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From Dong Lin <lindon...@gmail.com>
Subject Re: [DISCUSS] KIP-232: Detect outdated metadata by adding ControllerMetadataEpoch field
Date Mon, 11 Dec 2017 22:55:24 GMT
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problem with consumer using outdated metadata and whether it is easier to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 work in general since the cached controller could be outdated too.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 general problem is that if a consumer's metadata is outdated, it may get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 detecting outdated metadata in a separate KIP in the future if you didn't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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