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From Jun Rao <...@confluent.io>
Subject Re: [DISCUSS] KIP-232: Detect outdated metadata by adding ControllerMetadataEpoch field
Date Fri, 15 Dec 2017 02:38:08 GMT
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.

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.

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?

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 problem with consumer using outdated metadata and whether it is easier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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