cadonna commented on code in PR #18476:
URL: https://github.com/apache/kafka/pull/18476#discussion_r1912971326


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/TaskTuple.java:
##########
@@ -0,0 +1,135 @@
+/*
+ * 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.coordinator.group.streams;
+
+import 
org.apache.kafka.coordinator.group.generated.StreamsGroupTargetAssignmentMemberValue;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+/**
+ * An immutable tuple containing active, standby and warm-up tasks.
+ *
+ * @param activeTasks           Active tasks.
+ *                              The key of the map is the subtopology ID and 
the value is the set of partition IDs.
+ * @param standbyTasks          Standby tasks.
+ *                              The key of the map is the subtopology ID and 
the value is the set of partition IDs.
+ * @param warmupTasks           Warm-up tasks.
+ *                              The key of the map is the subtopology ID and 
the value is the set of partition IDs.
+ */
+public record TaskTuple(Map<String, Set<Integer>> activeTasks,

Review Comment:
   What about simply `Tasks`?



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilder.java:
##########
@@ -0,0 +1,447 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.BiPredicate;
+
+/**
+ * The CurrentAssignmentBuilder class encapsulates the reconciliation engine 
of the streams group protocol. Given the current state of a
+ * member and a desired or target assignment state, the state machine takes 
the necessary steps to converge them.
+ */
+public class CurrentAssignmentBuilder {
+
+    /**
+     * The streams group member which is reconciled.
+     */
+    private final StreamsGroupMember member;
+
+    /**
+     * The target assignment epoch.
+     */
+    private int targetAssignmentEpoch;
+
+    /**
+     * The target assignment.
+     */
+    private TaskTuple targetAssignment;
+
+    /**
+     * A function which returns the current process ID of an active task or 
null if the active task
+     * is not assigned. The current process ID is the process ID of the 
current owner.
+     */
+    private BiFunction<String, Integer, String> currentActiveTaskProcessId;
+
+    /**
+     * A function which returns the current process IDs of a standby task or 
null if the standby
+     * task is not assigned. The current process IDs are the process IDs of 
all current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentStandbyTaskProcessIds;
+
+    /**
+     * A function which returns the current process IDs of a warmup task or 
null if the warmup task
+     * is not assigned. The current process IDs are the process IDs of all 
current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentWarmupTaskProcessIds;
+
+    /**
+     * The tasks owned by the member. This may be provided by the member in 
the StreamsGroupHeartbeat request.
+     */
+    private Optional<TaskTuple> ownedTasks = Optional.empty();
+
+    /**
+     * Constructs the CurrentAssignmentBuilder based on the current state of 
the provided streams group member.
+     *
+     * @param member The streams group member that must be reconciled.
+     */
+    public CurrentAssignmentBuilder(StreamsGroupMember member) {
+        this.member = Objects.requireNonNull(member);
+    }
+
+    /**
+     * Sets the target assignment epoch and the target assignment that the 
streams group member must be reconciled to.
+     *
+     * @param targetAssignmentEpoch The target assignment epoch.
+     * @param targetAssignment      The target assignment.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withTargetAssignment(
+        int targetAssignmentEpoch,
+        TaskTuple targetAssignment
+    ) {

Review Comment:
   Here and elsewhere:
   ```suggestion
       public CurrentAssignmentBuilder withTargetAssignment(int 
targetAssignmentEpoch,
                                                            TaskTuple 
targetAssignment) {
   ```



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/TaskTuple.java:
##########
@@ -0,0 +1,135 @@
+/*
+ * 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.coordinator.group.streams;
+
+import 
org.apache.kafka.coordinator.group.generated.StreamsGroupTargetAssignmentMemberValue;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+/**
+ * An immutable tuple containing active, standby and warm-up tasks.
+ *
+ * @param activeTasks           Active tasks.
+ *                              The key of the map is the subtopology ID and 
the value is the set of partition IDs.
+ * @param standbyTasks          Standby tasks.
+ *                              The key of the map is the subtopology ID and 
the value is the set of partition IDs.
+ * @param warmupTasks           Warm-up tasks.
+ *                              The key of the map is the subtopology ID and 
the value is the set of partition IDs.
+ */
+public record TaskTuple(Map<String, Set<Integer>> activeTasks,
+                        Map<String, Set<Integer>> standbyTasks,
+                        Map<String, Set<Integer>> warmupTasks) {
+
+    public TaskTuple {
+        activeTasks = 
Collections.unmodifiableMap(Objects.requireNonNull(activeTasks));
+        standbyTasks = 
Collections.unmodifiableMap(Objects.requireNonNull(standbyTasks));
+        warmupTasks = 
Collections.unmodifiableMap(Objects.requireNonNull(warmupTasks));
+    }
+
+    /**
+     * An empty task tuple.
+     */
+    public static final TaskTuple EMPTY = new TaskTuple(
+        Collections.emptyMap(),
+        Collections.emptyMap(),
+        Collections.emptyMap()
+    );
+
+    /**
+     * @return true if all collections in the tuple are empty.
+     */
+    public boolean isEmpty() {
+        return activeTasks.isEmpty() && standbyTasks.isEmpty() && 
warmupTasks.isEmpty();
+    }
+
+    /**
+     * Merges this task tuple with another task tuple.
+     *
+     * @param other The other task tuple.
+     * @return A new task tuple, containing all active tasks, standby tasks 
and warm-up tasks from both tuples.
+     */
+    public TaskTuple merge(TaskTuple other) {
+        Map<String, Set<Integer>> mergedActiveTasks = merge(activeTasks, 
other.activeTasks);
+        Map<String, Set<Integer>> mergedStandbyTasks = merge(standbyTasks, 
other.standbyTasks);
+        Map<String, Set<Integer>> mergedWarmupTasks = merge(warmupTasks, 
other.warmupTasks);
+        return new TaskTuple(mergedActiveTasks, mergedStandbyTasks, 
mergedWarmupTasks);
+    }
+
+    private Map<String, Set<Integer>> merge(final Map<String, Set<Integer>> 
tasks1, final Map<String, Set<Integer>> tasks2) {

Review Comment:
   This could be static to emphasize that this object is not changed.



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/TaskTuple.java:
##########
@@ -0,0 +1,135 @@
+/*
+ * 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.coordinator.group.streams;
+
+import 
org.apache.kafka.coordinator.group.generated.StreamsGroupTargetAssignmentMemberValue;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+/**
+ * An immutable tuple containing active, standby and warm-up tasks.
+ *
+ * @param activeTasks           Active tasks.
+ *                              The key of the map is the subtopology ID and 
the value is the set of partition IDs.
+ * @param standbyTasks          Standby tasks.
+ *                              The key of the map is the subtopology ID and 
the value is the set of partition IDs.
+ * @param warmupTasks           Warm-up tasks.
+ *                              The key of the map is the subtopology ID and 
the value is the set of partition IDs.
+ */
+public record TaskTuple(Map<String, Set<Integer>> activeTasks,
+                        Map<String, Set<Integer>> standbyTasks,
+                        Map<String, Set<Integer>> warmupTasks) {
+
+    public TaskTuple {
+        activeTasks = 
Collections.unmodifiableMap(Objects.requireNonNull(activeTasks));
+        standbyTasks = 
Collections.unmodifiableMap(Objects.requireNonNull(standbyTasks));
+        warmupTasks = 
Collections.unmodifiableMap(Objects.requireNonNull(warmupTasks));

Review Comment:
   Why making the maps unmodifiable but not the sets? To avoid the iterations?



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/streams/TaskAssignmentTestUtil.java:
##########
@@ -17,37 +17,41 @@
 package org.apache.kafka.coordinator.group.streams;
 
 import java.util.AbstractMap;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
-import java.util.Objects;
 import java.util.Set;
 
 public class TaskAssignmentTestUtil {
 
-    public static Assignment mkAssignment(final Map<String, Set<Integer>> 
activeTasks,
-                                          final Map<String, Set<Integer>> 
standbyTasks,
-                                          final Map<String, Set<Integer>> 
warmupTasks) {
-        return new Assignment(
-            Collections.unmodifiableMap(Objects.requireNonNull(activeTasks)),
-            Collections.unmodifiableMap(Objects.requireNonNull(standbyTasks)),
-            Collections.unmodifiableMap(Objects.requireNonNull(warmupTasks))
-        );
+    public enum TaskRole {
+        ACTIVE,
+        STANDBY,
+        WARMUP
+    }
+
+    @SafeVarargs
+    public static TaskTuple mkTaskTuple(TaskRole taskRole, Map.Entry<String, 
Set<Integer>>... entries) {
+        return switch (taskRole) {
+            case ACTIVE -> new TaskTuple(mkTasksPerSubtopology(entries), new 
HashMap<>(), new HashMap<>());
+            case STANDBY -> new TaskTuple(new HashMap<>(), 
mkTasksPerSubtopology(entries), new HashMap<>());
+            case WARMUP -> new TaskTuple(new HashMap<>(), new HashMap<>(), 
mkTasksPerSubtopology(entries));
+        };
     }
 
-    public static Map.Entry<String, Set<Integer>> mkTasks(String subtopologyId,
-                                                          Integer... tasks) {
+    public static Map.Entry<String, Set<Integer>> mkTasks(
+        String subtopologyId,
+        Integer... tasks
+    ) {

Review Comment:
   Why this change? The style was better before.



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilder.java:
##########
@@ -0,0 +1,447 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.BiPredicate;
+
+/**
+ * The CurrentAssignmentBuilder class encapsulates the reconciliation engine 
of the streams group protocol. Given the current state of a
+ * member and a desired or target assignment state, the state machine takes 
the necessary steps to converge them.
+ */
+public class CurrentAssignmentBuilder {
+
+    /**
+     * The streams group member which is reconciled.
+     */
+    private final StreamsGroupMember member;
+
+    /**
+     * The target assignment epoch.
+     */
+    private int targetAssignmentEpoch;
+
+    /**
+     * The target assignment.
+     */
+    private TaskTuple targetAssignment;
+
+    /**
+     * A function which returns the current process ID of an active task or 
null if the active task
+     * is not assigned. The current process ID is the process ID of the 
current owner.
+     */
+    private BiFunction<String, Integer, String> currentActiveTaskProcessId;
+
+    /**
+     * A function which returns the current process IDs of a standby task or 
null if the standby
+     * task is not assigned. The current process IDs are the process IDs of 
all current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentStandbyTaskProcessIds;
+
+    /**
+     * A function which returns the current process IDs of a warmup task or 
null if the warmup task
+     * is not assigned. The current process IDs are the process IDs of all 
current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentWarmupTaskProcessIds;
+
+    /**
+     * The tasks owned by the member. This may be provided by the member in 
the StreamsGroupHeartbeat request.
+     */
+    private Optional<TaskTuple> ownedTasks = Optional.empty();
+
+    /**
+     * Constructs the CurrentAssignmentBuilder based on the current state of 
the provided streams group member.
+     *
+     * @param member The streams group member that must be reconciled.
+     */
+    public CurrentAssignmentBuilder(StreamsGroupMember member) {
+        this.member = Objects.requireNonNull(member);
+    }
+
+    /**
+     * Sets the target assignment epoch and the target assignment that the 
streams group member must be reconciled to.
+     *
+     * @param targetAssignmentEpoch The target assignment epoch.
+     * @param targetAssignment      The target assignment.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withTargetAssignment(
+        int targetAssignmentEpoch,
+        TaskTuple targetAssignment
+    ) {
+        this.targetAssignmentEpoch = targetAssignmentEpoch;
+        this.targetAssignment = Objects.requireNonNull(targetAssignment);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process ID of an 
active task. This is
+     * used by the state machine to determine if an active task is free or 
still used by another
+     * member, and if there is still a task on a specific process that is not 
yet revoked.
+     *
+     * @param currentActiveTaskProcessId A BiFunction which gets the memberId 
of a subtopology id /
+     *                                   partition id pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentActiveTaskProcessId(
+        BiFunction<String, Integer, String> currentActiveTaskProcessId
+    ) {
+        this.currentActiveTaskProcessId = 
Objects.requireNonNull(currentActiveTaskProcessId);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process IDs of a 
standby task. This is
+     * used by the state machine to determine if there is still a task on a 
specific process that is
+     * not yet revoked.
+     *
+     * @param currentStandbyTaskProcessIds A BiFunction which gets the 
memberIds of a subtopology
+     *                                     ids / partition ids pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentStandbyTaskProcessIds(
+        BiFunction<String, Integer, Set<String>> currentStandbyTaskProcessIds
+    ) {
+        this.currentStandbyTaskProcessIds = 
Objects.requireNonNull(currentStandbyTaskProcessIds);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process IDs of a 
warmup task. This is
+     * used by the state machine to determine if there is still a task on a 
specific process that is
+     * not yet revoked.
+     *
+     * @param currentWarmupTaskProcessIds A BiFunction which gets the 
memberIds of a subtopology ids
+     *                                    / partition ids pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentWarmupTaskProcessIds(
+        BiFunction<String, Integer, Set<String>> currentWarmupTaskProcessIds
+    ) {
+        this.currentWarmupTaskProcessIds = 
Objects.requireNonNull(currentWarmupTaskProcessIds);
+        return this;
+    }
+
+    /**
+     * Sets the tasks currently owned by the member. This comes directly from 
the last StreamsGroupHeartbeat request. This is used to
+     * determine if the member has revoked the necessary tasks. Passing null 
into this function means that the member did not provide
+     * its owned tasks in this heartbeat.
+     *
+     * @param ownedAssignment A collection of active, standby and warm-up tasks
+     * @return This object.
+     */
+    protected CurrentAssignmentBuilder withOwnedAssignment(
+        TaskTuple ownedAssignment
+    ) {
+        this.ownedTasks = Optional.ofNullable(ownedAssignment);
+        return this;
+    }
+
+    /**
+     * Builds the next state for the member or keep the current one if it is 
not possible to move forward with the current state.
+     *
+     * @return A new StreamsGroupMember or the current one.
+     */
+    public StreamsGroupMember build() {
+        switch (member.state()) {
+            case STABLE:
+                // When the member is in the STABLE state, we verify if a newer
+                // epoch (or target assignment) is available. If it is, we can
+                // reconcile the member towards it. Otherwise, we return.
+                if (member.memberEpoch() != targetAssignmentEpoch) {

Review Comment:
   Do we need to verify for the invariant that a member epoch is always <= 
target assignment epoch?



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilderTest.java:
##########
@@ -0,0 +1,744 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+import 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.TaskRole;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.EnumSource;
+
+import java.util.Collections;
+
+import static 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.mkTaskTuple;
+import static 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.mkTasks;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class CurrentAssignmentBuilderTest {
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testStableToStable(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member =
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.STABLE)
+                .setProcessId("process")

Review Comment:
   This could also be a class constant that can be re-used in other parts.



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilderTest.java:
##########
@@ -0,0 +1,744 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+import 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.TaskRole;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.EnumSource;
+
+import java.util.Collections;
+
+import static 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.mkTaskTuple;
+import static 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.mkTasks;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class CurrentAssignmentBuilderTest {
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testStableToStable(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();

Review Comment:
   Those could be class constants.



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilder.java:
##########
@@ -0,0 +1,447 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.BiPredicate;
+
+/**
+ * The CurrentAssignmentBuilder class encapsulates the reconciliation engine 
of the streams group protocol. Given the current state of a
+ * member and a desired or target assignment state, the state machine takes 
the necessary steps to converge them.
+ */
+public class CurrentAssignmentBuilder {
+
+    /**
+     * The streams group member which is reconciled.
+     */
+    private final StreamsGroupMember member;
+
+    /**
+     * The target assignment epoch.
+     */
+    private int targetAssignmentEpoch;
+
+    /**
+     * The target assignment.
+     */
+    private TaskTuple targetAssignment;
+
+    /**
+     * A function which returns the current process ID of an active task or 
null if the active task
+     * is not assigned. The current process ID is the process ID of the 
current owner.
+     */
+    private BiFunction<String, Integer, String> currentActiveTaskProcessId;
+
+    /**
+     * A function which returns the current process IDs of a standby task or 
null if the standby
+     * task is not assigned. The current process IDs are the process IDs of 
all current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentStandbyTaskProcessIds;
+
+    /**
+     * A function which returns the current process IDs of a warmup task or 
null if the warmup task
+     * is not assigned. The current process IDs are the process IDs of all 
current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentWarmupTaskProcessIds;
+
+    /**
+     * The tasks owned by the member. This may be provided by the member in 
the StreamsGroupHeartbeat request.
+     */
+    private Optional<TaskTuple> ownedTasks = Optional.empty();
+
+    /**
+     * Constructs the CurrentAssignmentBuilder based on the current state of 
the provided streams group member.
+     *
+     * @param member The streams group member that must be reconciled.
+     */
+    public CurrentAssignmentBuilder(StreamsGroupMember member) {
+        this.member = Objects.requireNonNull(member);
+    }
+
+    /**
+     * Sets the target assignment epoch and the target assignment that the 
streams group member must be reconciled to.
+     *
+     * @param targetAssignmentEpoch The target assignment epoch.
+     * @param targetAssignment      The target assignment.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withTargetAssignment(
+        int targetAssignmentEpoch,
+        TaskTuple targetAssignment
+    ) {
+        this.targetAssignmentEpoch = targetAssignmentEpoch;
+        this.targetAssignment = Objects.requireNonNull(targetAssignment);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process ID of an 
active task. This is
+     * used by the state machine to determine if an active task is free or 
still used by another
+     * member, and if there is still a task on a specific process that is not 
yet revoked.
+     *
+     * @param currentActiveTaskProcessId A BiFunction which gets the memberId 
of a subtopology id /
+     *                                   partition id pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentActiveTaskProcessId(
+        BiFunction<String, Integer, String> currentActiveTaskProcessId
+    ) {
+        this.currentActiveTaskProcessId = 
Objects.requireNonNull(currentActiveTaskProcessId);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process IDs of a 
standby task. This is
+     * used by the state machine to determine if there is still a task on a 
specific process that is
+     * not yet revoked.
+     *
+     * @param currentStandbyTaskProcessIds A BiFunction which gets the 
memberIds of a subtopology
+     *                                     ids / partition ids pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentStandbyTaskProcessIds(
+        BiFunction<String, Integer, Set<String>> currentStandbyTaskProcessIds
+    ) {
+        this.currentStandbyTaskProcessIds = 
Objects.requireNonNull(currentStandbyTaskProcessIds);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process IDs of a 
warmup task. This is
+     * used by the state machine to determine if there is still a task on a 
specific process that is
+     * not yet revoked.
+     *
+     * @param currentWarmupTaskProcessIds A BiFunction which gets the 
memberIds of a subtopology ids
+     *                                    / partition ids pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentWarmupTaskProcessIds(
+        BiFunction<String, Integer, Set<String>> currentWarmupTaskProcessIds
+    ) {
+        this.currentWarmupTaskProcessIds = 
Objects.requireNonNull(currentWarmupTaskProcessIds);
+        return this;
+    }
+
+    /**
+     * Sets the tasks currently owned by the member. This comes directly from 
the last StreamsGroupHeartbeat request. This is used to
+     * determine if the member has revoked the necessary tasks. Passing null 
into this function means that the member did not provide
+     * its owned tasks in this heartbeat.
+     *
+     * @param ownedAssignment A collection of active, standby and warm-up tasks
+     * @return This object.
+     */
+    protected CurrentAssignmentBuilder withOwnedAssignment(
+        TaskTuple ownedAssignment
+    ) {
+        this.ownedTasks = Optional.ofNullable(ownedAssignment);
+        return this;
+    }
+
+    /**
+     * Builds the next state for the member or keep the current one if it is 
not possible to move forward with the current state.
+     *
+     * @return A new StreamsGroupMember or the current one.
+     */
+    public StreamsGroupMember build() {
+        switch (member.state()) {
+            case STABLE:
+                // When the member is in the STABLE state, we verify if a newer
+                // epoch (or target assignment) is available. If it is, we can
+                // reconcile the member towards it. Otherwise, we return.
+                if (member.memberEpoch() != targetAssignmentEpoch) {
+                    return computeNextAssignment(
+                        member.memberEpoch(),
+                        member.assignedTasks()
+                    );
+                } else {
+                    return member;
+                }
+
+            case UNREVOKED_TASKS:
+                // When the member is in the UNREVOKED_TASKS state, we wait
+                // until the member has revoked the necessary tasks. They are
+                // considered revoked when they are not anymore reported in the
+                // owned tasks set in the StreamsGroupHeartbeat API.
+
+                // If the member provides its owned tasks, we verify if it 
still
+                // owns any of the revoked tasks. If it did not provide it's
+                // owned tasks, or we still own some of the revoked tasks, we
+                // cannot progress.
+                if (
+                    ownedTasks.isEmpty() || 
ownedTasks.get().containsAny(member.tasksPendingRevocation())
+                ) {
+                    return member;
+                }
+
+                // When the member has revoked all the pending tasks, it can
+                // transition to the next epoch (current + 1) and we can 
reconcile
+                // its state towards the latest target assignment.
+                return computeNextAssignment(
+                    member.memberEpoch() + 1,
+                    member.assignedTasks()
+                );
+
+            case UNRELEASED_TASKS:
+                // When the member is in the UNRELEASED_TASKS, we reconcile the
+                // member towards the latest target assignment. This will 
assign any
+                // of the unreleased tasks when they become available.
+                return computeNextAssignment(
+                    member.memberEpoch(),
+                    member.assignedTasks()
+                );
+
+            case UNKNOWN:
+                // We could only end up in this state if a new state is added 
in the
+                // future and the group coordinator is downgraded. In this 
case, the
+                // best option is to fence the member to force it to rejoin 
the group
+                // without any tasks and to reconcile it again from scratch.
+                if ((ownedTasks.isEmpty() || !ownedTasks.get().isEmpty())) {

Review Comment:
   Why is the case owned tasks data structure exists but is empty treated 
differently?



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilderTest.java:
##########
@@ -0,0 +1,744 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+import 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.TaskRole;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.EnumSource;
+
+import java.util.Collections;
+
+import static 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.mkTaskTuple;
+import static 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.mkTasks;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class CurrentAssignmentBuilderTest {
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testStableToStable(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member =
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.STABLE)
+                .setProcessId("process")
+                .setMemberEpoch(10)
+                .setPreviousMemberEpoch(10)
+                .setAssignedTasks(
+                    mkTaskTuple(
+                        taskRole,
+                        mkTasks(subtopologyId1, 1, 2),
+                        mkTasks(subtopologyId2, 3, 4)))
+                .setTasksPendingRevocation(TaskTuple.EMPTY)
+                .build();
+
+        StreamsGroupMember updatedMember = new CurrentAssignmentBuilder(member)
+            .withTargetAssignment(11, mkTaskTuple(taskRole,

Review Comment:
   Should the target assignment epoch here be formulated by means of the member 
epoch to document the invariant member epoch <= target assignment epoch?



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilder.java:
##########
@@ -0,0 +1,447 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.BiPredicate;
+
+/**
+ * The CurrentAssignmentBuilder class encapsulates the reconciliation engine 
of the streams group protocol. Given the current state of a
+ * member and a desired or target assignment state, the state machine takes 
the necessary steps to converge them.
+ */
+public class CurrentAssignmentBuilder {
+
+    /**
+     * The streams group member which is reconciled.
+     */
+    private final StreamsGroupMember member;
+
+    /**
+     * The target assignment epoch.
+     */
+    private int targetAssignmentEpoch;
+
+    /**
+     * The target assignment.
+     */
+    private TaskTuple targetAssignment;
+
+    /**
+     * A function which returns the current process ID of an active task or 
null if the active task
+     * is not assigned. The current process ID is the process ID of the 
current owner.
+     */
+    private BiFunction<String, Integer, String> currentActiveTaskProcessId;
+
+    /**
+     * A function which returns the current process IDs of a standby task or 
null if the standby
+     * task is not assigned. The current process IDs are the process IDs of 
all current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentStandbyTaskProcessIds;
+
+    /**
+     * A function which returns the current process IDs of a warmup task or 
null if the warmup task
+     * is not assigned. The current process IDs are the process IDs of all 
current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentWarmupTaskProcessIds;
+
+    /**
+     * The tasks owned by the member. This may be provided by the member in 
the StreamsGroupHeartbeat request.
+     */
+    private Optional<TaskTuple> ownedTasks = Optional.empty();
+
+    /**
+     * Constructs the CurrentAssignmentBuilder based on the current state of 
the provided streams group member.
+     *
+     * @param member The streams group member that must be reconciled.
+     */
+    public CurrentAssignmentBuilder(StreamsGroupMember member) {
+        this.member = Objects.requireNonNull(member);
+    }
+
+    /**
+     * Sets the target assignment epoch and the target assignment that the 
streams group member must be reconciled to.
+     *
+     * @param targetAssignmentEpoch The target assignment epoch.
+     * @param targetAssignment      The target assignment.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withTargetAssignment(
+        int targetAssignmentEpoch,
+        TaskTuple targetAssignment
+    ) {
+        this.targetAssignmentEpoch = targetAssignmentEpoch;
+        this.targetAssignment = Objects.requireNonNull(targetAssignment);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process ID of an 
active task. This is
+     * used by the state machine to determine if an active task is free or 
still used by another
+     * member, and if there is still a task on a specific process that is not 
yet revoked.
+     *
+     * @param currentActiveTaskProcessId A BiFunction which gets the memberId 
of a subtopology id /
+     *                                   partition id pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentActiveTaskProcessId(
+        BiFunction<String, Integer, String> currentActiveTaskProcessId
+    ) {
+        this.currentActiveTaskProcessId = 
Objects.requireNonNull(currentActiveTaskProcessId);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process IDs of a 
standby task. This is
+     * used by the state machine to determine if there is still a task on a 
specific process that is
+     * not yet revoked.
+     *
+     * @param currentStandbyTaskProcessIds A BiFunction which gets the 
memberIds of a subtopology
+     *                                     ids / partition ids pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentStandbyTaskProcessIds(
+        BiFunction<String, Integer, Set<String>> currentStandbyTaskProcessIds
+    ) {
+        this.currentStandbyTaskProcessIds = 
Objects.requireNonNull(currentStandbyTaskProcessIds);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process IDs of a 
warmup task. This is
+     * used by the state machine to determine if there is still a task on a 
specific process that is
+     * not yet revoked.
+     *
+     * @param currentWarmupTaskProcessIds A BiFunction which gets the 
memberIds of a subtopology ids
+     *                                    / partition ids pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentWarmupTaskProcessIds(
+        BiFunction<String, Integer, Set<String>> currentWarmupTaskProcessIds
+    ) {
+        this.currentWarmupTaskProcessIds = 
Objects.requireNonNull(currentWarmupTaskProcessIds);
+        return this;
+    }
+
+    /**
+     * Sets the tasks currently owned by the member. This comes directly from 
the last StreamsGroupHeartbeat request. This is used to
+     * determine if the member has revoked the necessary tasks. Passing null 
into this function means that the member did not provide
+     * its owned tasks in this heartbeat.
+     *
+     * @param ownedAssignment A collection of active, standby and warm-up tasks
+     * @return This object.
+     */
+    protected CurrentAssignmentBuilder withOwnedAssignment(
+        TaskTuple ownedAssignment
+    ) {
+        this.ownedTasks = Optional.ofNullable(ownedAssignment);
+        return this;
+    }
+
+    /**
+     * Builds the next state for the member or keep the current one if it is 
not possible to move forward with the current state.
+     *
+     * @return A new StreamsGroupMember or the current one.
+     */
+    public StreamsGroupMember build() {
+        switch (member.state()) {
+            case STABLE:
+                // When the member is in the STABLE state, we verify if a newer
+                // epoch (or target assignment) is available. If it is, we can
+                // reconcile the member towards it. Otherwise, we return.
+                if (member.memberEpoch() != targetAssignmentEpoch) {
+                    return computeNextAssignment(
+                        member.memberEpoch(),
+                        member.assignedTasks()
+                    );
+                } else {
+                    return member;
+                }
+
+            case UNREVOKED_TASKS:
+                // When the member is in the UNREVOKED_TASKS state, we wait
+                // until the member has revoked the necessary tasks. They are
+                // considered revoked when they are not anymore reported in the
+                // owned tasks set in the StreamsGroupHeartbeat API.
+
+                // If the member provides its owned tasks, we verify if it 
still
+                // owns any of the revoked tasks. If it did not provide it's
+                // owned tasks, or we still own some of the revoked tasks, we
+                // cannot progress.
+                if (
+                    ownedTasks.isEmpty() || 
ownedTasks.get().containsAny(member.tasksPendingRevocation())
+                ) {
+                    return member;
+                }
+
+                // When the member has revoked all the pending tasks, it can
+                // transition to the next epoch (current + 1) and we can 
reconcile
+                // its state towards the latest target assignment.
+                return computeNextAssignment(
+                    member.memberEpoch() + 1,
+                    member.assignedTasks()
+                );
+
+            case UNRELEASED_TASKS:
+                // When the member is in the UNRELEASED_TASKS, we reconcile the
+                // member towards the latest target assignment. This will 
assign any
+                // of the unreleased tasks when they become available.
+                return computeNextAssignment(
+                    member.memberEpoch(),
+                    member.assignedTasks()
+                );
+
+            case UNKNOWN:
+                // We could only end up in this state if a new state is added 
in the
+                // future and the group coordinator is downgraded. In this 
case, the
+                // best option is to fence the member to force it to rejoin 
the group
+                // without any tasks and to reconcile it again from scratch.
+                if ((ownedTasks.isEmpty() || !ownedTasks.get().isEmpty())) {
+                    throw new FencedMemberEpochException(
+                        "The streams group member is in a unknown state. "
+                            + "The member must abandon all its tasks and 
rejoin.");
+                }
+
+                return computeNextAssignment(
+                    targetAssignmentEpoch,
+                    member.assignedTasks()
+                );
+        }
+
+        return member;
+    }
+
+    /**
+     * Takes the current currentAssignment and the targetAssignment, and 
generates three
+     * collections:
+     *
+     * - the resultAssignedTasks: the tasks that are assigned in both the 
current and target
+     * assignments.
+     * - the resultTasksPendingRevocation: the tasks that are assigned in the 
current
+     * assignment but not in the target assignment.
+     * - the resultTasksPendingAssignment: the tasks that are assigned in the 
target assignment but
+     * not in the current assignment, and can be assigned currently (i.e., 
they are not owned by
+     * another member, as defined by the `isUnreleasedTask` predicate).
+     */
+    private boolean computeAssignmentDifference(
+        Map<String, Set<Integer>> currentAssignment,
+        Map<String, Set<Integer>> targetAssignment,
+        Map<String, Set<Integer>> resultAssignedTasks,
+        Map<String, Set<Integer>> resultTasksPendingRevocation,
+        Map<String, Set<Integer>> resultTasksPendingAssignment,
+        BiPredicate<String, Integer> isUnreleasedTask
+    ) {
+        boolean hasUnreleasedTasks = false;
+
+        Set<String> allSubtopologyIds = new 
HashSet<>(targetAssignment.keySet());
+        allSubtopologyIds.addAll(currentAssignment.keySet());
+
+        for (String subtopologyId : allSubtopologyIds) {
+            Set<Integer> currentPartitions = 
currentAssignment.getOrDefault(subtopologyId, Collections.emptySet());
+            Set<Integer> targetPartitions = 
targetAssignment.getOrDefault(subtopologyId, Collections.emptySet());
+
+            // Result Assigned Partitions = Current Partitions ∩ Target 
Partitions
+            // i.e. we remove all partitions from the current assignment that 
are not in the target
+            //         assignment
+            Set<Integer> resultAssignedPartitions = new 
HashSet<>(currentPartitions);
+            resultAssignedPartitions.retainAll(targetPartitions);
+
+            // Result Partitions Pending Revocation = Current Partitions - 
Result Assigned Partitions
+            // i.e. we will ask the member to revoke all partitions in its 
current assignment that
+            //      are not in the target assignment
+            Set<Integer> resultPartitionsPendingRevocation = new 
HashSet<>(currentPartitions);
+            
resultPartitionsPendingRevocation.removeAll(resultAssignedPartitions);
+
+            // Result Partitions Pending Assignment = Target Partitions - 
Result Assigned Partitions - Unreleased Partitions
+            // i.e. we will ask the member to assign all partitions in its 
target assignment,
+            //      except those that are already assigned, and those that are 
unrelead

Review Comment:
   ```suggestion
               //      except those that are already assigned, and those that 
are unreleased
   ```



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilder.java:
##########
@@ -0,0 +1,447 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.BiPredicate;
+
+/**
+ * The CurrentAssignmentBuilder class encapsulates the reconciliation engine 
of the streams group protocol. Given the current state of a
+ * member and a desired or target assignment state, the state machine takes 
the necessary steps to converge them.
+ */
+public class CurrentAssignmentBuilder {
+
+    /**
+     * The streams group member which is reconciled.
+     */
+    private final StreamsGroupMember member;
+
+    /**
+     * The target assignment epoch.
+     */
+    private int targetAssignmentEpoch;
+
+    /**
+     * The target assignment.
+     */
+    private TaskTuple targetAssignment;
+
+    /**
+     * A function which returns the current process ID of an active task or 
null if the active task
+     * is not assigned. The current process ID is the process ID of the 
current owner.
+     */
+    private BiFunction<String, Integer, String> currentActiveTaskProcessId;
+
+    /**
+     * A function which returns the current process IDs of a standby task or 
null if the standby
+     * task is not assigned. The current process IDs are the process IDs of 
all current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentStandbyTaskProcessIds;
+
+    /**
+     * A function which returns the current process IDs of a warmup task or 
null if the warmup task
+     * is not assigned. The current process IDs are the process IDs of all 
current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentWarmupTaskProcessIds;
+
+    /**
+     * The tasks owned by the member. This may be provided by the member in 
the StreamsGroupHeartbeat request.
+     */
+    private Optional<TaskTuple> ownedTasks = Optional.empty();
+
+    /**
+     * Constructs the CurrentAssignmentBuilder based on the current state of 
the provided streams group member.
+     *
+     * @param member The streams group member that must be reconciled.
+     */
+    public CurrentAssignmentBuilder(StreamsGroupMember member) {
+        this.member = Objects.requireNonNull(member);
+    }
+
+    /**
+     * Sets the target assignment epoch and the target assignment that the 
streams group member must be reconciled to.
+     *
+     * @param targetAssignmentEpoch The target assignment epoch.
+     * @param targetAssignment      The target assignment.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withTargetAssignment(
+        int targetAssignmentEpoch,
+        TaskTuple targetAssignment
+    ) {
+        this.targetAssignmentEpoch = targetAssignmentEpoch;
+        this.targetAssignment = Objects.requireNonNull(targetAssignment);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process ID of an 
active task. This is
+     * used by the state machine to determine if an active task is free or 
still used by another
+     * member, and if there is still a task on a specific process that is not 
yet revoked.
+     *
+     * @param currentActiveTaskProcessId A BiFunction which gets the memberId 
of a subtopology id /
+     *                                   partition id pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentActiveTaskProcessId(
+        BiFunction<String, Integer, String> currentActiveTaskProcessId
+    ) {
+        this.currentActiveTaskProcessId = 
Objects.requireNonNull(currentActiveTaskProcessId);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process IDs of a 
standby task. This is
+     * used by the state machine to determine if there is still a task on a 
specific process that is
+     * not yet revoked.
+     *
+     * @param currentStandbyTaskProcessIds A BiFunction which gets the 
memberIds of a subtopology
+     *                                     ids / partition ids pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentStandbyTaskProcessIds(
+        BiFunction<String, Integer, Set<String>> currentStandbyTaskProcessIds
+    ) {
+        this.currentStandbyTaskProcessIds = 
Objects.requireNonNull(currentStandbyTaskProcessIds);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process IDs of a 
warmup task. This is
+     * used by the state machine to determine if there is still a task on a 
specific process that is
+     * not yet revoked.
+     *
+     * @param currentWarmupTaskProcessIds A BiFunction which gets the 
memberIds of a subtopology ids
+     *                                    / partition ids pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentWarmupTaskProcessIds(
+        BiFunction<String, Integer, Set<String>> currentWarmupTaskProcessIds
+    ) {
+        this.currentWarmupTaskProcessIds = 
Objects.requireNonNull(currentWarmupTaskProcessIds);
+        return this;
+    }
+
+    /**
+     * Sets the tasks currently owned by the member. This comes directly from 
the last StreamsGroupHeartbeat request. This is used to
+     * determine if the member has revoked the necessary tasks. Passing null 
into this function means that the member did not provide
+     * its owned tasks in this heartbeat.
+     *
+     * @param ownedAssignment A collection of active, standby and warm-up tasks
+     * @return This object.
+     */
+    protected CurrentAssignmentBuilder withOwnedAssignment(
+        TaskTuple ownedAssignment
+    ) {
+        this.ownedTasks = Optional.ofNullable(ownedAssignment);
+        return this;
+    }
+
+    /**
+     * Builds the next state for the member or keep the current one if it is 
not possible to move forward with the current state.
+     *
+     * @return A new StreamsGroupMember or the current one.
+     */
+    public StreamsGroupMember build() {
+        switch (member.state()) {
+            case STABLE:
+                // When the member is in the STABLE state, we verify if a newer
+                // epoch (or target assignment) is available. If it is, we can
+                // reconcile the member towards it. Otherwise, we return.
+                if (member.memberEpoch() != targetAssignmentEpoch) {
+                    return computeNextAssignment(
+                        member.memberEpoch(),
+                        member.assignedTasks()
+                    );
+                } else {
+                    return member;
+                }
+
+            case UNREVOKED_TASKS:
+                // When the member is in the UNREVOKED_TASKS state, we wait
+                // until the member has revoked the necessary tasks. They are
+                // considered revoked when they are not anymore reported in the
+                // owned tasks set in the StreamsGroupHeartbeat API.
+
+                // If the member provides its owned tasks, we verify if it 
still
+                // owns any of the revoked tasks. If it did not provide it's
+                // owned tasks, or we still own some of the revoked tasks, we
+                // cannot progress.
+                if (
+                    ownedTasks.isEmpty() || 
ownedTasks.get().containsAny(member.tasksPendingRevocation())
+                ) {
+                    return member;
+                }
+
+                // When the member has revoked all the pending tasks, it can
+                // transition to the next epoch (current + 1) and we can 
reconcile
+                // its state towards the latest target assignment.
+                return computeNextAssignment(
+                    member.memberEpoch() + 1,

Review Comment:
   Could it be that this member epoch + 1 is never used because in 
`buildNewMember()` the member epoch is set to the target assignment epoch in 
this case?
   If yes, I believe we need to refactor a bit this code to make it more 
readable.



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilder.java:
##########
@@ -0,0 +1,447 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.BiPredicate;
+
+/**
+ * The CurrentAssignmentBuilder class encapsulates the reconciliation engine 
of the streams group protocol. Given the current state of a
+ * member and a desired or target assignment state, the state machine takes 
the necessary steps to converge them.
+ */
+public class CurrentAssignmentBuilder {
+
+    /**
+     * The streams group member which is reconciled.
+     */
+    private final StreamsGroupMember member;
+
+    /**
+     * The target assignment epoch.
+     */
+    private int targetAssignmentEpoch;
+
+    /**
+     * The target assignment.
+     */
+    private TaskTuple targetAssignment;
+
+    /**
+     * A function which returns the current process ID of an active task or 
null if the active task
+     * is not assigned. The current process ID is the process ID of the 
current owner.
+     */
+    private BiFunction<String, Integer, String> currentActiveTaskProcessId;
+
+    /**
+     * A function which returns the current process IDs of a standby task or 
null if the standby
+     * task is not assigned. The current process IDs are the process IDs of 
all current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentStandbyTaskProcessIds;
+
+    /**
+     * A function which returns the current process IDs of a warmup task or 
null if the warmup task
+     * is not assigned. The current process IDs are the process IDs of all 
current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentWarmupTaskProcessIds;
+
+    /**
+     * The tasks owned by the member. This may be provided by the member in 
the StreamsGroupHeartbeat request.
+     */
+    private Optional<TaskTuple> ownedTasks = Optional.empty();
+
+    /**
+     * Constructs the CurrentAssignmentBuilder based on the current state of 
the provided streams group member.
+     *
+     * @param member The streams group member that must be reconciled.
+     */
+    public CurrentAssignmentBuilder(StreamsGroupMember member) {
+        this.member = Objects.requireNonNull(member);
+    }
+
+    /**
+     * Sets the target assignment epoch and the target assignment that the 
streams group member must be reconciled to.
+     *
+     * @param targetAssignmentEpoch The target assignment epoch.
+     * @param targetAssignment      The target assignment.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withTargetAssignment(
+        int targetAssignmentEpoch,
+        TaskTuple targetAssignment
+    ) {
+        this.targetAssignmentEpoch = targetAssignmentEpoch;
+        this.targetAssignment = Objects.requireNonNull(targetAssignment);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process ID of an 
active task. This is
+     * used by the state machine to determine if an active task is free or 
still used by another
+     * member, and if there is still a task on a specific process that is not 
yet revoked.
+     *
+     * @param currentActiveTaskProcessId A BiFunction which gets the memberId 
of a subtopology id /

Review Comment:
   Also member ID does not sound right. Isn't it the process ID which is the ID 
of the Streams client that can contain more than one member?



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilderTest.java:
##########
@@ -0,0 +1,744 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+import 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.TaskRole;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.EnumSource;
+
+import java.util.Collections;
+
+import static 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.mkTaskTuple;
+import static 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.mkTasks;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class CurrentAssignmentBuilderTest {
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testStableToStable(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member =
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.STABLE)
+                .setProcessId("process")
+                .setMemberEpoch(10)
+                .setPreviousMemberEpoch(10)
+                .setAssignedTasks(
+                    mkTaskTuple(
+                        taskRole,
+                        mkTasks(subtopologyId1, 1, 2),
+                        mkTasks(subtopologyId2, 3, 4)))
+                .setTasksPendingRevocation(TaskTuple.EMPTY)
+                .build();
+
+        StreamsGroupMember updatedMember = new CurrentAssignmentBuilder(member)
+            .withTargetAssignment(11, mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1, 2),
+                mkTasks(subtopologyId2, 3, 4)))
+            .withCurrentActiveTaskProcessId((subtopologyId, partitionId) -> 
"process")
+            .withCurrentStandbyTaskProcessIds(
+                (subtopologyId, partitionId) -> Collections.emptySet())
+            .withCurrentWarmupTaskProcessIds((subtopologyId, partitionId) -> 
Collections.emptySet())
+            .build();
+
+        assertEquals(
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.STABLE)
+                .setProcessId("process")
+                .setMemberEpoch(11)
+                .setPreviousMemberEpoch(10)

Review Comment:
   Should those values depend on previous values?



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilder.java:
##########
@@ -0,0 +1,447 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.BiPredicate;
+
+/**
+ * The CurrentAssignmentBuilder class encapsulates the reconciliation engine 
of the streams group protocol. Given the current state of a
+ * member and a desired or target assignment state, the state machine takes 
the necessary steps to converge them.
+ */
+public class CurrentAssignmentBuilder {
+
+    /**
+     * The streams group member which is reconciled.
+     */
+    private final StreamsGroupMember member;
+
+    /**
+     * The target assignment epoch.
+     */
+    private int targetAssignmentEpoch;
+
+    /**
+     * The target assignment.
+     */
+    private TaskTuple targetAssignment;
+
+    /**
+     * A function which returns the current process ID of an active task or 
null if the active task
+     * is not assigned. The current process ID is the process ID of the 
current owner.
+     */
+    private BiFunction<String, Integer, String> currentActiveTaskProcessId;
+
+    /**
+     * A function which returns the current process IDs of a standby task or 
null if the standby
+     * task is not assigned. The current process IDs are the process IDs of 
all current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentStandbyTaskProcessIds;
+
+    /**
+     * A function which returns the current process IDs of a warmup task or 
null if the warmup task
+     * is not assigned. The current process IDs are the process IDs of all 
current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentWarmupTaskProcessIds;
+
+    /**
+     * The tasks owned by the member. This may be provided by the member in 
the StreamsGroupHeartbeat request.
+     */
+    private Optional<TaskTuple> ownedTasks = Optional.empty();
+
+    /**
+     * Constructs the CurrentAssignmentBuilder based on the current state of 
the provided streams group member.
+     *
+     * @param member The streams group member that must be reconciled.
+     */
+    public CurrentAssignmentBuilder(StreamsGroupMember member) {
+        this.member = Objects.requireNonNull(member);
+    }
+
+    /**
+     * Sets the target assignment epoch and the target assignment that the 
streams group member must be reconciled to.
+     *
+     * @param targetAssignmentEpoch The target assignment epoch.
+     * @param targetAssignment      The target assignment.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withTargetAssignment(
+        int targetAssignmentEpoch,
+        TaskTuple targetAssignment
+    ) {
+        this.targetAssignmentEpoch = targetAssignmentEpoch;
+        this.targetAssignment = Objects.requireNonNull(targetAssignment);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process ID of an 
active task. This is
+     * used by the state machine to determine if an active task is free or 
still used by another
+     * member, and if there is still a task on a specific process that is not 
yet revoked.
+     *
+     * @param currentActiveTaskProcessId A BiFunction which gets the memberId 
of a subtopology id /
+     *                                   partition id pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentActiveTaskProcessId(
+        BiFunction<String, Integer, String> currentActiveTaskProcessId
+    ) {
+        this.currentActiveTaskProcessId = 
Objects.requireNonNull(currentActiveTaskProcessId);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process IDs of a 
standby task. This is
+     * used by the state machine to determine if there is still a task on a 
specific process that is
+     * not yet revoked.
+     *
+     * @param currentStandbyTaskProcessIds A BiFunction which gets the 
memberIds of a subtopology
+     *                                     ids / partition ids pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentStandbyTaskProcessIds(
+        BiFunction<String, Integer, Set<String>> currentStandbyTaskProcessIds
+    ) {
+        this.currentStandbyTaskProcessIds = 
Objects.requireNonNull(currentStandbyTaskProcessIds);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process IDs of a 
warmup task. This is
+     * used by the state machine to determine if there is still a task on a 
specific process that is
+     * not yet revoked.
+     *
+     * @param currentWarmupTaskProcessIds A BiFunction which gets the 
memberIds of a subtopology ids
+     *                                    / partition ids pair.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withCurrentWarmupTaskProcessIds(
+        BiFunction<String, Integer, Set<String>> currentWarmupTaskProcessIds
+    ) {
+        this.currentWarmupTaskProcessIds = 
Objects.requireNonNull(currentWarmupTaskProcessIds);
+        return this;
+    }
+
+    /**
+     * Sets the tasks currently owned by the member. This comes directly from 
the last StreamsGroupHeartbeat request. This is used to
+     * determine if the member has revoked the necessary tasks. Passing null 
into this function means that the member did not provide
+     * its owned tasks in this heartbeat.
+     *
+     * @param ownedAssignment A collection of active, standby and warm-up tasks
+     * @return This object.
+     */
+    protected CurrentAssignmentBuilder withOwnedAssignment(
+        TaskTuple ownedAssignment
+    ) {
+        this.ownedTasks = Optional.ofNullable(ownedAssignment);
+        return this;
+    }
+
+    /**
+     * Builds the next state for the member or keep the current one if it is 
not possible to move forward with the current state.
+     *
+     * @return A new StreamsGroupMember or the current one.
+     */
+    public StreamsGroupMember build() {
+        switch (member.state()) {
+            case STABLE:
+                // When the member is in the STABLE state, we verify if a newer
+                // epoch (or target assignment) is available. If it is, we can
+                // reconcile the member towards it. Otherwise, we return.
+                if (member.memberEpoch() != targetAssignmentEpoch) {
+                    return computeNextAssignment(
+                        member.memberEpoch(),
+                        member.assignedTasks()
+                    );
+                } else {
+                    return member;
+                }
+
+            case UNREVOKED_TASKS:
+                // When the member is in the UNREVOKED_TASKS state, we wait
+                // until the member has revoked the necessary tasks. They are
+                // considered revoked when they are not anymore reported in the
+                // owned tasks set in the StreamsGroupHeartbeat API.
+
+                // If the member provides its owned tasks, we verify if it 
still
+                // owns any of the revoked tasks. If it did not provide it's
+                // owned tasks, or we still own some of the revoked tasks, we
+                // cannot progress.
+                if (
+                    ownedTasks.isEmpty() || 
ownedTasks.get().containsAny(member.tasksPendingRevocation())
+                ) {
+                    return member;
+                }
+
+                // When the member has revoked all the pending tasks, it can
+                // transition to the next epoch (current + 1) and we can 
reconcile
+                // its state towards the latest target assignment.
+                return computeNextAssignment(
+                    member.memberEpoch() + 1,
+                    member.assignedTasks()
+                );
+
+            case UNRELEASED_TASKS:
+                // When the member is in the UNRELEASED_TASKS, we reconcile the
+                // member towards the latest target assignment. This will 
assign any
+                // of the unreleased tasks when they become available.
+                return computeNextAssignment(
+                    member.memberEpoch(),
+                    member.assignedTasks()
+                );
+
+            case UNKNOWN:
+                // We could only end up in this state if a new state is added 
in the
+                // future and the group coordinator is downgraded. In this 
case, the
+                // best option is to fence the member to force it to rejoin 
the group
+                // without any tasks and to reconcile it again from scratch.
+                if ((ownedTasks.isEmpty() || !ownedTasks.get().isEmpty())) {
+                    throw new FencedMemberEpochException(
+                        "The streams group member is in a unknown state. "
+                            + "The member must abandon all its tasks and 
rejoin.");
+                }
+
+                return computeNextAssignment(
+                    targetAssignmentEpoch,
+                    member.assignedTasks()
+                );
+        }
+
+        return member;
+    }
+
+    /**
+     * Takes the current currentAssignment and the targetAssignment, and 
generates three
+     * collections:
+     *
+     * - the resultAssignedTasks: the tasks that are assigned in both the 
current and target
+     * assignments.
+     * - the resultTasksPendingRevocation: the tasks that are assigned in the 
current
+     * assignment but not in the target assignment.
+     * - the resultTasksPendingAssignment: the tasks that are assigned in the 
target assignment but
+     * not in the current assignment, and can be assigned currently (i.e., 
they are not owned by
+     * another member, as defined by the `isUnreleasedTask` predicate).
+     */
+    private boolean computeAssignmentDifference(
+        Map<String, Set<Integer>> currentAssignment,
+        Map<String, Set<Integer>> targetAssignment,
+        Map<String, Set<Integer>> resultAssignedTasks,
+        Map<String, Set<Integer>> resultTasksPendingRevocation,
+        Map<String, Set<Integer>> resultTasksPendingAssignment,
+        BiPredicate<String, Integer> isUnreleasedTask
+    ) {
+        boolean hasUnreleasedTasks = false;
+
+        Set<String> allSubtopologyIds = new 
HashSet<>(targetAssignment.keySet());
+        allSubtopologyIds.addAll(currentAssignment.keySet());
+
+        for (String subtopologyId : allSubtopologyIds) {
+            Set<Integer> currentPartitions = 
currentAssignment.getOrDefault(subtopologyId, Collections.emptySet());

Review Comment:
   Could you please rename the variables to use `Tasks` instead of 
`Partitions`? With `Partitions` it is a bit confusing. 



##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilder.java:
##########
@@ -0,0 +1,447 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.BiPredicate;
+
+/**
+ * The CurrentAssignmentBuilder class encapsulates the reconciliation engine 
of the streams group protocol. Given the current state of a
+ * member and a desired or target assignment state, the state machine takes 
the necessary steps to converge them.
+ */
+public class CurrentAssignmentBuilder {
+
+    /**
+     * The streams group member which is reconciled.
+     */
+    private final StreamsGroupMember member;
+
+    /**
+     * The target assignment epoch.
+     */
+    private int targetAssignmentEpoch;
+
+    /**
+     * The target assignment.
+     */
+    private TaskTuple targetAssignment;
+
+    /**
+     * A function which returns the current process ID of an active task or 
null if the active task
+     * is not assigned. The current process ID is the process ID of the 
current owner.
+     */
+    private BiFunction<String, Integer, String> currentActiveTaskProcessId;
+
+    /**
+     * A function which returns the current process IDs of a standby task or 
null if the standby
+     * task is not assigned. The current process IDs are the process IDs of 
all current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentStandbyTaskProcessIds;
+
+    /**
+     * A function which returns the current process IDs of a warmup task or 
null if the warmup task
+     * is not assigned. The current process IDs are the process IDs of all 
current owners.
+     */
+    private BiFunction<String, Integer, Set<String>> 
currentWarmupTaskProcessIds;
+
+    /**
+     * The tasks owned by the member. This may be provided by the member in 
the StreamsGroupHeartbeat request.
+     */
+    private Optional<TaskTuple> ownedTasks = Optional.empty();
+
+    /**
+     * Constructs the CurrentAssignmentBuilder based on the current state of 
the provided streams group member.
+     *
+     * @param member The streams group member that must be reconciled.
+     */
+    public CurrentAssignmentBuilder(StreamsGroupMember member) {
+        this.member = Objects.requireNonNull(member);
+    }
+
+    /**
+     * Sets the target assignment epoch and the target assignment that the 
streams group member must be reconciled to.
+     *
+     * @param targetAssignmentEpoch The target assignment epoch.
+     * @param targetAssignment      The target assignment.
+     * @return This object.
+     */
+    public CurrentAssignmentBuilder withTargetAssignment(
+        int targetAssignmentEpoch,
+        TaskTuple targetAssignment
+    ) {
+        this.targetAssignmentEpoch = targetAssignmentEpoch;
+        this.targetAssignment = Objects.requireNonNull(targetAssignment);
+        return this;
+    }
+
+    /**
+     * Sets a BiFunction which allows to retrieve the current process ID of an 
active task. This is
+     * used by the state machine to determine if an active task is free or 
still used by another
+     * member, and if there is still a task on a specific process that is not 
yet revoked.
+     *
+     * @param currentActiveTaskProcessId A BiFunction which gets the memberId 
of a subtopology id /

Review Comment:
   Here and elsewhere
   ```suggestion
        * @param currentActiveTaskProcessId A BiFunction which gets the 
memberId of a subtopology ID /
   ```



##########
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/streams/CurrentAssignmentBuilderTest.java:
##########
@@ -0,0 +1,744 @@
+/*
+ * 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.coordinator.group.streams;
+
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.errors.FencedMemberEpochException;
+import 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.TaskRole;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.EnumSource;
+
+import java.util.Collections;
+
+import static 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.mkTaskTuple;
+import static 
org.apache.kafka.coordinator.group.streams.TaskAssignmentTestUtil.mkTasks;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class CurrentAssignmentBuilderTest {
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testStableToStable(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member =
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.STABLE)
+                .setProcessId("process")
+                .setMemberEpoch(10)
+                .setPreviousMemberEpoch(10)
+                .setAssignedTasks(
+                    mkTaskTuple(
+                        taskRole,
+                        mkTasks(subtopologyId1, 1, 2),
+                        mkTasks(subtopologyId2, 3, 4)))
+                .setTasksPendingRevocation(TaskTuple.EMPTY)
+                .build();
+
+        StreamsGroupMember updatedMember = new CurrentAssignmentBuilder(member)
+            .withTargetAssignment(11, mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1, 2),
+                mkTasks(subtopologyId2, 3, 4)))
+            .withCurrentActiveTaskProcessId((subtopologyId, partitionId) -> 
"process")
+            .withCurrentStandbyTaskProcessIds(
+                (subtopologyId, partitionId) -> Collections.emptySet())
+            .withCurrentWarmupTaskProcessIds((subtopologyId, partitionId) -> 
Collections.emptySet())
+            .build();
+
+        assertEquals(
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.STABLE)
+                .setProcessId("process")
+                .setMemberEpoch(11)
+                .setPreviousMemberEpoch(10)
+                .setAssignedTasks(mkTaskTuple(
+                    taskRole,
+                    mkTasks(subtopologyId1, 1, 2),
+                    mkTasks(subtopologyId2, 3, 4)))
+                .setTasksPendingRevocation(TaskTuple.EMPTY)
+                .build(),
+            updatedMember
+        );
+    }
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testStableToStableWithNewTasks(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member = new StreamsGroupMember.Builder("member")
+            .setState(MemberState.STABLE)
+            .setProcessId("process")
+            .setMemberEpoch(10)
+            .setPreviousMemberEpoch(10)
+            .setAssignedTasks(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1, 2),
+                mkTasks(subtopologyId2, 3, 4)))
+            .setTasksPendingRevocation(TaskTuple.EMPTY)
+            .build();
+
+        StreamsGroupMember updatedMember = new CurrentAssignmentBuilder(member)
+            .withTargetAssignment(11, mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1, 2, 4),
+                mkTasks(subtopologyId2, 3, 4, 7)))
+            .withCurrentActiveTaskProcessId((subtopologyId, partitionId) -> 
null)
+            .withCurrentStandbyTaskProcessIds(
+                (subtopologyId, partitionId) -> Collections.emptySet())
+            .withCurrentWarmupTaskProcessIds((subtopologyId, partitionId) -> 
Collections.emptySet())
+            .build();
+
+        assertEquals(
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.STABLE)
+                .setProcessId("process")
+                .setMemberEpoch(11)
+                .setPreviousMemberEpoch(10)
+                .setAssignedTasks(mkTaskTuple(taskRole,
+                    mkTasks(subtopologyId1, 1, 2, 4),
+                    mkTasks(subtopologyId2, 3, 4, 7)))
+                .setTasksPendingRevocation(TaskTuple.EMPTY)
+                .build(),
+            updatedMember
+        );
+    }
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testStableToUnrevokedTasks(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member = new StreamsGroupMember.Builder("member")
+            .setState(MemberState.STABLE)
+            .setProcessId("process")
+            .setMemberEpoch(10)
+            .setPreviousMemberEpoch(10)
+            .setAssignedTasks(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1, 2),
+                mkTasks(subtopologyId2, 3, 4)))
+            .setTasksPendingRevocation(TaskTuple.EMPTY)
+            .build();
+
+        StreamsGroupMember updatedMember = new CurrentAssignmentBuilder(member)
+            .withTargetAssignment(11, mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 2, 3),
+                mkTasks(subtopologyId2, 4, 5)))
+            .withCurrentActiveTaskProcessId((subtopologyId, partitionId) -> 
null)
+            .withCurrentStandbyTaskProcessIds(
+                (subtopologyId, partitionId) -> Collections.emptySet())
+            .withCurrentWarmupTaskProcessIds((subtopologyId, partitionId) -> 
Collections.emptySet())
+            .build();
+
+        assertEquals(
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.UNREVOKED_TASKS)
+                .setProcessId("process")
+                .setMemberEpoch(10)
+                .setPreviousMemberEpoch(10)
+                .setAssignedTasks(mkTaskTuple(taskRole,
+                    mkTasks(subtopologyId1, 2),
+                    mkTasks(subtopologyId2, 4)))
+                .setTasksPendingRevocation(mkTaskTuple(taskRole,
+                    mkTasks(subtopologyId1, 1),
+                    mkTasks(subtopologyId2, 3)))
+                .build(),
+            updatedMember
+        );
+    }
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testStableToUnreleasedTasks(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member = new StreamsGroupMember.Builder("member")
+            .setState(MemberState.STABLE)
+            .setProcessId("process")
+            .setMemberEpoch(10)
+            .setPreviousMemberEpoch(10)
+            .setAssignedTasks(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1, 2),
+                mkTasks(subtopologyId2, 3, 4)))
+            .setTasksPendingRevocation(TaskTuple.EMPTY)
+            .build();
+
+        StreamsGroupMember updatedMember = new CurrentAssignmentBuilder(member)
+            .withTargetAssignment(11, mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1, 2, 4),
+                mkTasks(subtopologyId2, 3, 4, 7)))
+            .withCurrentActiveTaskProcessId((subtopologyId, partitionId) -> 
"process")
+            .withCurrentStandbyTaskProcessIds(
+                (subtopologyId, partitionId) -> Collections.emptySet())
+            .withCurrentWarmupTaskProcessIds((subtopologyId, partitionId) -> 
Collections.emptySet())
+            .build();
+
+        assertEquals(
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.UNRELEASED_TASKS)
+                .setProcessId("process")
+                .setMemberEpoch(11)
+                .setPreviousMemberEpoch(10)
+                .setAssignedTasks(mkTaskTuple(taskRole,
+                    mkTasks(subtopologyId1, 1, 2),
+                    mkTasks(subtopologyId2, 3, 4)))
+                .setTasksPendingRevocation(TaskTuple.EMPTY)
+                .build(),
+            updatedMember
+        );
+    }
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void 
testStableToUnreleasedTasksWithOwnedTasksNotHavingRevokedTasks(TaskRole 
taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member = new StreamsGroupMember.Builder("member")
+            .setState(MemberState.STABLE)
+            .setProcessId("process")
+            .setMemberEpoch(10)
+            .setPreviousMemberEpoch(10)
+            .setAssignedTasks(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1, 2),
+                mkTasks(subtopologyId2, 3, 4)))
+            .setTasksPendingRevocation(TaskTuple.EMPTY)
+            .build();
+
+        StreamsGroupMember updatedMember = new CurrentAssignmentBuilder(member)
+            .withTargetAssignment(11, mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1, 2),
+                mkTasks(subtopologyId2, 3, 5)))
+            .withCurrentActiveTaskProcessId((subtopologyId, __) ->
+                subtopologyId2.equals(subtopologyId) ? "process" : null
+            )
+            .withCurrentStandbyTaskProcessIds(
+                (subtopologyId, partitionId) -> Collections.emptySet())
+            .withCurrentWarmupTaskProcessIds((subtopologyId, partitionId) -> 
Collections.emptySet())
+            .withOwnedAssignment(mkTaskTuple(taskRole))
+            .build();
+
+        assertEquals(
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.UNRELEASED_TASKS)
+                .setProcessId("process")
+                .setMemberEpoch(11)
+                .setPreviousMemberEpoch(10)
+                .setAssignedTasks(mkTaskTuple(taskRole,
+                    mkTasks(subtopologyId1, 1, 2),
+                    mkTasks(subtopologyId2, 3)))
+                .setTasksPendingRevocation(TaskTuple.EMPTY)
+                .build(),
+            updatedMember
+        );
+    }
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testUnrevokedTasksToStable(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member = new StreamsGroupMember.Builder("member")
+            .setState(MemberState.UNREVOKED_TASKS)
+            .setProcessId("process")
+            .setMemberEpoch(10)
+            .setPreviousMemberEpoch(10)
+            .setAssignedTasks(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 2, 3),
+                mkTasks(subtopologyId2, 5, 6)))
+            .setTasksPendingRevocation(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1),
+                mkTasks(subtopologyId2, 4)))
+            .build();
+
+        StreamsGroupMember updatedMember = new CurrentAssignmentBuilder(member)
+            .withTargetAssignment(11, mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 2, 3),
+                mkTasks(subtopologyId2, 5, 6)))
+            .withCurrentActiveTaskProcessId((subtopologyId, partitionId) -> 
null)
+            .withCurrentStandbyTaskProcessIds(
+                (subtopologyId, partitionId) -> Collections.emptySet())
+            .withCurrentWarmupTaskProcessIds((subtopologyId, partitionId) -> 
Collections.emptySet())
+            .withOwnedAssignment(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 2, 3),
+                mkTasks(subtopologyId2, 5, 6)))
+            .build();
+
+        assertEquals(
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.STABLE)
+                .setProcessId("process")
+                .setMemberEpoch(11)
+                .setPreviousMemberEpoch(10)
+                .setAssignedTasks(mkTaskTuple(taskRole,
+                    mkTasks(subtopologyId1, 2, 3),
+                    mkTasks(subtopologyId2, 5, 6)))
+                .setTasksPendingRevocation(TaskTuple.EMPTY)
+                .build(),
+            updatedMember
+        );
+    }
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testRemainsInUnrevokedTasks(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member = new StreamsGroupMember.Builder("member")
+            .setState(MemberState.UNREVOKED_TASKS)
+            .setProcessId("process")
+            .setMemberEpoch(10)
+            .setPreviousMemberEpoch(10)
+            .setAssignedTasks(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 2, 3),
+                mkTasks(subtopologyId2, 5, 6)))
+            .setTasksPendingRevocation(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1),
+                mkTasks(subtopologyId2, 4)))
+            .build();
+
+        CurrentAssignmentBuilder currentAssignmentBuilder = new 
CurrentAssignmentBuilder(
+            member)
+            .withTargetAssignment(12, mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 3),
+                mkTasks(subtopologyId2, 6)))
+            .withCurrentActiveTaskProcessId((subtopologyId, partitionId) -> 
null)
+            .withCurrentStandbyTaskProcessIds(
+                (subtopologyId, partitionId) -> Collections.emptySet())
+            .withCurrentWarmupTaskProcessIds(
+                (subtopologyId, partitionId) -> Collections.emptySet());
+
+        assertEquals(
+            member,
+            currentAssignmentBuilder
+                .withOwnedAssignment(null)
+                .build()
+        );
+
+        assertEquals(
+            member,
+            currentAssignmentBuilder
+                .withOwnedAssignment(mkTaskTuple(taskRole,
+                    mkTasks(subtopologyId1, 1, 2, 3),
+                    mkTasks(subtopologyId2, 5, 6)))
+                .build()
+        );
+
+        assertEquals(
+            member,
+            currentAssignmentBuilder
+                .withOwnedAssignment(mkTaskTuple(taskRole,
+                    mkTasks(subtopologyId1, 2, 3),
+                    mkTasks(subtopologyId2, 4, 5, 6)))
+                .build()
+        );
+    }
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testUnrevokedTasksToUnrevokedTasks(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member = new StreamsGroupMember.Builder("member")
+            .setState(MemberState.UNREVOKED_TASKS)
+            .setProcessId("process")
+            .setMemberEpoch(10)
+            .setPreviousMemberEpoch(10)
+            .setAssignedTasks(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 2, 3),
+                mkTasks(subtopologyId2, 5, 6)))
+            .setTasksPendingRevocation(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 1),
+                mkTasks(subtopologyId2, 4)))
+            .build();
+
+        StreamsGroupMember updatedMember = new CurrentAssignmentBuilder(member)
+            .withTargetAssignment(12, mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 3),
+                mkTasks(subtopologyId2, 6)))
+            .withCurrentActiveTaskProcessId((subtopologyId, partitionId) -> 
null)
+            .withOwnedAssignment(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 2, 3),
+                mkTasks(subtopologyId2, 5, 6)))
+            .build();
+
+        assertEquals(
+            new StreamsGroupMember.Builder("member")
+                .setState(MemberState.UNREVOKED_TASKS)
+                .setProcessId("process")
+                .setMemberEpoch(11)
+                .setPreviousMemberEpoch(10)
+                .setAssignedTasks(mkTaskTuple(taskRole,
+                    mkTasks(subtopologyId1, 3),
+                    mkTasks(subtopologyId2, 6)))
+                .setTasksPendingRevocation(mkTaskTuple(taskRole,
+                    mkTasks(subtopologyId1, 2),
+                    mkTasks(subtopologyId2, 5)))
+                .build(),
+            updatedMember
+        );
+    }
+
+    @ParameterizedTest
+    @EnumSource(TaskRole.class)
+    public void testUnrevokedTasksToUnreleasedTasks(TaskRole taskRole) {
+        String subtopologyId1 = Uuid.randomUuid().toString();
+        String subtopologyId2 = Uuid.randomUuid().toString();
+
+        StreamsGroupMember member = new StreamsGroupMember.Builder("member")
+            .setState(MemberState.UNREVOKED_TASKS)
+            .setProcessId("process")
+            .setMemberEpoch(11)
+            .setPreviousMemberEpoch(10)
+            .setAssignedTasks(mkTaskTuple(taskRole,
+                mkTasks(subtopologyId1, 2, 3),
+                mkTasks(subtopologyId2, 5, 6)))
+            .setTasksPendingRevocation(TaskTuple.EMPTY)

Review Comment:
   Shouldn't this be non-empty? The member is in `UNREVOKED_TASKS`. It should 
have pending tasks.



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