMovementWhenClientTagsDoNotMatch() {
+        final ClientTagAwareStandbyTaskAssignor standbyTaskAssignor = new 
ClientTagAwareStandbyTaskAssignor();
+        final ClientState source = createClientStateWithCapacity(1, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_1)));
+        final ClientState destination = createClientStateWithCapacity(1, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_1)));
+
+        assertFalse(standbyTaskAssignor.isAllowedTaskMovement(source, 
destination));
+    }
+
+    @Test
+    public void 
shouldDistributeStandbyTasksWhenActiveTasksAreLocatedOnSameZone() {
+        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, 
TASK_1_0)),
+            mkEntry(UUID_2, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_1)))),
+            mkEntry(UUID_3, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_1)))),
+
+            mkEntry(UUID_4, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_2)), TASK_0_1, 
TASK_1_1)),
+            mkEntry(UUID_5, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_2)))),
+            mkEntry(UUID_6, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_2)))),
+
+            mkEntry(UUID_7, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_3)), TASK_0_2, 
TASK_1_2)),
+            mkEntry(UUID_8, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_3)))),
+            mkEntry(UUID_9, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_3))))
+        );
+
+        final Set<TaskId> allActiveTasks = findAllActiveTasks(clientStates);
+        final AssignmentConfigs assignmentConfigs = newAssignmentConfigs(2, 
ZONE_TAG, CLUSTER_TAG);
+
+        new ClientTagAwareStandbyTaskAssignor().assign(clientStates, 
allActiveTasks, allActiveTasks, assignmentConfigs);
+
+        
assertTrue(clientStates.values().stream().allMatch(ClientState::reachedCapacity));
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_0,
+                clientStates,
+                asList(
+                    mkSet(UUID_9, UUID_5), mkSet(UUID_6, UUID_8)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_0,
+                clientStates,
+                asList(
+                    mkSet(UUID_9, UUID_5), mkSet(UUID_6, UUID_8)
+                )
+            )
+        );
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_1,
+                clientStates,
+                asList(
+                    mkSet(UUID_2, UUID_9), mkSet(UUID_3, UUID_8)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_1,
+                clientStates,
+                asList(
+                    mkSet(UUID_2, UUID_9), mkSet(UUID_3, UUID_8)
+                )
+            )
+        );
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_2,
+                clientStates,
+                asList(
+                    mkSet(UUID_5, UUID_3), mkSet(UUID_2, UUID_6)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_2,
+                clientStates,
+                asList(
+                    mkSet(UUID_5, UUID_3), mkSet(UUID_2, UUID_6)
+                )
+            )
+        );
+    }
+
+    @Test
+    public void 
shouldDistributeStandbyTasksWhenActiveTasksAreLocatedOnSameCluster() {
+        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, 
TASK_1_0)),
+            mkEntry(UUID_2, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_1)), TASK_0_1, 
TASK_1_1)),
+            mkEntry(UUID_3, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_1)), TASK_0_2, 
TASK_1_2)),
+
+            mkEntry(UUID_4, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_2)))),
+            mkEntry(UUID_5, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_2)))),
+            mkEntry(UUID_6, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_2)))),
+
+            mkEntry(UUID_7, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_1), mkEntry(CLUSTER_TAG, CLUSTER_3)))),
+            mkEntry(UUID_8, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_2), mkEntry(CLUSTER_TAG, CLUSTER_3)))),
+            mkEntry(UUID_9, createClientStateWithCapacity(2, 
mkMap(mkEntry(ZONE_TAG, ZONE_3), mkEntry(CLUSTER_TAG, CLUSTER_3))))
+        );
+
+        final Set<TaskId> allActiveTasks = findAllActiveTasks(clientStates);
+        final AssignmentConfigs assignmentConfigs = newAssignmentConfigs(2, 
ZONE_TAG, CLUSTER_TAG);
+
+        new ClientTagAwareStandbyTaskAssignor().assign(clientStates, 
allActiveTasks, allActiveTasks, assignmentConfigs);
+
+        
assertTrue(clientStates.values().stream().allMatch(ClientState::reachedCapacity));
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_0,
+                clientStates,
+                asList(
+                    mkSet(UUID_9, UUID_5), mkSet(UUID_6, UUID_8)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_0,
+                clientStates,
+                asList(
+                    mkSet(UUID_9, UUID_5), mkSet(UUID_6, UUID_8)
+                )
+            )
+        );
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_1,
+                clientStates,
+                asList(
+                    mkSet(UUID_4, UUID_9), mkSet(UUID_6, UUID_7)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_1,
+                clientStates,
+                asList(
+                    mkSet(UUID_4, UUID_9), mkSet(UUID_6, UUID_7)
+                )
+            )
+        );
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_2,
+                clientStates,
+                asList(
+                    mkSet(UUID_5, UUID_7), mkSet(UUID_4, UUID_8)
+                )
+            )
+        );
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_1_2,
+                clientStates,
+                asList(
+                    mkSet(UUID_5, UUID_7), mkSet(UUID_4, UUID_8)
+                )
+            )
+        );
+    }
+
+    @Test
+    public void shouldDoThePartialRackAwareness() {
+        final Map<UUID, ClientState> clientStates = mkMap(
+            mkEntry(UUID_1, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_1), mkEntry(ZONE_TAG, ZONE_1)), TASK_0_0)),
+            mkEntry(UUID_2, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_1), mkEntry(ZONE_TAG, ZONE_2)))),
+            mkEntry(UUID_3, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_1), mkEntry(ZONE_TAG, ZONE_3)))),
+
+            mkEntry(UUID_4, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_2), mkEntry(ZONE_TAG, ZONE_1)))),
+            mkEntry(UUID_5, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_2), mkEntry(ZONE_TAG, ZONE_2)))),
+            mkEntry(UUID_6, createClientStateWithCapacity(1, 
mkMap(mkEntry(CLUSTER_TAG, CLUSTER_2), mkEntry(ZONE_TAG, ZONE_3)), TASK_1_0))
+        );
+
+        final Set<TaskId> allActiveTasks = findAllActiveTasks(clientStates);
+        final AssignmentConfigs assignmentConfigs = newAssignmentConfigs(2, 
CLUSTER_TAG, ZONE_TAG);
+
+        new ClientTagAwareStandbyTaskAssignor().assign(clientStates, 
allActiveTasks, allActiveTasks, assignmentConfigs);
+
+        assertTrue(
+            standbyClientsHonorRackAwareness(
+                TASK_0_0,
+                clientStates,
+                asList(
+                    mkSet(UUID_5, UUID_3),
+                    mkSet(UUID_5, UUID_6)

Review comment:
       And I think we should add some comments here, to have a simple 
explanation like we I did above to explain why we have these results. Thanks.




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