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From Jason Gustafson <ja...@confluent.io>
Subject Re: [DISCUSS] KIP-232: Detect outdated metadata by adding ControllerMetadataEpoch field
Date Tue, 19 Dec 2017 17:06:23 GMT
Hey Dong,


> I think it is a good idea to let coordinator do the additional sanity check
> to ensure the leader epoch from OffsetCommitRequest never decreases. This
> can help us detect bug. The next question will be what should we do if
> OffsetCommitRequest provides a smaller leader epoch. One possible solution
> is to return a non-retriable error to consumer which will then be thrown to
> user application. But I am not sure it is worth doing it given its impact
> on the user. Maybe it will be safer to simply have an error message in the
> server log and allow offset commit to succeed. What do you think?


I think the check would only have value if you return an error when it
fails. It seems primarily useful to detect buggy consumer logic, so a
non-retriable error makes sense to me. Clients which don't implement this
capability can use the sentinel value and keep the current behavior.

It seems that FetchResponse includes leader epoch via the path
> FetchResponse -> MemoryRecords -> MutableRecordBatch -> DefaultRecordBatch
> -> partitionLeaderEpoch. Could this be an existing case where we expose the
> leader epoch to clients?


Right, in this case the client has no direct dependence on the field, but
it could still be argued that it is exposed (I had actually considered
stuffing this field into an opaque blob of bytes in the message format
which the client wasn't allowed to touch, but it didn't happen in the end).
I'm not opposed to using the leader epoch field here, I was just mentioning
that it does tie clients a bit tighter to something which could be
considered a Kafka internal implementation detail. It makes the protocol a
bit less intuitive as well since it is rather difficult to explain the edge
case it is protecting. That said, we've hit other scenarios where being
able to detect stale metadata in the client would be helpful, so I think it
might be worth the tradeoff.

-Jason

On Mon, Dec 18, 2017 at 6:09 PM, Dong Lin <lindong28@gmail.com> wrote:

> Hey Jason,
>
> Thanks much for reviewing the KIP.
>
> I think it is a good idea to let coordinator do the additional sanity check
> to ensure the leader epoch from OffsetCommitRequest never decreases. This
> can help us detect bug. The next question will be what should we do if
> OffsetCommitRequest provides a smaller leader epoch. One possible solution
> is to return a non-retriable error to consumer which will then be thrown to
> user application. But I am not sure it is worth doing it given its impact
> on the user. Maybe it will be safer to simply have an error message in the
> server log and allow offset commit to succeed. What do you think?
>
> It seems that FetchResponse includes leader epoch via the path
> FetchResponse -> MemoryRecords -> MutableRecordBatch -> DefaultRecordBatch
> -> partitionLeaderEpoch. Could this be an existing case where we expose the
> leader epoch to clients?
>
> Thanks,
> Dong
>
>
>
> On Mon, Dec 18, 2017 at 3:27 PM, Jason Gustafson <jason@confluent.io>
> wrote:
>
> > Hi Dong,
> >
> > Thanks for the KIP. Good job identifying the problem. One minor question
> I
> > had is whether the coordinator should enforce that the leader epoch
> > associated with an offset commit can only go forward for each partition?
> > Currently it looks like we just depend on the client for this, but since
> > we're caching the leader epoch anyway, it seems like a cheap safety
> > condition. To support old clients, you can always allow the commit if the
> > leader epoch is unknown.
> >
> > I agree that we shouldn't expose the leader epoch in OffsetAndMetadata in
> > the consumer API for what it's worth. As you have noted, it is more of an
> > implementation detail. By the same argument, it's also a bit unfortunate
> > that we have to expose it in the request API since that is nearly as
> > binding in terms of how it limits future iterations. I could be wrong,
> but
> > this appears to be the first case where clients will depend on the
> concept
> > of leader epoch. Might not be a big deal considering how deeply embedded
> > leader epochs already are in the inter-broker RPCs and the message format
> > itself, but just wanted to mention the fact that good encapsulation
> applies
> > to the client request API as well.
> >
> > Thanks,
> > Jason
> >
> > On Mon, Dec 18, 2017 at 1:58 PM, Dong Lin <lindong28@gmail.com> wrote:
> >
> > > Hey Jun,
> > >
> > > Thanks much for your comments. These are very thoughtful ideas. Please
> > see
> > > my comments below.
> > >
> > > On Thu, Dec 14, 2017 at 6:38 PM, Jun Rao <jun@confluent.io> wrote:
> > >
> > > > Hi, Dong,
> > > >
> > > > Thanks for the update. A few more comments below.
> > > >
> > > > 10. It seems that we need to return the leader epoch in the fetch
> > > response
> > > > as well When fetching data, we could be fetching data from a leader
> > epoch
> > > > older than what's returned in the metadata response. So, we want to
> use
> > > the
> > > > leader epoch associated with the offset being fetched for committing
> > > > offsets.
> > > >
> > >
> > > It seems that we may have two separate issues here. The first issue is
> > that
> > > consumer uses metadata that is older than the one it uses before. The
> > > second issue is that consumer uses metadata which is newer than the
> > > corresponding leader epoch in the leader broker. We know that the
> > > OffsetOutOfRangeException described in this KIP can be prevented by
> > > avoiding the first issue. On the other hand, it seems that the
> > > OffsetOffsetOutOfRangeException can still happen even if we avoid the
> > > second issue -- if consumer uses an older version of metadata, the
> leader
> > > epoch in its metadata may equal the leader epoch in the broker even if
> > the
> > > leader epoch in the broker is oudated.
> > >
> > > Given this understanding, I am not sure why we need to return the
> leader
> > > epoch in the fetch response. As long as consumer's metadata is not
> going
> > > back in version, I think we are good. Did I miss something here?
> > >
> > >
> > > >
> > > > 11. Should we now extend OffsetAndMetadata used in the offset commit
> > api
> > > in
> > > > KafkaConsumer to include leader epoch? Similarly, should we return
> > leader
> > > > epoch in endOffsets(), beginningOffsets() and position()? We probably
> > > need
> > > > to think about how to make the api backward compatible.
> > > >
> > >
> > > After thinking through this carefully, I think we probably don't want
> to
> > > extend OffsetAndMetadata to include leader epoch because leader epoch
> is
> > > kind of implementation detail which ideally should be hidden from user.
> > The
> > > consumer can include leader epoch in the OffsetCommitRequest after
> taking
> > > offset from commitSync(final Map<TopicPartition, OffsetAndMetadata>
> > > offsets). Similarly consumer can store leader epoch from
> > > OffsetFetchResponse and only provide offset to user via
> > > consumer.committed(topicPartition). This solution seems to work well
> and
> > > we
> > > don't have to make changes to consumer's public API. Does this sound
> OK?
> > >
> > >
> > > >
> > > > 12. It seems that we now need to store leader epoch in the offset
> > topic.
> > > > Could you include the new schema for the value of the offset topic
> and
> > > add
> > > > upgrade notes?
> > >
> > >
> > > You are right. I have updated the KIP to specify the new schema for the
> > > value of the offset topic. Can you take another look?
> > >
> > > For existing messages in the offset topic, leader_epoch will be
> missing.
> > We
> > > will use leader_epoch = -1 to indicate the missing leader_epoch. Then
> the
> > > consumer behavior will be the same as it is now because any
> leader_epoch
> > in
> > > the MetadataResponse will be larger than the leader_epoch = -1 in the
> > > OffetFetchResponse. Thus we don't need specific procedure for upgrades
> > due
> > > to this change in the offset topic schema. By "upgrade nodes", do you
> > mean
> > > the sentences we need to include in the upgrade.html in the PR later?
> > >
> > >
> > > >
> > > > Jun
> > > >
> > > >
> > > > On Tue, Dec 12, 2017 at 5:19 PM, Dong Lin <lindong28@gmail.com>
> wrote:
> > > >
> > > > > Hey Jun,
> > > > >
> > > > > I see. Sounds good. Yeah it is probably simpler to leave this to
> > > another
> > > > > KIP in the future.
> > > > >
> > > > > Thanks for all the comments. Since there is no further comment in
> the
> > > > > community, I will open the voting thread.
> > > > >
> > > > > Thanks,
> > > > > Dong
> > > > >
> > > > > On Mon, Dec 11, 2017 at 5:37 PM, Jun Rao <jun@confluent.io> wrote:
> > > > >
> > > > > > Hi, Dong,
> > > > > >
> > > > > > The case that I am thinking is network partitioning. Suppose one
> > > > deploys
> > > > > a
> > > > > > stretched cluster across multiple AZs in the same region. If the
> > > > machines
> > > > > > in one AZ can't communicate to brokers in other AZs due to a
> > network
> > > > > issue,
> > > > > > the brokers in that AZ won't get any new metadata.
> > > > > >
> > > > > > We can potentially solve this problem by requiring some kind of
> > > regular
> > > > > > heartbeats between the controller and the broker. This may need
> > some
> > > > more
> > > > > > thoughts. So, it's probably fine to leave this to another KIP in
> > the
> > > > > > future.
> > > > > >
> > > > > > Thanks,
> > > > > >
> > > > > > Jun
> > > > > >
> > > > > > On Mon, Dec 11, 2017 at 2:55 PM, Dong Lin <lindong28@gmail.com>
> > > wrote:
> > > > > >
> > > > > > > Hey Jun,
> > > > > > >
> > > > > > > Thanks for the comment. I am open to improve this KIP to
> address
> > > more
> > > > > > > problems. I probably need more help in understanding what is
> the
> > > > > current
> > > > > > > problem with consumer using outdated metadata and whether it is
> > > > easier
> > > > > to
> > > > > > > address it together with this KIP.
> > > > > > >
> > > > > > > I agree that a consumer can potentially talk to old leader for
> a
> > > long
> > > > > > time
> > > > > > > even after this KIP. But after this KIP, the consumer probably
> > > should
> > > > > not
> > > > > > > get OffetOutofRangeException and therefore will not cause
> offset
> > > > rewind
> > > > > > > issue. So the only problem is that consumer will not be able to
> > > fetch
> > > > > > data
> > > > > > > until it has updated metadata. It seems that this situation can
> > > only
> > > > > > happen
> > > > > > > if the broker is too slow in processing LeaderAndIsrRequest
> since
> > > > > > otherwise
> > > > > > > the consumer will be forced to update metadata due to
> > > > > > > NotLeaderForPartitionException. So the problem we are having
> > here
> > > is
> > > > > > that
> > > > > > > consumer will not be able to fetch data if some broker is too
> > slow
> > > in
> > > > > > > processing LeaderAndIsrRequest.
> > > > > > >
> > > > > > > Because Kafka propagates LeaderAndIsrRequest asynchronously to
> > all
> > > > > > brokers
> > > > > > > in the cluster, there will always be a period of time when
> > consumer
> > > > can
> > > > > > not
> > > > > > > fetch data for the partition during the leadership change. Thus
> > it
> > > > > seems
> > > > > > > more like a broker-side performance issue instead of
> client-side
> > > > > > > correctness issue. My gut feel is that it is not causing a
> much a
> > > > > problem
> > > > > > > as the problem to be fixed in this KIP. And if we were to
> address
> > > it,
> > > > > we
> > > > > > > probably need to make change in the broker side, e.g. with
> > > > prioritized
> > > > > > > queue for controller-related requests, which may be kind of
> > > > orthogonal
> > > > > to
> > > > > > > this KIP. I am not very sure it will be easier to address it
> with
> > > the
> > > > > > > change in this KIP. Do you have any recommendation?
> > > > > > >
> > > > > > > Thanks,
> > > > > > > Dong
> > > > > > >
> > > > > > >
> > > > > > > On Mon, Dec 11, 2017 at 1:51 PM, Jun Rao <jun@confluent.io>
> > wrote:
> > > > > > >
> > > > > > > > Hi, Dong,
> > > > > > > >
> > > > > > > > Thanks for the reply.
> > > > > > > >
> > > > > > > > My suggestion of forcing the metadata refresh from the
> > controller
> > > > may
> > > > > > not
> > > > > > > > work in general since the cached controller could be outdated
> > > too.
> > > > > The
> > > > > > > > general problem is that if a consumer's metadata is outdated,
> > it
> > > > may
> > > > > > get
> > > > > > > > stuck with the old leader for a long time. We can address the
> > > issue
> > > > > of
> > > > > > > > detecting outdated metadata in a separate KIP in the future
> if
> > > you
> > > > > > didn't
> > > > > > > > intend to address it in this KIP.
> > > > > > > >
> > > > > > > > Thanks,
> > > > > > > >
> > > > > > > > Jun
> > > > > > > >
> > > > > > > >
> > > > > > > > On Sat, Dec 9, 2017 at 10:12 PM, Dong Lin <
> lindong28@gmail.com
> > >
> > > > > wrote:
> > > > > > > >
> > > > > > > > > Hey Jun,
> > > > > > > > >
> > > > > > > > > Thanks much for your comments. Given that client needs to
> > > > > > de-serialize
> > > > > > > > the
> > > > > > > > > metadata anyway, the extra overhead of checking the
> > > per-partition
> > > > > > > version
> > > > > > > > > for every partition should not be a big concern. Thus it
> > makes
> > > > > sense
> > > > > > to
> > > > > > > > use
> > > > > > > > > leader epoch as the per-partition version instead of
> > creating a
> > > > > > global
> > > > > > > > > metadata version. I will update the KIP to do that.
> > > > > > > > >
> > > > > > > > > Regarding the detection of outdated metadata, I think it is
> > > > > possible
> > > > > > to
> > > > > > > > > ensure that client gets latest metadata by fetching from
> > > > > controller.
> > > > > > > Note
> > > > > > > > > that this requires extra logic in the controller such that
> > > > > controller
> > > > > > > > > updates metadata directly in memory without requiring
> > > > > > > > > UpdateMetadataRequest. But I am not sure the main
> motivation
> > of
> > > > > this
> > > > > > at
> > > > > > > > > this moment. But this makes controller more like a
> bottleneck
> > > in
> > > > > the
> > > > > > > > > cluster which we probably want to avoid.
> > > > > > > > >
> > > > > > > > > I think we can probably keep the current way of ensuring
> > > metadata
> > > > > > > > > freshness. Currently client will be forced to refresh
> > metadata
> > > if
> > > > > > > broker
> > > > > > > > > returns error (e.g. NotLeaderForPartition) due to outdated
> > > > metadata
> > > > > > or
> > > > > > > if
> > > > > > > > > the metadata does not contain the partition that the client
> > > > needs.
> > > > > In
> > > > > > > the
> > > > > > > > > future, as you previously suggested, we can include
> > > per-partition
> > > > > > > > > leaderEpoch in the FetchRequest/ProduceRequest such that
> > broker
> > > > can
> > > > > > > > return
> > > > > > > > > error if the epoch is smaller than cached epoch in the
> > broker.
> > > > > Given
> > > > > > > that
> > > > > > > > > this adds more complexity to Kafka, I think we can probably
> > > think
> > > > > > about
> > > > > > > > > that leader when we have a specific use-case or problem to
> > > solve
> > > > > with
> > > > > > > > > up-to-date metadata. Does this sound OK?
> > > > > > > > >
> > > > > > > > > Thanks,
> > > > > > > > > Dong
> > > > > > > > >
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > On Fri, Dec 8, 2017 at 3:53 PM, Jun Rao <jun@confluent.io>
> > > > wrote:
> > > > > > > > >
> > > > > > > > > > Hi, Dong,
> > > > > > > > > >
> > > > > > > > > > Thanks for the reply. A few more points below.
> > > > > > > > > >
> > > > > > > > > > For dealing with how to prevent a consumer switching
> from a
> > > new
> > > > > > > leader
> > > > > > > > to
> > > > > > > > > > an old leader, you suggestion that refreshes metadata on
> > > > consumer
> > > > > > > > restart
> > > > > > > > > > until it sees a metadata version >= the one associated
> with
> > > the
> > > > > > > offset
> > > > > > > > > > works too, as long as we guarantee that the cached
> metadata
> > > > > > versions
> > > > > > > on
> > > > > > > > > the
> > > > > > > > > > brokers only go up.
> > > > > > > > > >
> > > > > > > > > > The second discussion point is on whether the metadata
> > > > versioning
> > > > > > > > should
> > > > > > > > > be
> > > > > > > > > > per partition or global. For the partition level
> > versioning,
> > > > you
> > > > > > were
> > > > > > > > > > concerned about the performance. Given that metadata
> > updates
> > > > are
> > > > > > > rare,
> > > > > > > > I
> > > > > > > > > am
> > > > > > > > > > not sure if it's a big concern though. Doing a million if
> > > tests
> > > > > is
> > > > > > > > > probably
> > > > > > > > > > going to take less than 1ms. Another thing is that the
> > > metadata
> > > > > > > version
> > > > > > > > > > seems to need to survive controller failover. In your
> > current
> > > > > > > > approach, a
> > > > > > > > > > consumer may not be able to wait on the right version of
> > the
> > > > > > metadata
> > > > > > > > > after
> > > > > > > > > > the consumer restart since the metadata version may have
> > been
> > > > > > > recycled
> > > > > > > > on
> > > > > > > > > > the server side due to a controller failover while the
> > > consumer
> > > > > is
> > > > > > > > down.
> > > > > > > > > > The partition level leaderEpoch survives controller
> failure
> > > and
> > > > > > won't
> > > > > > > > > have
> > > > > > > > > > this issue.
> > > > > > > > > >
> > > > > > > > > > Lastly, neither your proposal nor mine addresses the
> issue
> > > how
> > > > to
> > > > > > > > > guarantee
> > > > > > > > > > a consumer to detect that is metadata is outdated.
> > Currently,
> > > > the
> > > > > > > > > consumer
> > > > > > > > > > is not guaranteed to fetch metadata from every broker
> > within
> > > > some
> > > > > > > > bounded
> > > > > > > > > > period of time. Maybe this is out of the scope of your
> KIP.
> > > But
> > > > > one
> > > > > > > > idea
> > > > > > > > > is
> > > > > > > > > > force the consumer to refresh metadata from the
> controller
> > > > > > > > periodically.
> > > > > > > > > >
> > > > > > > > > > Jun
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > On Thu, Dec 7, 2017 at 11:25 AM, Dong Lin <
> > > lindong28@gmail.com
> > > > >
> > > > > > > wrote:
> > > > > > > > > >
> > > > > > > > > > > Hey Jun,
> > > > > > > > > > >
> > > > > > > > > > > Thanks much for the comments. Great point particularly
> > > > > regarding
> > > > > > > > (3). I
> > > > > > > > > > > haven't thought about this before.
> > > > > > > > > > >
> > > > > > > > > > > It seems that there are two possible ways where the
> > version
> > > > > > number
> > > > > > > > can
> > > > > > > > > be
> > > > > > > > > > > used. One solution is for client to check the version
> > > number
> > > > at
> > > > > > the
> > > > > > > > > time
> > > > > > > > > > it
> > > > > > > > > > > receives MetadataResponse. And if the version number in
> > the
> > > > > > > > > > > MetadataResponse is smaller than the version number in
> > the
> > > > > > client's
> > > > > > > > > > cache,
> > > > > > > > > > > the client will be forced to fetch metadata again.
> > Another
> > > > > > > solution,
> > > > > > > > > as
> > > > > > > > > > > you have suggested, is for broker to check the version
> > > number
> > > > > at
> > > > > > > the
> > > > > > > > > time
> > > > > > > > > > > it receives a request from client. The broker will
> reject
> > > the
> > > > > > > request
> > > > > > > > > if
> > > > > > > > > > > the version is smaller than the version in broker's
> > cache.
> > > > > > > > > > >
> > > > > > > > > > > I am not very sure that the second solution can address
> > the
> > > > > > problem
> > > > > > > > > here.
> > > > > > > > > > > In the scenario described in the JIRA ticket, broker's
> > > cache
> > > > > may
> > > > > > be
> > > > > > > > > > > outdated because it has not processed the
> > > LeaderAndIsrRequest
> > > > > > from
> > > > > > > > the
> > > > > > > > > > > controller. Thus it may still process client's request
> > even
> > > > if
> > > > > > the
> > > > > > > > > > version
> > > > > > > > > > > in client's request is actually outdated. Does this
> make
> > > > sense?
> > > > > > > > > > >
> > > > > > > > > > > IMO, it seems that we can address problem (3) by saving
> > the
> > > > > > > metadata
> > > > > > > > > > > version together with the offset. After consumer
> starts,
> > it
> > > > > will
> > > > > > > keep
> > > > > > > > > > > fetching metadata until the metadata version >= the
> > version
> > > > > saved
> > > > > > > > with
> > > > > > > > > > the
> > > > > > > > > > > offset of this partition.
> > > > > > > > > > >
> > > > > > > > > > > Regarding problems (1) and (2): Currently we use the
> > > version
> > > > > > number
> > > > > > > > in
> > > > > > > > > > the
> > > > > > > > > > > MetadataResponse to ensure that the metadata does not
> go
> > > back
> > > > > in
> > > > > > > > time.
> > > > > > > > > > > There are two alternative solutions to address problems
> > (1)
> > > > and
> > > > > > > (2).
> > > > > > > > > One
> > > > > > > > > > > solution is for client to enumerate all partitions in
> the
> > > > > > > > > > MetadataResponse,
> > > > > > > > > > > compare their epoch with those in the cached metadata,
> > and
> > > > > > rejects
> > > > > > > > the
> > > > > > > > > > > MetadataResponse iff any leader epoch is smaller. The
> > main
> > > > > > concern
> > > > > > > is
> > > > > > > > > > that
> > > > > > > > > > > MetadataResponse currently cached information of all
> > > > partitions
> > > > > > in
> > > > > > > > the
> > > > > > > > > > > entire cluster. It may slow down client's performance
> if
> > we
> > > > > were
> > > > > > to
> > > > > > > > do
> > > > > > > > > > it.
> > > > > > > > > > > The other solution is for client to enumerate
> partitions
> > > for
> > > > > only
> > > > > > > > > topics
> > > > > > > > > > > registered in the org.apache.kafka.clients.Metadata,
> > which
> > > > > will
> > > > > > be
> > > > > > > > an
> > > > > > > > > > > empty
> > > > > > > > > > > set for producer and the set of subscribed partitions
> for
> > > > > > consumer.
> > > > > > > > But
> > > > > > > > > > > this degrades to all topics if consumer subscribes to
> > > topics
> > > > in
> > > > > > the
> > > > > > > > > > cluster
> > > > > > > > > > > by pattern.
> > > > > > > > > > >
> > > > > > > > > > > Note that client will only be forced to update metadata
> > if
> > > > the
> > > > > > > > version
> > > > > > > > > in
> > > > > > > > > > > the MetadataResponse is smaller than the version in the
> > > > cached
> > > > > > > > > metadata.
> > > > > > > > > > In
> > > > > > > > > > > general it should not be a problem. It can be a problem
> > > only
> > > > if
> > > > > > > some
> > > > > > > > > > broker
> > > > > > > > > > > is particularly slower than other brokers in processing
> > > > > > > > > > > UpdateMetadataRequest. When this is the case, it means
> > that
> > > > the
> > > > > > > > broker
> > > > > > > > > is
> > > > > > > > > > > also particularly slower in processing
> > LeaderAndIsrRequest,
> > > > > which
> > > > > > > can
> > > > > > > > > > cause
> > > > > > > > > > > problem anyway because some partition will probably
> have
> > no
> > > > > > leader
> > > > > > > > > during
> > > > > > > > > > > this period. I am not sure problems (1) and (2) cause
> > more
> > > > > > problem
> > > > > > > > than
> > > > > > > > > > > what we already have.
> > > > > > > > > > >
> > > > > > > > > > > Thanks,
> > > > > > > > > > > Dong
> > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > On Wed, Dec 6, 2017 at 6:42 PM, Jun Rao <
> > jun@confluent.io>
> > > > > > wrote:
> > > > > > > > > > >
> > > > > > > > > > > > Hi, Dong,
> > > > > > > > > > > >
> > > > > > > > > > > > Great finding on the issue. It's a real problem. A
> few
> > > > > comments
> > > > > > > > about
> > > > > > > > > > the
> > > > > > > > > > > > KIP. (1) I am not sure about updating
> > > > > controller_metadata_epoch
> > > > > > > on
> > > > > > > > > > every
> > > > > > > > > > > > UpdateMetadataRequest. Currently, the controller can
> > send
> > > > > > > > > > > > UpdateMetadataRequest when there is no actual
> metadata
> > > > > change.
> > > > > > > > Doing
> > > > > > > > > > this
> > > > > > > > > > > > may require unnecessary metadata refresh on the
> client.
> > > (2)
> > > > > > > > > > > > controller_metadata_epoch is global across all
> topics.
> > > This
> > > > > > means
> > > > > > > > > that
> > > > > > > > > > a
> > > > > > > > > > > > client may be forced to update its metadata even when
> > the
> > > > > > > metadata
> > > > > > > > > for
> > > > > > > > > > > the
> > > > > > > > > > > > topics that it cares haven't changed. (3) It doesn't
> > seem
> > > > > that
> > > > > > > the
> > > > > > > > > KIP
> > > > > > > > > > > > handles the corner case when a consumer is restarted.
> > > Say a
> > > > > > > > consumer
> > > > > > > > > > > reads
> > > > > > > > > > > > from the new leader, commits the offset and then is
> > > > > restarted.
> > > > > > On
> > > > > > > > > > > restart,
> > > > > > > > > > > > the consumer gets an outdated metadata and fetches
> from
> > > the
> > > > > old
> > > > > > > > > leader.
> > > > > > > > > > > > Then, the consumer will get into the offset out of
> > range
> > > > > issue.
> > > > > > > > > > > >
> > > > > > > > > > > > Given the above, I am thinking of the following
> > approach.
> > > > We
> > > > > > > > actually
> > > > > > > > > > > > already have metadata versioning at the partition
> > level.
> > > > Each
> > > > > > > > leader
> > > > > > > > > > has
> > > > > > > > > > > a
> > > > > > > > > > > > leader epoch which is monotonically increasing. We
> can
> > > > > > > potentially
> > > > > > > > > > > > propagate leader epoch back in the metadata response
> > and
> > > > the
> > > > > > > > clients
> > > > > > > > > > can
> > > > > > > > > > > > cache that. This solves the issue of (1) and (2). To
> > > solve
> > > > > (3),
> > > > > > > > when
> > > > > > > > > > > saving
> > > > > > > > > > > > an offset, we could save both an offset and the
> > > > corresponding
> > > > > > > > leader
> > > > > > > > > > > epoch.
> > > > > > > > > > > > When fetching the data, the consumer provides both
> the
> > > > offset
> > > > > > and
> > > > > > > > the
> > > > > > > > > > > > leader epoch. A leader will only serve the request if
> > its
> > > > > > leader
> > > > > > > > > epoch
> > > > > > > > > > is
> > > > > > > > > > > > equal to or greater than the leader epoch from the
> > > > consumer.
> > > > > To
> > > > > > > > > achieve
> > > > > > > > > > > > this, we need to change the fetch request protocol
> and
> > > the
> > > > > > offset
> > > > > > > > > > commit
> > > > > > > > > > > > api, which requires some more thoughts.
> > > > > > > > > > > >
> > > > > > > > > > > > Thanks,
> > > > > > > > > > > >
> > > > > > > > > > > > Jun
> > > > > > > > > > > >
> > > > > > > > > > > >
> > > > > > > > > > > > On Wed, Dec 6, 2017 at 10:57 AM, Dong Lin <
> > > > > lindong28@gmail.com
> > > > > > >
> > > > > > > > > wrote:
> > > > > > > > > > > >
> > > > > > > > > > > > > Bump up the thread.
> > > > > > > > > > > > >
> > > > > > > > > > > > > It will be great to have more comments on whether
> we
> > > > should
> > > > > > do
> > > > > > > it
> > > > > > > > > or
> > > > > > > > > > > > > whether there is better way to address the
> motivation
> > > of
> > > > > this
> > > > > > > > KIP.
> > > > > > > > > > > > >
> > > > > > > > > > > > > On Mon, Dec 4, 2017 at 3:09 PM, Dong Lin <
> > > > > > lindong28@gmail.com>
> > > > > > > > > > wrote:
> > > > > > > > > > > > >
> > > > > > > > > > > > > > I don't have an interesting rejected alternative
> > > > solution
> > > > > > to
> > > > > > > > put
> > > > > > > > > in
> > > > > > > > > > > the
> > > > > > > > > > > > > > KIP. If there is good alternative solution from
> > > anyone
> > > > in
> > > > > > > this
> > > > > > > > > > > thread,
> > > > > > > > > > > > I
> > > > > > > > > > > > > am
> > > > > > > > > > > > > > happy to discuss this and update the KIP
> > accordingly.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Thanks,
> > > > > > > > > > > > > > Dong
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > On Mon, Dec 4, 2017 at 1:12 PM, Ted Yu <
> > > > > > yuzhihong@gmail.com>
> > > > > > > > > > wrote:
> > > > > > > > > > > > > >
> > > > > > > > > > > > > >> It is clearer now.
> > > > > > > > > > > > > >>
> > > > > > > > > > > > > >> I noticed that Rejected Alternatives section is
> > > empty.
> > > > > > > > > > > > > >> Have you considered any alternative ?
> > > > > > > > > > > > > >>
> > > > > > > > > > > > > >> Cheers
> > > > > > > > > > > > > >>
> > > > > > > > > > > > > >> On Mon, Dec 4, 2017 at 1:07 PM, Dong Lin <
> > > > > > > lindong28@gmail.com
> > > > > > > > >
> > > > > > > > > > > wrote:
> > > > > > > > > > > > > >>
> > > > > > > > > > > > > >> > Ted, thanks for catching this. I have updated
> > the
> > > > > > sentence
> > > > > > > > to
> > > > > > > > > > make
> > > > > > > > > > > > it
> > > > > > > > > > > > > >> > readable.
> > > > > > > > > > > > > >> >
> > > > > > > > > > > > > >> > Thanks,
> > > > > > > > > > > > > >> > Dong
> > > > > > > > > > > > > >> >
> > > > > > > > > > > > > >> > On Sat, Dec 2, 2017 at 3:05 PM, Ted Yu <
> > > > > > > yuzhihong@gmail.com
> > > > > > > > >
> > > > > > > > > > > wrote:
> > > > > > > > > > > > > >> >
> > > > > > > > > > > > > >> > > bq. It the controller_epoch of the incoming
> > > > > > > > > MetadataResponse,
> > > > > > > > > > or
> > > > > > > > > > > > if
> > > > > > > > > > > > > >> the
> > > > > > > > > > > > > >> > > controller_epoch is the same but the
> > > > > > > > > controller_metadata_epoch
> > > > > > > > > > > > > >> > >
> > > > > > > > > > > > > >> > > Can you update the above sentence so that
> the
> > > > > > intention
> > > > > > > is
> > > > > > > > > > > > clearer ?
> > > > > > > > > > > > > >> > >
> > > > > > > > > > > > > >> > > Thanks
> > > > > > > > > > > > > >> > >
> > > > > > > > > > > > > >> > > On Fri, Dec 1, 2017 at 6:33 PM, Dong Lin <
> > > > > > > > > lindong28@gmail.com
> > > > > > > > > > >
> > > > > > > > > > > > > wrote:
> > > > > > > > > > > > > >> > >
> > > > > > > > > > > > > >> > > > Hi all,
> > > > > > > > > > > > > >> > > >
> > > > > > > > > > > > > >> > > > I have created KIP-232: Detect outdated
> > > metadata
> > > > > by
> > > > > > > > adding
> > > > > > > > > > > > > >> > > > ControllerMetadataEpoch field:
> > > > > > > > > > > > > >> > > > https://cwiki.apache.org/
> > > > > > > confluence/display/KAFKA/KIP-
> > > > > > > > > > > > > >> > > > 232%3A+Detect+outdated+
> metadata+by+adding+
> > > > > > > > > > > > > >> > ControllerMetadataEpoch+field
> > > > > > > > > > > > > >> > > > .
> > > > > > > > > > > > > >> > > >
> > > > > > > > > > > > > >> > > > The KIP proposes to add fields in
> > > > MetadataResponse
> > > > > > and
> > > > > > > > > > > > > >> > > > UpdateMetadataRequest so that client can
> > > reject
> > > > > > > outdated
> > > > > > > > > > > > metadata
> > > > > > > > > > > > > >> and
> > > > > > > > > > > > > >> > > avoid
> > > > > > > > > > > > > >> > > > unnecessary OffsetOutOfRangeException.
> > > Otherwise
> > > > > > there
> > > > > > > > is
> > > > > > > > > > > > > currently
> > > > > > > > > > > > > >> > race
> > > > > > > > > > > > > >> > > > condition that can cause consumer to reset
> > > > offset
> > > > > > > which
> > > > > > > > > > > > negatively
> > > > > > > > > > > > > >> > affect
> > > > > > > > > > > > > >> > > > the consumer's availability.
> > > > > > > > > > > > > >> > > >
> > > > > > > > > > > > > >> > > > Feedback and suggestions are welcome!
> > > > > > > > > > > > > >> > > >
> > > > > > > > > > > > > >> > > > Regards,
> > > > > > > > > > > > > >> > > > Dong
> > > > > > > > > > > > > >> > > >
> > > > > > > > > > > > > >> > >
> > > > > > > > > > > > > >> >
> > > > > > > > > > > > > >>
> > > > > > > > > > > > > >
> > > > > > > > > > > > > >
> > > > > > > > > > > > >
> > > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > >
> > > > > > > > >
> > > > > > > >
> > > > > > >
> > > > > >
> > > > >
> > > >
> > >
> >
>

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