lianetm commented on code in PR #14690:
URL: https://github.com/apache/kafka/pull/14690#discussion_r1395108554


##########
clients/src/main/java/org/apache/kafka/clients/consumer/internals/MembershipManagerImpl.java:
##########
@@ -181,63 +348,560 @@ public void 
updateState(ConsumerGroupHeartbeatResponseData response) {
     public void transitionToFenced() {
         resetEpoch();
         transitionTo(MemberState.FENCED);
+
+        // Release assignment
+        CompletableFuture<Void> callbackResult = 
invokeOnPartitionsLostCallback(subscriptions.assignedPartitions());
+        callbackResult.whenComplete((result, error) -> {
+            if (error != null) {
+                log.debug("OnPartitionsLost callback invocation failed while 
releasing assignment" +
+                        "after member got fenced. Member will rejoin the group 
anyways.", error);
+            }
+            subscriptions.assignFromSubscribed(Collections.emptySet());
+            transitionToJoining();
+        });
+
+        clearPendingAssignmentsAndLocalNamesCache();
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
-    public void transitionToFailed() {
-        log.error("Member {} transitioned to {} state", memberId, 
MemberState.FAILED);
-        transitionTo(MemberState.FAILED);
+    public void transitionToFatal() {
+        log.error("Member {} transitioned to {} state", memberId, 
MemberState.FATAL);
+
+        // Update epoch to indicate that the member is not in the group 
anymore, so that the
+        // onPartitionsLost is called to release assignment.
+        memberEpoch = ConsumerGroupHeartbeatRequest.LEAVE_GROUP_MEMBER_EPOCH;
+        invokeOnPartitionsLostCallback(subscriptions.assignedPartitions());
+
+        clearPendingAssignmentsAndLocalNamesCache();
+        transitionTo(MemberState.FATAL);
     }
 
+    /**
+     * {@inheritDoc}
+     */
     @Override
-    public boolean shouldSendHeartbeat() {
-        return state() != MemberState.FAILED;
+    public void transitionToJoining() {
+        resetEpoch();
+        transitionTo(MemberState.JOINING);
+        clearPendingAssignmentsAndLocalNamesCache();
+        // Reset member ID of the reconciliation in progress (if any), to make 
sure that if the
+        // reconciliation completes while the member is rejoining but hasn't 
received the new
+        // member ID yet, the reconciliation result is discarded.
+        memberIdOnReconciliationStart = null;
     }
 
     /**
-     * Transition to {@link MemberState#STABLE} only if there are no target 
assignments left to
-     * reconcile. Transition to {@link MemberState#RECONCILING} otherwise.
+     * {@inheritDoc}
      */
-    private boolean maybeTransitionToStable() {
-        if (!hasPendingTargetAssignment()) {
-            transitionTo(MemberState.STABLE);
+    @Override
+    public CompletableFuture<Void> leaveGroup() {
+        transitionTo(MemberState.PREPARE_LEAVING);
+
+        CompletableFuture<Void> callbackResult = 
invokeOnPartitionsRevokedOrLostToReleaseAssignment();
+        callbackResult.whenComplete((result, error) -> {
+
+            // Clear the subscription, no matter if the callback execution 
failed or succeeded.
+            subscriptions.assignFromSubscribed(Collections.emptySet());
+
+            // Transition to ensure that a heartbeat request is sent out to 
effectively leave the
+            // group (even in the case where the member had no assignment to 
release or when the
+            // callback execution failed.)
+            transitionToSendingLeaveGroup();
+
+        });
+
+        clearPendingAssignmentsAndLocalNamesCache();
+
+        // Return callback future to indicate that the leave group is done 
when the callbacks
+        // complete, without waiting for the heartbeat to be sent out. (Best 
effort to send it
+        // but do not hold the leave group operation for it)
+        return callbackResult;
+    }
+
+    /**
+     * Release member assignment by calling the user defined callbacks for 
onPartitionsRevoked or
+     * onPartitionsLost.
+     * <ul>
+     *     <li>If the member is part of the group (epoch > 0), this will 
invoke onPartitionsRevoked.
+     *     This will be the case when releasing assignment because the member 
is intentionally
+     *     leaving the group (after a call to unsubscribe)</li>
+     *
+     *     <li>If the member is not part of the group (epoch <=0), this will 
invoke onPartitionsLost.
+     *     This will be the case when releasing assignment after being fenced 
.</li>
+     * </ul>
+     *
+     * @return Future that will complete when the callback execution completes.
+     */
+    private CompletableFuture<Void> 
invokeOnPartitionsRevokedOrLostToReleaseAssignment() {
+        SortedSet<TopicPartition> droppedPartitions = new 
TreeSet<>(COMPARATOR);
+        droppedPartitions.addAll(subscriptions.assignedPartitions());
+
+        CompletableFuture<Void> callbackResult;
+        if (droppedPartitions.isEmpty()) {
+            // No assignment to release
+            callbackResult = CompletableFuture.completedFuture(null);
         } else {
-            transitionTo(MemberState.RECONCILING);
+            // Release assignment
+            if (memberEpoch > 0) {
+                // Member is part of the group. Invoke onPartitionsRevoked.
+                callbackResult = revokePartitions(droppedPartitions);
+            } else {
+                // Member is not part of the group anymore. Invoke 
onPartitionsLost.
+                callbackResult = 
invokeOnPartitionsLostCallback(droppedPartitions);
+            }
+            // Remove all topic IDs and names from local cache
+            callbackResult.whenComplete((result, error) -> 
clearPendingAssignmentsAndLocalNamesCache());
+        }
+        return callbackResult;
+    }
+
+    /**
+     * Reset member epoch to the value required for the leave the group 
heartbeat request, and
+     * transition to the {@link MemberState#LEAVING} state so that a heartbeat
+     * request is sent out with it.
+     */
+    private void transitionToSendingLeaveGroup() {
+        memberEpoch = ConsumerGroupHeartbeatRequest.LEAVE_GROUP_MEMBER_EPOCH;
+        currentAssignment = new HashSet<>();
+        transitionTo(MemberState.LEAVING);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public boolean shouldHeartbeatNow() {
+        MemberState state = state();
+        return state == MemberState.ACKNOWLEDGING || state == 
MemberState.LEAVING;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void onHeartbeatRequestSent() {
+        MemberState state = state();
+        if (state == MemberState.ACKNOWLEDGING) {
+            if (allPendingAssignmentsReconciled()) {
+                transitionTo(MemberState.STABLE);
+            } else {
+                transitionTo(MemberState.RECONCILING);
+            }
+        } else if (state == MemberState.LEAVING) {
+            transitionTo(MemberState.UNSUBSCRIBED);
         }
-        return state.equals(MemberState.STABLE);
     }
 
     /**
-     * Take new target assignment received from the server and set it as 
targetAssignment to be
-     * processed. Following the consumer group protocol, the server won't send 
a new target
-     * member while a previous one hasn't been acknowledged by the member, so 
this will fail
-     * if a target assignment already exists.
+     * @return True if there are no assignments waiting to be resolved from 
metadata or reconciled.
+     */
+    private boolean allPendingAssignmentsReconciled() {
+        return assignmentUnresolved.isEmpty() && 
assignmentReadyToReconcile.isEmpty();
+    }
+
+    @Override
+    public boolean shouldSkipHeartbeat() {
+        MemberState state = state();
+        return state == MemberState.UNSUBSCRIBED || state == MemberState.FATAL;
+    }
+
+    /**
+     * Reconcile the assignment that has been received from the server and for 
which topic names
+     * are resolved, kept in the {@link #assignmentReadyToReconcile}. This 
will commit if needed,
+     * trigger the callbacks and update the subscription state. Note that only 
one reconciliation
+     * can be in progress at a time. If there is already another one in 
progress when this is
+     * triggered, it will be no-op, and the assignment will be reconciled on 
the next
+     * reconciliation loop.
+     */
+    boolean reconcile() {
+        // Make copy of the assignment to reconcile as it could change as new 
assignments or metadata updates are received
+        SortedSet<TopicPartition> assignedPartitions = new 
TreeSet<>(COMPARATOR);
+        assignedPartitions.addAll(assignmentReadyToReconcile);
+
+        SortedSet<TopicPartition> ownedPartitions = new TreeSet<>(COMPARATOR);
+        ownedPartitions.addAll(subscriptions.assignedPartitions());
+
+        boolean sameAssignmentReceived = 
assignedPartitions.equals(ownedPartitions);
+
+        if (reconciliationInProgress || sameAssignmentReceived) {
+            String reason;
+            if (reconciliationInProgress) {
+                reason = "Another reconciliation is already in progress. 
Assignment " +
+                        assignmentReadyToReconcile + " will be handled in the 
next reconciliation loop.";
+            } else {
+                reason = "Target assignment ready to reconcile is equals to 
the member current assignment.";
+            }
+            log.debug("Ignoring reconciliation attempt. " + reason);
+            return false;
+        }
+
+        markReconciliationInProgress();
+
+
+        // Partitions to assign (not previously owned)
+        SortedSet<TopicPartition> addedPartitions = new TreeSet<>(COMPARATOR);
+        addedPartitions.addAll(assignedPartitions);
+        addedPartitions.removeAll(ownedPartitions);
+
+        // Partitions to revoke
+        SortedSet<TopicPartition> revokedPartitions = new 
TreeSet<>(COMPARATOR);
+        revokedPartitions.addAll(ownedPartitions);
+        revokedPartitions.removeAll(assignedPartitions);
+
+        log.info("Updating assignment with\n" +
+                        "\tAssigned partitions:                       {}\n" +
+                        "\tCurrent owned partitions:                  {}\n" +
+                        "\tAdded partitions (assigned - owned):       {}\n" +
+                        "\tRevoked partitions (owned - assigned):     {}\n",
+                assignedPartitions,
+                ownedPartitions,
+                addedPartitions,
+                revokedPartitions
+        );
+
+        CompletableFuture<Void> revocationResult;
+        if (!revokedPartitions.isEmpty()) {
+            revocationResult = revokePartitions(revokedPartitions);

Review Comment:
   Agree, we should bundle them together when triggering. It will require 
re-thinking those events a bit to make sure we still enforce the execution 
order. Filed [KAFKA-15835](https://issues.apache.org/jira/browse/KAFKA-15835) 
and I will address that as a follow-up right after this (including also the 
commit before revocation)



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