KAFKA-10199: Consider tasks in state updater when computing offset sums #13925

cadonna · 2023-06-28T14:19:20Z

With the state updater, the task manager needs also to look into the tasks owned by the state updater when computing the sum of offsets of the state. This sum of offsets is used by the high availability assignor to assign warm-up replicas.
If the task manager does not take into account tasks in the state updater, a warm-up replica will never report back that the state for the corresponding task has caught up. Consequently, the warm-up replica will never be dismissed and probing rebalances will never end..

Committer Checklist (excluded from commit message)

Verify design and implementation
Verify test coverage and CI build status
Verify documentation (including upgrade notes)

With the state updater, the task manager needs also to look into the tasks owned by the state updater when computing the sum of offsets of the state. This sum of offsets is used by the high availability assignor to assign warm-up replicas. If the task manager does not take into account tasks in the state updater, a warm-up replica will never report back that the state for the corresponding task has caught up. Consequently, the warm-up replica will never be dismissed and probing rebalances will never end..

lucasbru

lgtm, just one minor change to consider

lucasbru · 2023-06-28T15:11:37Z

streams/src/main/java/org/apache/kafka/streams/processor/internals/TaskManager.java

-                    }
-                } catch (final IOException e) {
-                    log.warn(String.format("Exception caught while trying to read checkpoint for task %s:", id), e);
+                createdAndClosedTasks.add(task.id());


nit: if you want to do it with fewer collections, you could inititialize lockedTaskDirectoriesOfNonOwnedTasks earlier, and just remove directly from that set in the if branch, instead of adding to createdAndClosedTasks in the else branch.

lucasbru · 2023-06-28T15:45:30Z

streams/src/main/java/org/apache/kafka/streams/processor/internals/TaskManager.java

-            // Closed and uninitialized tasks don't have any offsets so we should read directly from the checkpoint
-            if (task != null && task.state() != State.CREATED && task.state() != State.CLOSED) {
+        final Map<TaskId, Task> tasks = allTasks();
+        final Set<TaskId> lockedTaskDirectoriesOfNonOwnedTasksAndClosedAndCreatedTasks =


Ah, I recommended this change thinking that lockedTaskDirectories always includes all ClosedAndCreatedTasks -- I think it does right? So it should be enough to assign this to lockedTaskDirectories.

I do not think there is guarantee that lockedTaskDirectories contains any tasks the client owns. lockedTaskDirectories are just the non-empty task directories in the state directory when a rebalance starts. However, a task directory is created when a task is created, i.e., it is in state CREATE. A task directory is not deleted when a task is closed, i.e., in state CLOSED. This might be a correlation and not a thought-out invariant. At least, the original code did not rely on this since it used union(HashSet::new, lockedTaskDirectories, tasks.allTaskIds()).
I am also somehow reluctant to rely on such -- IMO -- brittle invariant.
As an example, in future we could decide to move the creation of the task directory to other parts of the code -- like when the task is initialized -- which would mean that there is a interval in which the task is in state CREATED but does not have a task directory.

Well, if there is no task directory, there is no checkpoint to process. So it's safe to not do anything in this case.

All you'd do by adding more tasks is to later skip on the check checkPointFile.exists().

What could make the simplified code break, is that we decide to not release the lock before transitioning to the CLOSED state.

So yeah, being defensive here and going through all CREATED and CLOSED tasks as well to make sure that they do not have state directories that are locked but not inside lockedTaskDirectories sound good to me as well.

OK, I agree with you!

Let's be defensive then!

cadonna · 2023-06-29T08:56:06Z

streams/src/main/java/org/apache/kafka/streams/processor/internals/TaskManager.java

        for (final TaskDirectory taskDir : stateDirectory.listNonEmptyTaskDirectories()) {
            final File dir = taskDir.file();
            final String namedTopology = taskDir.namedTopology();
            try {
                final TaskId id = parseTaskDirectoryName(dir.getName(), namedTopology);
                if (stateDirectory.lock(id)) {
                    lockedTaskDirectories.add(id);
-                    if (!tasks.contains(id)) {
+                    if (!allTasks.containsKey(id)) {


For this debug log, we did only consider tasks owned by the stream thread.

mjsax · 2023-06-30T00:12:00Z

streams/src/main/java/org/apache/kafka/streams/processor/internals/TaskManager.java

@@ -1141,25 +1141,30 @@ public Map<TaskId, Long> getTaskOffsetSums() {
        // Not all tasks will create directories, and there may be directories for tasks we don't currently own,
        // so we consider all tasks that are either owned or on disk. This includes stateless tasks, which should
        // just have an empty changelogOffsets map.
-        for (final TaskId id : union(HashSet::new, lockedTaskDirectories, tasks.allTaskIds())) {


It seems with the state updated enabled, tasks is actually only containing "running tasks". It seems appropriate the rename this variable to runningTasks (can also happen in a follow up PR).

I am actually also wondering if we still need this Tasks container any longer to begin with? The purpose of the Tasks container was to simplify TaskManager that manages both active and standby tasks. With the state updated (from my understanding) the TaskManager only manages active tasks, while standby tasks will be owned by the state-updated-thread (would it still be useful for the state-updated-thread to use Tasks container, given that is also own active tasks as long as they are restoring?

It seems with the state updated enabled, tasks is actually only containing "running tasks". It seems appropriate the rename this variable to runningTasks (can also happen in a follow up PR).

The old code path with disabled state updater does still exist and we can disable the state updater if we encounter a major bug after releasing. So, I would postpone such renamings to the removal of the old code path.

I am actually also wondering if we still need this Tasks container any longer to begin with?

I would keep it, because it allows to cleanly set a specific state of the task manager in unit tests. Anyways, I would wait for the upcoming thread refactoring to make such changes.

would it still be useful for the state-updated-thread to use Tasks container, given that is also own active tasks as long as they are restoring?

I do not think so, since access by the state updater would imply that the tasks registry (aka tasks container) needs to be concurrently accessed. For this reason, we defined a invariant, that a task can only be owned either by the stream thread or by the state updater, but not both. Sharing the tasks registry between stream thread and state updater would break that invariant. If you meant to use an separate instance of the tasks registry for the state updater, that would be not useful IMO.

wcarlson5

LGTM

cadonna · 2023-07-03T14:32:17Z

Build failures are unrelated:

Build / JDK 17 and Scala 2.13 / org.apache.kafka.controller.QuorumControllerTest.testBalancePartitionLeaders()
Build / JDK 8 and Scala 2.12 / org.apache.kafka.controller.QuorumControllerTest.testBalancePartitionLeaders()
Build / JDK 11 and Scala 2.13 / org.apache.kafka.connect.mirror.integration.MirrorConnectorsIntegrationExactlyOnceTest.testOffsetTranslationBehindReplicationFlow()
Build / JDK 20 and Scala 2.13 / org.apache.kafka.connect.mirror.integration.IdentityReplicationIntegrationTest.testReplicateTargetDefault()
Build / JDK 20 and Scala 2.13 / org.apache.kafka.connect.mirror.integration.MirrorConnectorsIntegrationBaseTest.testReplicateSourceDefault()
Build / JDK 20 and Scala 2.13 / org.apache.kafka.connect.mirror.integration.MirrorConnectorsIntegrationBaseTest.testReplicateSourceDefault()
Build / JDK 20 and Scala 2.13 / org.apache.kafka.connect.mirror.integration.MirrorConnectorsIntegrationExactlyOnceTest.testOffsetTranslationBehindReplicationFlow()
Build / JDK 20 and Scala 2.13 / org.apache.kafka.connect.integration.BlockingConnectorTest.testBlockInConnectorValidate
Build / JDK 20 and Scala 2.13 / org.apache.kafka.connect.integration.ConnectWorkerIntegrationTest.testAddAndRemoveWorker
Build / JDK 20 and Scala 2.13 / integration.kafka.server.FetchFromFollowerIntegrationTest.testRackAwareRangeAssignor()
Build / JDK 20 and Scala 2.13 / kafka.admin.TopicCommandIntegrationTest.testDescribeUnderMinIsrPartitionsMixed(String).quorum=kraft
Build / JDK 20 and Scala 2.13 / kafka.api.ConsumerBounceTest.testCloseDuringRebalance()

cadonna requested a review from lucasbru June 28, 2023 14:19

lucasbru approved these changes Jun 28, 2023

View reviewed changes

Save on collections

c4bbd2c

lucasbru reviewed Jun 28, 2023

View reviewed changes

mjsax added the streams label Jun 28, 2023

Improve code and tests

cd76885

cadonna commented Jun 29, 2023

View reviewed changes

Be defensive

bb70eab

lucasbru approved these changes Jun 29, 2023

View reviewed changes

cadonna mentioned this pull request Jun 29, 2023

KAFKA-10199: Enable state updater by default #13927

Merged

3 tasks

mjsax reviewed Jun 30, 2023

View reviewed changes

mjsax approved these changes Jun 30, 2023

View reviewed changes

wcarlson5 approved these changes Jun 30, 2023

View reviewed changes

cadonna merged commit 5c2492b into apache:trunk Jul 3, 2023
1 check failed

cadonna deleted the fix_stateupdater_bug_offsetsum branch July 5, 2023 07:25

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

KAFKA-10199: Consider tasks in state updater when computing offset sums #13925

KAFKA-10199: Consider tasks in state updater when computing offset sums #13925

cadonna commented Jun 28, 2023

lucasbru left a comment

lucasbru Jun 28, 2023

lucasbru Jun 28, 2023

cadonna Jun 28, 2023 •

edited

lucasbru Jun 28, 2023 •

edited

lucasbru Jun 29, 2023

cadonna Jun 29, 2023

cadonna Jun 29, 2023

cadonna Jun 29, 2023

mjsax Jun 30, 2023

cadonna Jul 3, 2023

wcarlson5 left a comment

cadonna commented Jul 3, 2023

KAFKA-10199: Consider tasks in state updater when computing offset sums #13925

KAFKA-10199: Consider tasks in state updater when computing offset sums #13925

Conversation

cadonna commented Jun 28, 2023

Committer Checklist (excluded from commit message)

lucasbru left a comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

cadonna Jun 28, 2023 • edited

Choose a reason for hiding this comment

lucasbru Jun 28, 2023 • edited

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

Choose a reason for hiding this comment

wcarlson5 left a comment

Choose a reason for hiding this comment

cadonna commented Jul 3, 2023

cadonna Jun 28, 2023 •

edited

lucasbru Jun 28, 2023 •

edited