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From Jun Rao <...@confluent.io>
Subject Re: [DISCUSS] KIP-232: Detect outdated metadata by adding ControllerMetadataEpoch field
Date Fri, 08 Dec 2017 23:53:49 GMT
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a consumer to detect that is metadata is outdated.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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