cadonna commented on code in PR #18551:
URL: https://github.com/apache/kafka/pull/18551#discussion_r1933929039


##########
clients/src/main/java/org/apache/kafka/clients/consumer/internals/StreamsMembershipManager.java:
##########
@@ -0,0 +1,1009 @@
+/*
+ * 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.clients.consumer.internals;
+
+import 
org.apache.kafka.clients.consumer.internals.events.StreamsOnAllTasksLostCallbackCompletedEvent;
+import 
org.apache.kafka.clients.consumer.internals.events.StreamsOnTasksAssignedCallbackCompletedEvent;
+import 
org.apache.kafka.clients.consumer.internals.events.StreamsOnTasksRevokedCallbackCompletedEvent;
+import 
org.apache.kafka.clients.consumer.internals.metrics.ConsumerRebalanceMetricsManager;
+import 
org.apache.kafka.clients.consumer.internals.metrics.RebalanceMetricsManager;
+import org.apache.kafka.common.KafkaException;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.Uuid;
+import org.apache.kafka.common.message.StreamsGroupHeartbeatResponseData;
+import org.apache.kafka.common.metrics.Metrics;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.requests.StreamsGroupHeartbeatRequest;
+import org.apache.kafka.common.requests.StreamsGroupHeartbeatResponse;
+import org.apache.kafka.common.utils.LogContext;
+import org.apache.kafka.common.utils.Time;
+
+import org.slf4j.Logger;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.SortedSet;
+import java.util.TreeSet;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+/**
+ * Tracks state the state of a single member in relationship to a group:
+ * <p/>
+ * Responsible for:
+ * <ul>
+ *   <li>Keeping member state</li>
+ *   <li>Keeping assignment for the member</li>
+ *   <li>Computing assignment for the group if the member is required to do 
so</li>
+ *   <li>Calling the assignment and revocation callbacks on the Streams 
client</li>
+ * </ul>
+ */
+public class StreamsMembershipManager implements RequestManager {
+
+    /**
+     * A data structure to represent the current task assignment, and current 
target task assignment of a member in a
+     * streams group.
+     * <p/>
+     * Besides the assigned tasks, it contains a local epoch that is bumped 
whenever the assignment changes, to ensure
+     * that two assignments with the same tasks but different local epochs are 
not considered equal.
+     */
+    private static class LocalAssignment {
+        public static final long NONE_EPOCH = -1;
+        public static final LocalAssignment NONE = new LocalAssignment(
+            NONE_EPOCH,
+            Collections.emptyMap(),
+            Collections.emptyMap(),
+            Collections.emptyMap()
+        );
+
+        public final long localEpoch;
+        public final Map<String, SortedSet<Integer>> activeTasks;
+        public final Map<String, SortedSet<Integer>> standbyTasks;
+        public final Map<String, SortedSet<Integer>> warmupTasks;
+
+        public LocalAssignment(final long localEpoch,
+                               final Map<String, SortedSet<Integer>> 
activeTasks,
+                               final Map<String, SortedSet<Integer>> 
standbyTasks,
+                               final Map<String, SortedSet<Integer>> 
warmupTasks) {
+            this.localEpoch = localEpoch;
+            this.activeTasks = activeTasks;
+            this.standbyTasks = standbyTasks;
+            this.warmupTasks = warmupTasks;
+            if (localEpoch == NONE_EPOCH &&
+                (!activeTasks.isEmpty() || !standbyTasks.isEmpty() || 
!warmupTasks.isEmpty())) {
+                throw new IllegalArgumentException("Local epoch must be set if 
tasks are assigned.");
+            }
+        }
+
+        Optional<LocalAssignment> updateWith(final Map<String, 
SortedSet<Integer>> activeTasks,
+                                             final Map<String, 
SortedSet<Integer>> standbyTasks,
+                                             final Map<String, 
SortedSet<Integer>> warmupTasks) {
+            if (localEpoch != NONE_EPOCH) {
+                if (activeTasks.equals(this.activeTasks) &&
+                    standbyTasks.equals(this.standbyTasks) &&
+                    warmupTasks.equals(this.warmupTasks)) {
+
+                    return Optional.empty();
+                }
+            }
+
+            long nextLocalEpoch = localEpoch + 1;
+            return Optional.of(new LocalAssignment(nextLocalEpoch, 
activeTasks, standbyTasks, warmupTasks));
+        }
+
+        @Override
+        public String toString() {
+            return "LocalAssignment{" +
+                "localEpoch=" + localEpoch +
+                ", activeTasks=" + activeTasks +
+                ", standbyTasks=" + standbyTasks +
+                ", warmupTasks=" + warmupTasks +
+                '}';
+        }
+
+        @Override
+        public boolean equals(Object o) {
+            if (this == o) return true;
+            if (o == null || getClass() != o.getClass()) return false;
+            LocalAssignment that = (LocalAssignment) o;
+            return localEpoch == that.localEpoch &&
+                Objects.equals(activeTasks, that.activeTasks) &&
+                Objects.equals(standbyTasks, that.standbyTasks) &&
+                Objects.equals(warmupTasks, that.warmupTasks);
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hash(localEpoch, activeTasks, standbyTasks, 
warmupTasks);
+        }
+    }
+
+    static final Utils.TopicPartitionComparator TOPIC_PARTITION_COMPARATOR = 
new Utils.TopicPartitionComparator();
+
+    private final Logger log;
+
+    private final StreamsRebalanceEventsProcessor 
streamsRebalanceEventsProcessor;
+
+    private final StreamsRebalanceData streamsRebalanceData;
+
+    private final SubscriptionState subscriptionState;
+
+    private MemberState state;
+
+    private final String groupId;
+
+    private final String memberId = Uuid.randomUuid().toString();
+
+    private final Optional<String> groupInstanceId = Optional.empty();
+
+    private int memberEpoch = 0;
+
+    private Optional<CompletableFuture<Void>> leaveGroupInProgress = 
Optional.empty();
+
+    private CompletableFuture<Void> staleMemberAssignmentRelease;
+
+    private boolean reconciliationInProgress;
+
+    private boolean rejoinedWhileReconciliationInProgress;
+
+    private final List<MemberStateListener> stateUpdatesListeners = new 
ArrayList<>();
+
+    private LocalAssignment targetAssignment = LocalAssignment.NONE;
+
+    private LocalAssignment currentAssignment = LocalAssignment.NONE;
+
+    private final AtomicBoolean subscriptionUpdated = new AtomicBoolean(false);
+
+    private final RebalanceMetricsManager metricsManager;
+
+    private final Time time;
+
+    private boolean isPollTimerExpired;
+
+    /**
+     * Constructs the Streams membership manager.
+     *
+     * @param groupId                           The ID of the group.
+     * @param streamsRebalanceEventsProcessor   The processor that handles 
Streams rebalance events like requests for
+     *                                          invocation of 
assignment/revocation callbacks.
+     * @param streamsRebalanceData              Data needed to participate in 
the Streams rebalance protocol.
+     * @param subscriptionState                 The subscription state of the 
member.
+     * @param logContext                        The log context.
+     * @param time                              The time.
+     * @param metrics                           The metrics.
+     */
+    public StreamsMembershipManager(final String groupId,
+                                    final StreamsRebalanceEventsProcessor 
streamsRebalanceEventsProcessor,
+                                    final StreamsRebalanceData 
streamsRebalanceData,
+                                    final SubscriptionState subscriptionState,
+                                    final LogContext logContext,
+                                    final Time time,
+                                    final Metrics metrics) {
+        log = logContext.logger(StreamsMembershipManager.class);
+        this.state = MemberState.UNSUBSCRIBED;
+        this.groupId = groupId;
+        this.streamsRebalanceEventsProcessor = streamsRebalanceEventsProcessor;
+        this.streamsRebalanceData = streamsRebalanceData;
+        this.subscriptionState = subscriptionState;
+        metricsManager = new ConsumerRebalanceMetricsManager(metrics);
+        this.time = time;
+    }
+
+    public String groupId() {
+        return groupId;
+    }
+
+    public String memberId() {
+        return memberId;
+    }
+
+    public Optional<String> groupInstanceId() {
+        return groupInstanceId;
+    }
+
+    public int memberEpoch() {
+        return memberEpoch;
+    }
+
+    public MemberState state() {
+        return state;
+    }
+
+    public boolean isLeavingGroup() {
+        MemberState state = state();
+        return state == MemberState.PREPARE_LEAVING || state == 
MemberState.LEAVING;
+    }
+
+    private boolean isNotInGroup() {
+        MemberState state = state();
+        return state == MemberState.UNSUBSCRIBED ||
+            state == MemberState.FENCED ||
+            state == MemberState.FATAL ||
+            state == MemberState.STALE;
+    }
+
+    public void registerStateListener(MemberStateListener listener) {
+        stateUpdatesListeners.add(Objects.requireNonNull(listener, "State 
updates listener cannot be null"));
+    }
+
+    void notifyEpochChange(Optional<Integer> epoch) {
+        stateUpdatesListeners.forEach(stateListener -> 
stateListener.onMemberEpochUpdated(epoch, memberId));
+    }
+
+    private void transitionToJoining() {
+        if (state == MemberState.FATAL) {
+            log.warn("No action taken to join the group with the updated 
subscription because " +
+                "the member is in FATAL state");
+            return;
+        }
+        if (reconciliationInProgress) {
+            rejoinedWhileReconciliationInProgress = true;
+        }
+        resetEpoch();
+        transitionTo(MemberState.JOINING);
+        clearCurrentTaskAssignment();
+    }
+
+    private void transitionToSendingLeaveGroup(boolean dueToExpiredPollTimer) {
+        if (state == MemberState.FATAL) {
+            log.warn("Member {} with epoch {} won't send leave group request 
because it is in " +
+                "FATAL state", memberId, memberEpoch);
+            return;
+        }
+        if (state == MemberState.UNSUBSCRIBED) {
+            log.warn("Member {} won't send leave group request because it is 
already out of the group.",
+                memberId);
+            return;
+        }
+
+        if (dueToExpiredPollTimer) {
+            isPollTimerExpired = true;
+            // Briefly transition through prepare leaving. The member does not 
have to release
+            // any assignment before sending the leave group given that is 
stale. It will invoke
+            // onTaskAssignment with empty assignment after sending the leave 
group on the STALE state.
+            transitionTo(MemberState.PREPARE_LEAVING);
+        }
+        finalizeLeaving();
+        transitionTo(MemberState.LEAVING);
+    }
+
+    private void finalizeLeaving() {
+        
updateMemberEpoch(StreamsGroupHeartbeatRequest.LEAVE_GROUP_MEMBER_EPOCH);
+        clearCurrentTaskAssignment();
+    }
+
+    private void transitionToStale() {
+        transitionTo(MemberState.STALE);
+
+        final CompletableFuture<Void> onAllTasksLostCallbackExecution =
+            
streamsRebalanceEventsProcessor.requestOnAllTasksLostCallbackInvocation();
+        staleMemberAssignmentRelease = 
onAllTasksLostCallbackExecution.whenComplete((result, error) -> {
+            if (error != null) {
+                log.error("Task revocation callback invocation failed " +
+                    "after member left group due to expired poll timer.", 
error);
+            }
+            clearTaskAndPartitionAssignment();
+            log.debug("Member {} sent leave group heartbeat and released its 
assignment. It will remain " +
+                    "in {} state until the poll timer is reset, and it will 
then rejoin the group",
+                memberId, MemberState.STALE);
+        });
+    }
+
+    public void transitionToFatal() {
+        MemberState previousState = state;
+        transitionTo(MemberState.FATAL);
+        log.error("Member {} with epoch {} transitioned to fatal state", 
memberId, memberEpoch);
+        notifyEpochChange(Optional.empty());
+
+        if (previousState == MemberState.UNSUBSCRIBED) {
+            log.debug("Member {} with epoch {} got fatal error from the broker 
but it already " +
+                "left the group, so onTaskAssignment callback won't be 
triggered.", memberId, memberEpoch);
+            return;
+        }
+
+        if (previousState == MemberState.LEAVING || previousState == 
MemberState.PREPARE_LEAVING) {
+            log.info("Member {} with epoch {} was leaving the group with state 
{} when it got a " +
+                "fatal error from the broker. It will discard the ongoing 
leave and remain in " +
+                "fatal state.", memberId, memberEpoch, previousState);
+            maybeCompleteLeaveInProgress();
+            return;
+        }
+
+        CompletableFuture<Void> onAllTasksLostCallbackExecuted = 
streamsRebalanceEventsProcessor.requestOnAllTasksLostCallbackInvocation();
+        onAllTasksLostCallbackExecuted.whenComplete((result, error) -> {
+            if (error != null) {
+                log.error("onTaskAssignment callback invocation failed while 
releasing assignment" +
+                    "after member failed with fatal error.", error);
+            }
+            clearTaskAndPartitionAssignment();
+        });
+    }
+
+    public void transitionToUnsubscribeIfLeaving() {
+        if (state == MemberState.LEAVING) {
+            log.warn("Heartbeat to leave group cannot be sent (most probably 
due to coordinator " +
+                    "not known/available). Member {} with epoch {} will 
transition to {}.",
+                memberId, memberEpoch, MemberState.UNSUBSCRIBED);
+            transitionTo(MemberState.UNSUBSCRIBED);
+            maybeCompleteLeaveInProgress();
+        }
+    }
+
+    private void transitionTo(MemberState nextState) {
+        if (!state.equals(nextState) && 
!nextState.getPreviousValidStates().contains(state)) {
+            throw new IllegalStateException(String.format("Invalid state 
transition from %s to %s",
+                state, nextState));
+        }
+
+        if (isCompletingRebalance(state, nextState)) {
+            metricsManager.recordRebalanceEnded(time.milliseconds());
+        }
+        if (isStartingRebalance(state, nextState)) {
+            metricsManager.recordRebalanceStarted(time.milliseconds());
+        }
+
+        log.info("Member {} with epoch {} transitioned from {} to {}.", 
memberId, memberEpoch, state, nextState);
+        this.state = nextState;
+    }
+
+    private static boolean isCompletingRebalance(MemberState currentState, 
MemberState nextState) {
+        return currentState == MemberState.RECONCILING &&
+            (nextState == MemberState.STABLE || nextState == 
MemberState.ACKNOWLEDGING);
+    }
+
+    private static boolean isStartingRebalance(MemberState currentState, 
MemberState nextState) {
+        return currentState != MemberState.RECONCILING && nextState == 
MemberState.RECONCILING;
+    }
+
+    private void resetEpoch() {
+        
updateMemberEpoch(StreamsGroupHeartbeatRequest.JOIN_GROUP_MEMBER_EPOCH);
+    }
+
+    private void updateMemberEpoch(int newEpoch) {
+        boolean newEpochReceived = this.memberEpoch != newEpoch;
+        this.memberEpoch = newEpoch;
+        if (newEpochReceived) {
+            if (memberEpoch > 0) {
+                notifyEpochChange(Optional.of(memberEpoch));
+            } else {
+                notifyEpochChange(Optional.empty());
+            }
+        }
+    }
+
+    private void clearCurrentTaskAssignment() {
+        currentAssignment = LocalAssignment.NONE;
+    }
+
+    private void clearTaskAndPartitionAssignment() {
+        subscriptionState.assignFromSubscribed(Collections.emptySet());
+        currentAssignment = LocalAssignment.NONE;
+        targetAssignment = LocalAssignment.NONE;
+    }
+
+    public boolean shouldSkipHeartbeat() {
+        return isNotInGroup();
+    }
+
+    public boolean shouldHeartbeatNow() {
+        MemberState state = state();
+        return state == MemberState.ACKNOWLEDGING || state == 
MemberState.LEAVING || state == MemberState.JOINING;
+    }
+
+    public void onSubscriptionUpdated() {
+        subscriptionUpdated.compareAndSet(false, true);
+    }
+
+    public void onConsumerPoll() {
+        if (subscriptionUpdated.compareAndSet(true, false) && state == 
MemberState.UNSUBSCRIBED) {
+            transitionToJoining();
+        }
+    }
+
+    public void onHeartbeatRequestGenerated() {
+        MemberState state = state();
+        if (state == MemberState.ACKNOWLEDGING) {
+            if (targetAssignmentReconciled()) {
+                transitionTo(MemberState.STABLE);
+            } else {
+                log.debug("Member {} with epoch {} transitioned to {} after a 
heartbeat was sent " +
+                    "to ack a previous reconciliation. New assignments are 
ready to " +
+                    "be reconciled.", memberId, memberEpoch, 
MemberState.RECONCILING);
+                transitionTo(MemberState.RECONCILING);
+            }
+        } else if (state == MemberState.LEAVING) {
+            if (isPollTimerExpired) {
+                log.debug("Member {} with epoch {} generated the heartbeat to 
leave due to expired poll timer. It will " +
+                    "remain stale (no heartbeat) until it rejoins the group on 
the next consumer " +
+                    "poll.", memberId, memberEpoch);
+                transitionToStale();
+            } else {
+                log.debug("Member {} with epoch {} generated the heartbeat to 
leave the group.", memberId, memberEpoch);
+                transitionTo(MemberState.UNSUBSCRIBED);
+            }
+        }
+    }
+
+    public void onHeartbeatSuccess(StreamsGroupHeartbeatResponse response) {
+        StreamsGroupHeartbeatResponseData responseData = response.data();
+        throwIfUnexpectedError(responseData);
+        if (state == MemberState.LEAVING) {
+            log.debug("Ignoring heartbeat response received from broker. 
Member {} with epoch {} is " +
+                "already leaving the group.", memberId, memberEpoch);
+            return;
+        }
+        if (state == MemberState.UNSUBSCRIBED && 
maybeCompleteLeaveInProgress()) {
+            log.debug("Member {} with epoch {} received a successful response 
to the heartbeat " +
+                "to leave the group and completed the leave operation. ", 
memberId, memberEpoch);
+            return;
+        }
+        if (isNotInGroup()) {
+            log.debug("Ignoring heartbeat response received from broker. 
Member {} is in {} state" +
+                " so it's not a member of the group. ", memberId, state);
+            return;
+        }
+        
+        updateMemberEpoch(responseData.memberEpoch());
+
+        final List<StreamsGroupHeartbeatResponseData.TaskIds> activeTasks = 
responseData.activeTasks();
+        final List<StreamsGroupHeartbeatResponseData.TaskIds> standbyTasks = 
responseData.standbyTasks();
+        final List<StreamsGroupHeartbeatResponseData.TaskIds> warmupTasks = 
responseData.warmupTasks();
+
+        if (activeTasks != null && standbyTasks != null && warmupTasks != 
null) {
+
+            if (!state.canHandleNewAssignment()) {
+                log.debug("Ignoring new assignment: active tasks {}, standby 
tasks {}, and warm-up tasks {} received " +
+                        "from server because member is in {} state.",
+                    activeTasks, standbyTasks, warmupTasks, state);
+                return;
+            }
+
+            processAssignmentReceived(
+                toTasksAssignment(activeTasks),
+                toTasksAssignment(standbyTasks),
+                toTasksAssignment(warmupTasks)
+            );
+        } else {
+            if (responseData.activeTasks() != null ||
+                responseData.standbyTasks() != null ||
+                responseData.warmupTasks() != null) {
+
+                throw new IllegalStateException("Invalid response data, task 
collections must be all null or all non-null: "
+                    + responseData);
+            }
+        }
+    }
+
+    public void onHeartbeatFailure(boolean retriable) {
+        if (!retriable) {
+            metricsManager.maybeRecordRebalanceFailed();
+        }
+        // The leave group request is sent out once (not retried), so we 
should complete the leave
+        // operation once the request completes, regardless of the response.
+        if (state == MemberState.UNSUBSCRIBED && 
maybeCompleteLeaveInProgress()) {
+            log.warn("Member {} with epoch {} received a failed response to 
the heartbeat to " +
+                "leave the group and completed the leave operation. ", 
memberId, memberEpoch);
+        }
+    }
+
+    public void onPollTimerExpired() {
+        transitionToSendingLeaveGroup(true);
+    }
+
+    public void onFenced() {
+        if (state == MemberState.PREPARE_LEAVING) {
+            log.info("Member {} with epoch {} got fenced but it is already 
preparing to leave " +
+                "the group, so it will stop sending heartbeat and won't 
attempt to send the " +
+                "leave request or rejoin.", memberId, memberEpoch);
+            finalizeLeaving();
+            transitionTo(MemberState.UNSUBSCRIBED);
+            maybeCompleteLeaveInProgress();
+            return;
+        }
+
+        if (state == MemberState.LEAVING) {
+            log.debug("Member {} with epoch {} got fenced before sending leave 
group heartbeat. " +
+                "It will not send the leave request and won't attempt to 
rejoin.", memberId, memberEpoch);
+            transitionTo(MemberState.UNSUBSCRIBED);
+            maybeCompleteLeaveInProgress();
+            return;
+        }
+        if (state == MemberState.UNSUBSCRIBED) {
+            log.debug("Member {} with epoch {} got fenced but it already left 
the group, so it " +
+                "won't attempt to rejoin.", memberId, memberEpoch);
+            return;
+        }
+        transitionTo(MemberState.FENCED);
+        resetEpoch();
+        log.debug("Member {} with epoch {} transitioned to {} state. It will 
release its " +
+            "assignment and rejoin the group.", memberId, memberEpoch, 
MemberState.FENCED);
+
+        CompletableFuture<Void> callbackResult = 
streamsRebalanceEventsProcessor.requestOnAllTasksLostCallbackInvocation();
+        callbackResult.whenComplete((result, error) -> {
+            if (error != null) {
+                log.error("onAllTasksLost callback invocation failed while 
releasing assignment" +
+                    " after member got fenced. Member will rejoin the group 
anyways.", error);
+            }
+            clearTaskAndPartitionAssignment();
+            if (state == MemberState.FENCED) {
+                transitionToJoining();
+            } else {
+                log.debug("Fenced member onAllTasksLost callback completed but 
the state has " +
+                    "already changed to {}, so the member won't rejoin the 
group", state);
+            }
+        });
+    }
+
+    private void throwIfUnexpectedError(StreamsGroupHeartbeatResponseData 
responseData) {
+        if (responseData.errorCode() != Errors.NONE.code()) {
+            String errorMessage = String.format(
+                "Unexpected error in Heartbeat response. Expected no error, 
but received: %s with message: '%s'",
+                Errors.forCode(responseData.errorCode()), 
responseData.errorMessage()
+            );
+            throw new IllegalArgumentException(errorMessage);
+        }
+    }
+
+    public void maybeRejoinStaleMember() {
+        isPollTimerExpired = false;
+        if (state == MemberState.STALE) {
+            log.debug("Expired poll timer has been reset so stale member {} 
will rejoin the group " +
+                "when it completes releasing its previous assignment.", 
memberId);
+            staleMemberAssignmentRelease.whenComplete((__, error) -> 
transitionToJoining());
+        }
+    }
+
+    private boolean maybeCompleteLeaveInProgress() {
+        if (leaveGroupInProgress.isPresent()) {
+            leaveGroupInProgress.get().complete(null);
+            leaveGroupInProgress = Optional.empty();
+            return true;
+        }
+        return false;
+    }
+
+    private static SortedSet<StreamsRebalanceData.TaskId> toTaskIdSet(final 
Map<String, SortedSet<Integer>> tasks) {
+        SortedSet<StreamsRebalanceData.TaskId> taskIdSet = new TreeSet<>();
+        for (final Map.Entry<String, SortedSet<Integer>> task : 
tasks.entrySet()) {
+            final String subtopologyId = task.getKey();
+            final SortedSet<Integer> partitions = task.getValue();
+            for (final int partition : partitions) {
+                taskIdSet.add(new StreamsRebalanceData.TaskId(subtopologyId, 
partition));
+            }
+        }
+        return taskIdSet;
+    }
+
+    private static Map<String, SortedSet<Integer>> toTasksAssignment(final 
List<StreamsGroupHeartbeatResponseData.TaskIds> taskIds) {
+        return taskIds.stream()
+            
.collect(Collectors.toMap(StreamsGroupHeartbeatResponseData.TaskIds::subtopologyId,
 taskId -> new TreeSet<>(taskId.partitions())));
+    }
+
+    /**
+     * Leaves the group when the member closes.
+     *
+     * <p>
+     * This method does the following:
+     * <ol>
+     *     <li>Transitions member state to {@link 
MemberState#PREPARE_LEAVING}.</li>
+     *     <li>Skips the invocation of the revocation callback or lost 
callback.</li>
+     *     <li>Clears the current and target assignment, unsubscribes from all 
topics and
+     *     transitions the member state to {@link MemberState#LEAVING}.</li>
+     * </ol>
+     * States {@link MemberState#PREPARE_LEAVING} and {@link 
MemberState#LEAVING} cause the heartbeat request manager
+     * to send a leave group heartbeat.
+     * </p>
+     *
+     * @return future that will complete when the heartbeat to leave the group 
has been sent out.
+     */
+    public CompletableFuture<Void> leaveGroupOnClose() {
+        return leaveGroup(true);
+    }
+
+    /**
+     * Leaves the group.
+     *
+     * <p>
+     * This method does the following:
+     * <ol>
+     *     <li>Transitions member state to {@link 
MemberState#PREPARE_LEAVING}.</li>
+     *     <li>Requests the invocation of the revocation callback or lost 
callback.</li>
+     *     <li>Once the callback completes, it clears the current and target 
assignment, unsubscribes from
+     *     all topics and transitions the member state to {@link 
MemberState#LEAVING}.</li>
+     * </ol>
+     * States {@link MemberState#PREPARE_LEAVING} and {@link 
MemberState#LEAVING} cause the heartbeat request manager
+     * to send a leave group heartbeat.
+     * </p>
+     *
+     * @return future that will complete when the revocation callback 
execution completes and the heartbeat
+     *         to leave the group has been sent out.
+     */
+    public CompletableFuture<Void> leaveGroup() {
+        return leaveGroup(false);
+    }
+
+    private CompletableFuture<Void> leaveGroup(final boolean isOnClose) {
+        if (isNotInGroup()) {
+            if (state == MemberState.FENCED) {
+                clearTaskAndPartitionAssignment();
+                transitionTo(MemberState.UNSUBSCRIBED);
+            }
+            subscriptionState.unsubscribe();
+            return CompletableFuture.completedFuture(null);
+        }
+
+        if (state == MemberState.PREPARE_LEAVING || state == 
MemberState.LEAVING) {
+            log.debug("Leave group operation already in progress for member 
{}", memberId);
+            return leaveGroupInProgress.get();
+        }
+
+        transitionTo(MemberState.PREPARE_LEAVING);
+        CompletableFuture<Void> onGroupLeft = new CompletableFuture<>();
+        leaveGroupInProgress = Optional.of(onGroupLeft);
+        if (!isOnClose) {
+            CompletableFuture<Void> onAllActiveTasksReleasedCallbackExecuted = 
releaseActiveTasks();
+            onAllActiveTasksReleasedCallbackExecuted
+                .whenComplete((__, callbackError) -> 
leavingAfterReleasingActiveTasks(callbackError));
+        } else {
+            leaving();
+        }
+
+        return onGroupLeft;
+    }
+
+    private CompletableFuture<Void> releaseActiveTasks() {
+        if (memberEpoch > 0) {
+            return 
revokeActiveTasks(toTaskIdSet(currentAssignment.activeTasks));
+        } else {
+            return releaseLostActiveTasks();
+        }
+    }
+
+    private void leavingAfterReleasingActiveTasks(Throwable callbackError) {
+        if (callbackError != null) {
+            log.error("Member {} callback to revoke task assignment failed. It 
will proceed " +
+                    "to clear its assignment and send a leave group heartbeat",
+                memberId, callbackError);
+        } else {
+            log.info("Member {} completed callback to revoke task assignment. 
It will proceed " +
+                    "to clear its assignment and send a leave group heartbeat",
+                memberId);
+        }
+        leaving();
+    }
+
+    private void leaving() {
+        clearTaskAndPartitionAssignment();
+        subscriptionState.unsubscribe();
+        transitionToSendingLeaveGroup(false);
+    }
+
+    /**
+     * This will process the assignment received if it is different from the 
member's current
+     * assignment. If a new assignment is received, this will make sure 
reconciliation is attempted
+     * on the next call of `poll`. If another reconciliation is currently in 
process, the first `poll`
+     * after that reconciliation will trigger the new reconciliation.
+     *
+     * @param activeTasks Target active tasks assignment received from the 
broker.
+     * @param standbyTasks Target standby tasks assignment received from the 
broker.
+     * @param warmupTasks Target warm-up tasks assignment received from the 
broker.
+     */
+    private void processAssignmentReceived(Map<String, SortedSet<Integer>> 
activeTasks,
+                                           Map<String, SortedSet<Integer>> 
standbyTasks,
+                                           Map<String, SortedSet<Integer>> 
warmupTasks) {
+        replaceTargetAssignmentWithNewAssignment(activeTasks, standbyTasks, 
warmupTasks);
+        if (!targetAssignmentReconciled()) {
+            transitionTo(MemberState.RECONCILING);
+        } else {
+            log.debug("Target assignment {} received from the broker is equals 
to the member " +
+                    "current assignment {}. Nothing to reconcile.",
+                targetAssignment, currentAssignment);
+            // Make sure we transition the member back to STABLE if it was 
RECONCILING (ex.
+            // member was RECONCILING unresolved assignments that were just 
removed by the
+            // broker), or JOINING (member joining received empty assignment).
+            if (state == MemberState.RECONCILING || state == 
MemberState.JOINING) {
+                transitionTo(MemberState.STABLE);
+            }
+        }
+    }
+
+    private boolean targetAssignmentReconciled() {
+        return currentAssignment.equals(targetAssignment);
+    }
+
+    private void replaceTargetAssignmentWithNewAssignment(Map<String, 
SortedSet<Integer>> activeTasks,
+                                                          Map<String, 
SortedSet<Integer>> standbyTasks,
+                                                          Map<String, 
SortedSet<Integer>> warmupTasks) {
+        targetAssignment.updateWith(activeTasks, standbyTasks, warmupTasks)
+            .ifPresent(updatedAssignment -> {
+                log.debug("Target assignment updated from {} to {}. Member 
will reconcile it on the next poll.",
+                    targetAssignment, updatedAssignment);
+                targetAssignment = updatedAssignment;
+            });
+    }
+
+    @Override
+    public NetworkClientDelegate.PollResult poll(long currentTimeMs) {
+        if (state == MemberState.RECONCILING) {
+            maybeReconcile();
+        }
+        return NetworkClientDelegate.PollResult.EMPTY;
+    }
+
+    private void maybeReconcile() {
+        if (targetAssignmentReconciled()) {
+            log.trace("Ignoring reconciliation attempt. Target assignment is 
equal to the " +
+                "current assignment.");
+            return;
+        }
+        if (reconciliationInProgress) {
+            log.trace("Ignoring reconciliation attempt. Another reconciliation 
is already in progress. Assignment " +
+                targetAssignment + " will be handled in the next 
reconciliation loop.");
+            return;
+        }
+
+        markReconciliationInProgress();
+
+        SortedSet<StreamsRebalanceData.TaskId> assignedActiveTasks = 
toTaskIdSet(targetAssignment.activeTasks);
+        SortedSet<StreamsRebalanceData.TaskId> ownedActiveTasks = 
toTaskIdSet(currentAssignment.activeTasks);
+        SortedSet<StreamsRebalanceData.TaskId> activeTasksToRevoke = new 
TreeSet<>(ownedActiveTasks);
+        activeTasksToRevoke.removeAll(assignedActiveTasks);
+        SortedSet<StreamsRebalanceData.TaskId> assignedStandbyTasks = 
toTaskIdSet(targetAssignment.standbyTasks);
+        SortedSet<StreamsRebalanceData.TaskId> ownedStandbyTasks = 
toTaskIdSet(currentAssignment.standbyTasks);
+        SortedSet<StreamsRebalanceData.TaskId> assignedWarmupTasks = 
toTaskIdSet(targetAssignment.warmupTasks);
+        SortedSet<StreamsRebalanceData.TaskId> ownedWarmupTasks = 
toTaskIdSet(currentAssignment.warmupTasks);
+
+        log.info("Assigned tasks with local epoch {}\n" +
+                "\tMember:                        {}\n" +
+                "\tAssigned active tasks:         {}\n" +
+                "\tOwned active tasks:            {}\n" +
+                "\tActive tasks to revoke:        {}\n" +
+                "\tAssigned standby tasks:        {}\n" +
+                "\tOwned standby tasks:           {}\n" +
+                "\tAssigned warm-up tasks:        {}\n" +
+                "\tOwned warm-up tasks:           {}\n",
+            targetAssignment.localEpoch,
+            memberId,
+            assignedActiveTasks,
+            ownedActiveTasks,
+            activeTasksToRevoke,
+            assignedStandbyTasks,
+            ownedStandbyTasks,
+            assignedWarmupTasks,
+            ownedWarmupTasks
+        );
+
+        SortedSet<TopicPartition> ownedTopicPartitionsFromSubscriptionState = 
new TreeSet<>(TOPIC_PARTITION_COMPARATOR);
+        
ownedTopicPartitionsFromSubscriptionState.addAll(subscriptionState.assignedPartitions());
+        SortedSet<TopicPartition> ownedTopicPartitionsFromAssignedTasks =
+            topicPartitionsForActiveTasks(currentAssignment.activeTasks);
+        if 
(!ownedTopicPartitionsFromAssignedTasks.equals(ownedTopicPartitionsFromSubscriptionState))
 {
+            throw new IllegalStateException("Owned partitions from 
subscription state and owned partitions from " +
+                "assigned active tasks are not equal. " +
+                "Owned partitions from subscription state: " + 
ownedTopicPartitionsFromSubscriptionState + ", " +
+                "Owned partitions from assigned active tasks: " + 
ownedTopicPartitionsFromAssignedTasks);
+        }
+        SortedSet<TopicPartition> assignedTopicPartitions = 
topicPartitionsForActiveTasks(targetAssignment.activeTasks);
+        SortedSet<TopicPartition> assignedTopicPartitionsNotPreviouslyOwned =
+            partitionsToAssignNotPreviouslyOwned(assignedTopicPartitions, 
ownedTopicPartitionsFromSubscriptionState);
+        SortedSet<TopicPartition> partitionsToRevoke = new 
TreeSet<>(ownedTopicPartitionsFromSubscriptionState);
+        partitionsToRevoke.removeAll(assignedTopicPartitions);
+
+        final CompletableFuture<Void> onTasksRevokedCallbackExecuted = 
revokeActiveTasks(activeTasksToRevoke);
+
+        final CompletableFuture<Void> 
onTasksRevokedAndAssignedCallbacksExecuted = 
onTasksRevokedCallbackExecuted.thenCompose(__ -> {
+            if (!maybeAbortReconciliation()) {
+                return assignTasks(assignedActiveTasks, ownedActiveTasks, 
assignedStandbyTasks, assignedWarmupTasks);
+            }
+            return CompletableFuture.completedFuture(null);
+        });
+
+        // The current target assignment is captured to ensure that 
acknowledging the current assignment is done with
+        // the same target assignment that was used when this reconciliation 
was initiated.
+        LocalAssignment currentTargetAssignment = targetAssignment;
+        onTasksRevokedAndAssignedCallbacksExecuted.whenComplete((__, 
callbackError) -> {
+            if (callbackError != null) {
+                log.error("Reconciliation failed: callback invocation failed 
for tasks {}",
+                    currentTargetAssignment, callbackError);
+                markReconciliationCompleted();
+            } else {
+                if (reconciliationInProgress && !maybeAbortReconciliation()) {
+                    
subscriptionState.enablePartitionsAwaitingCallback(assignedTopicPartitionsNotPreviouslyOwned);
+                    currentAssignment = currentTargetAssignment;
+                    transitionTo(MemberState.ACKNOWLEDGING);
+                    markReconciliationCompleted();
+                }
+            }
+        });
+    }
+
+    private CompletableFuture<Void> revokeActiveTasks(final 
SortedSet<StreamsRebalanceData.TaskId> activeTasksToRevoke) {
+        if (activeTasksToRevoke.isEmpty()) {
+            return CompletableFuture.completedFuture(null);
+        }
+
+        log.info("Revoking previously assigned active tasks {}", 
activeTasksToRevoke.stream()
+            .map(StreamsRebalanceData.TaskId::toString)
+            .collect(Collectors.joining(", ")));
+
+        final SortedSet<TopicPartition> partitionsToRevoke = 
topicPartitionsForActiveTasks(activeTasksToRevoke);
+        log.debug("Marking partitions pending for revocation: {}", 
partitionsToRevoke);
+        subscriptionState.markPendingRevocation(partitionsToRevoke);
+
+        return 
streamsRebalanceEventsProcessor.requestOnTasksRevokedCallbackInvocation(activeTasksToRevoke);
+    }
+
+    private CompletableFuture<Void> assignTasks(final 
SortedSet<StreamsRebalanceData.TaskId> activeTasksToAssign,
+                                                final 
SortedSet<StreamsRebalanceData.TaskId> ownedActiveTasks,
+                                                final 
SortedSet<StreamsRebalanceData.TaskId> standbyTasksToAssign,
+                                                final 
SortedSet<StreamsRebalanceData.TaskId> warmupTasksToAssign) {
+        log.info("Assigning " +
+                (activeTasksToAssign.isEmpty() ? "no active tasks, " : "active 
tasks {}, ") +

Review Comment:
   Ah, you are right! I will change this.



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