Hi, David,
Thanks for the reply. Just one more comment.
80. Since PartitionAssignor.AssignmentMemberSpec uses topic name for both
subscribedTopics and targetPartitions, in
ConsumerGroupPrepareAssignmentResponse, should Members.SubscribedTopicIds
and Members.TopicPartitions.TopicId be topic names too? Similarly, since
PartitionAssignor.MemberAssignment uses topic name, should
ConsumerGroupInstallAssignmentRequest.Members.Partitions.TopicId be topic
name too?
Thanks,
Jun
On Mon, Oct 17, 2022 at 2:35 AM David Jacot <dja...@confluent.io.invalid>
wrote:
> Hi Jun,
>
> Thanks for your comments. Please find my answers below.
>
> 60. Sure. Let me use a concrete example to illustrate it. Let's assume
> that KStreams has a member A with a task reading from foo-0 and a
> member B with a standby task reading from foo-0. As you know, the
> standby task information for B is encoded in the assignment metadata
> whereas the assigned partition for A is in the assignment. Now, let's
> imagine that the assignor decides to promote the standby task to
> become the active task for foo-0. The assignor will create a new
> assignment for the group and will encode the standby task in A's
> metadata and assign foo-0 to B. A will be requested to revoke foo-0
> and, while it does so, B will get its new metadata but without foo-0
> because foo-0 is not revoked yet. From the point of view of B, it will
> see that the standby task is no longer there. Without providing the
> full set of assigned partitions, it would not know what to do here. If
> foo-0 is targeted to be assigned to B, B should wait until it is
> before stopping the standby task. If foo-0 is not, it can stop the
> standby immediately. Long story short, KStreams needs to get the full
> assignment + metadata in order to reason about the correct end state.
>
> How do we provide this? We have added the pending partitions in the
> ConsumerGroupHeartbeatResponse in conjunction to the assigned
> partitions. With this, the Consumer knows the full set of partitions.
> PartitionAssignor#onAssign will be called when the member transitions
> to a new epoch with the target partitions and the metadata for the
> member. Then, RebalanceListener#onAssignedPartitions is called when
> partitions are incrementally assigned. At most, it will be called N
> times where N is the number of assigned partitions in the current
> epoch.
>
> For context, this was discussed with Guohzang in this thread.
>
> I hope that this clarifies the intent.
>
> 62. Got it. I have reworked that part as well. Unfortunately, I think
> that we should keep using a Set here because it is already there.
> Deprecating the current one to change the type is not worth it.
>
> 63. Fixed as well.
>
> 68.2 Right. The error is only used by the client side assignor. The
> client assignor will report a non-zero error code while installing a
> new assignment if it was not able to compute a new assignment for the
> group. In this case, the assignment is not changed but the error is
> reported to all the members. KStreams will have a few of such errors.
> They are listed in the KIP.
>
> When a new assignment is installed with the
> ConsumerGroupInstallAssignment API, the coordinator validates it and
> rejects the installation directly if it is not valid. In this case,
> only the member trying to install the assignment gets an error. The
> other members are not aware of it as they keep their current assignor.
>
> 71.1 That makes sense. Using SubscribedTopicNames is indeed more
> intuitive. I have changed the HB request to use it in order to be
> consistent. I have also added the topic names in the
> ConsumerGroupDescribe response.
>
> Best,
> David
>
> On Fri, Oct 14, 2022 at 8:05 PM Jun Rao <j...@confluent.io.invalid> wrote:
> >
> > Hi, David,
> >
> > Thanks for the reply.
> >
> > 60. Hmm, could you explain why KStreams needs the full set of partition
> > assignments? I am also not sure how this will be implemented based on the
> > protocol. Since HeartBeatResponse sends the assigned partitions in phases
> > (those that don't need to wait for revocation from other members first,
> > followed by the full assignment list), how does a member know which
> > response has the full assignment?
> >
> > 62. I was referring to Admin#describeConsumerGroups. It seems that
> > ownedPartitions is still of type List<TopicIdPartition>. Also, the
> > existing topicPartitions() returns Set<TopicPartition>, not a collection.
> >
> > 63. This is also in Admin#describeConsumerGroups. The comment seems
> > inconsistent with the field name.
> > /**
> > * The reason reported by the assignor.
> > */
> > byte error;
> >
> > 67. Thanks for the explanation. Make sense. The existing name may be ok.
> >
> > 68.2 Are you saying the error is only intended for the client assignor?
> But
> > the coordinator generates the error based on the server side validation,
> > right? Should we provide some info to tell the client why the validation
> > fails?
> >
> > 71.1 Hmm, for SubscribedTopicIds, should we use topic name in the
> > subscription part? That seems more intuitive---a subscription shouldn't
> > change just because a topic is recreated. For the assigned partitions,
> > perhaps we could include both topicId and name just like
> FetchOffsetRequest.
> >
> > Thanks,
> >
> > Jun
> >
> > On Fri, Oct 14, 2022 at 2:49 AM Luke Chen <show...@gmail.com> wrote:
> >
> > > Thanks for the update.
> > > Yes, I think using similar way as KIP-868 to fix this issue makes
> sense.
> > > Let's consider it in the future.
> > >
> > > Luke
> > >
> > > On Fri, Oct 14, 2022 at 5:16 PM David Jacot
> <dja...@confluent.io.invalid>
> > > wrote:
> > >
> > > > Hi Luke,
> > > >
> > > > Thanks for your questions.
> > > >
> > > > > 1. We will store the "targetAssignment" into log now. But as we
> know,
> > > > there's max batch size limit (default 1MB), which means, we cannot
> > > support
> > > > 1M partitions in one group (actually, it should be less than 60k
> > > partitions
> > > > since we'll store {topicID+partition id}) by default now. How will we
> > > > handle that? Do we expect users to adjust the max batch size to
> support
> > > > large partitions in groups, which we don't need this change for old
> > > > protocol?
> > > >
> > > > That's right. I have a few ideas to remove this limitation in the
> > > > future but I decided to keep them for future improvement. The KIP is
> > > > large enough and as the current protocol suffers from the exact same
> > > > limitation, it is not a regression.
> > > >
> > > > For the future, my thinking is to split the assignment and to only
> > > > write deltas to the log instead of re-writing all of it. We would
> need
> > > > to use transactions for this in the coordinator (similarly to
> > > > KIP-868). The challenge is that we have to ensure that those deltas
> > > > are all written or completely roll backed. Otherwise, we would have a
> > > > weird state with the compaction. This needs obviously more thinking.
> > > >
> > > > > I'm wondering why we should persist the "targetAssignment" data?
> If we
> > > > want
> > > > to work for coordinator failover, could the new coordinator try to
> > > request
> > > > for currently owned partitions from each consumer when failed over?
> I'm
> > > not
> > > > sure if the consumer will auto send owned partitions to the new
> > > > coordinator. If not, maybe we can return an error to client
> > > > ConsumerGroupHeartbeat API with REQUIRE_OWNED_PARTITION error, and
> ask
> > > > client to append the currently owned partitions to new coordinates
> for
> > > new
> > > > assignment computation. Does that make sense?
> > > >
> > > > The entire reconciliation process depends on it so if we lose it
> > > > during a failover, members could be in a weird state. For instance,
> > > > they could be in the middle of a transition from their current
> > > > assignment to their new target and thus would be blocked. Relying on
> > > > members to reconstruct it back does not really work because they
> don't
> > > > have all the information to do so (e.g. new metadata) so we would
> have
> > > > to recompute a new one. This implies that we need to get the owned
> > > > partitions from all members and that would take a few seconds until
> > > > all members come back in the best case, up to the session timeout in
> > > > the worst case. Imagine that a member joins or fails during this
> time,
> > > > the whole process would be stuck. I am afraid storing it is the best
> > > > way here.
> > > >
> > > > Best,
> > > > David
> > > >
> > > >
> > > > On Fri, Oct 14, 2022 at 5:11 AM Luke Chen <show...@gmail.com> wrote:
> > > > >
> > > > > Hi David,
> > > > >
> > > > > A few more questions:
> > > > > 1. We will store the "targetAssignment" into log now. But as we
> know,
> > > > > there's max batch size limit (default 1MB), which means, we cannot
> > > > support
> > > > > 1M partitions in one group (actually, it should be less than 60k
> > > > partitions
> > > > > since we'll store {topicID+partition id}) by default now. How will
> we
> > > > > handle that? Do we expect users to adjust the max batch size to
> support
> > > > > large partitions in groups, which we don't need this change for old
> > > > > protocol?
> > > > >
> > > > > I'm wondering why we should persist the "targetAssignment" data?
> If we
> > > > want
> > > > > to work for coordinator failover, could the new coordinator try to
> > > > request
> > > > > for currently owned partitions from each consumer when failed
> over? I'm
> > > > not
> > > > > sure if the consumer will auto send owned partitions to the new
> > > > > coordinator. If not, maybe we can return an error to client
> > > > > ConsumerGroupHeartbeat API with REQUIRE_OWNED_PARTITION error, and
> ask
> > > > > client to append the currently owned partitions to new coordinates
> for
> > > > new
> > > > > assignment computation. Does that make sense?
> > > > >
> > > > > Luke
> > > > >
> > > > > On Fri, Oct 14, 2022 at 12:22 AM Jun Rao <j...@confluent.io.invalid
> >
> > > > wrote:
> > > > >
> > > > > > Hi, David,
> > > > > >
> > > > > > Thanks for the reply and the updated KIP. A few more comments on
> the
> > > > > > interfaces and the protocols.
> > > > > >
> > > > > > 60. On the consumer side, do we need both
> > > > PartitionAssignor.onAssignment
> > > > > > and ConsumerRebalanceListener.onPartitionsAssigned? My
> understanding
> > > is
> > > > > > that the former was added for cooperative rebalance, which is now
> > > > handled
> > > > > > by the coordinator. If we do need both, should we make them more
> > > > consistent
> > > > > > (e.g. topic name vs topic id, list vs set vs collection)?
> > > > > >
> > > > > > 61. group.local.assignors: Could we make it clear that it's the
> full
> > > > class
> > > > > > name that implements PartitionAssignor?
> > > > > >
> > > > > > 62. MemberAssignment: It currently has the following method.
> > > > > > public Set<TopicPartition> topicPartitions()
> > > > > > We are adding List<TopicIdPartition> ownedPartitions. Should we
> keep
> > > > the
> > > > > > naming and the return type consistent?
> > > > > >
> > > > > > 63. MemberAssignment.error: should that be reason?
> > > > > >
> > > > > > 64. group.remote.assignor: The client may not know what
> assignors the
> > > > > > broker supports. Should we default this to what the broker
> determines
> > > > (e.g.
> > > > > > first assignor listed in group.consumer.assignors)?
> > > > > >
> > > > > > 65. After the text "When A heartbeats again and acknowledges the
> > > > > > revocation, the group coordinator transitions him to epoch 2 and
> > > > releases
> > > > > > foo-2.", we have the following.
> > > > > > B - epoch=2, partitions=[foo-2], pending-partitions=[]
> > > > > > Should foo-2 be in pending-partitions?
> > > > > >
> > > > > > 66. In the Online Migration example, is the first occurence of
> "C -
> > > > > > epoch=23, partitions=[foo-2, foo-5, foo-4],
> pending-partitions=[]"
> > > > correct?
> > > > > > It seems that should happen after C receives a SyncGroup
> response? If
> > > > so,
> > > > > > subsequent examples have the same issue.
> > > > > >
> > > > > > 67. ConsumerGroupHeartbeatRequest.RebalanceTimeoutMs : Which
> config
> > > > > > controls this? How is this used by the group coordinator since
> there
> > > > is no
> > > > > > sync barrier anymore?
> > > > > >
> > > > > > 68. ConsumerGroupHeartbeatResponse:
> > > > > > 68.1 AssignedTopicPartitions and PendingTopicPartitions are of
> type
> > > > > > []TopicPartition. Should they be TopicPartitions?
> > > > > > 68.2 Assignment.error. Should we also have an errorMessage field?
> > > > > >
> > > > > > 69. ConsumerGroupPrepareAssignmentResponse.Members.Assignor:
> Should
> > > it
> > > > > > include the selected assignor name?
> > > > > >
> > > > > > 70. ConsumerGroupInstallAssignmentRequest.GroupEpoch: Should we
> let
> > > the
> > > > > > client set this? Intuitively, it seems the coordinator should
> manage
> > > > the
> > > > > > group epoch.
> > > > > >
> > > > > > 71. ConsumerGroupDescribeResponse:
> > > > > > 71.1 Members.Assignment.Partitions. Should we include the topic
> name
> > > > too
> > > > > > since it's convenient for building tools? Ditto for
> TargetAssignment.
> > > > > > 71.2 Members: Should we include SubscribedTopicRegex too?
> > > > > >
> > > > > > 72. OffsetFetchRequest: Is GenerationIdOrMemberEpoch needed since
> > > > tools may
> > > > > > also want to issue this request?
> > > > > >
> > > > > > Thanks,
> > > > > >
> > > > > > Jun
> > > > > >
> > > >
> > >
>
