Hey Colin,
Thanks for reviewing the KIP.
If I understand you right, you maybe suggesting that we can use a global
metadataEpoch that is incremented every time controller updates metadata.
The problem with this solution is that, if a topic is deleted and created
again, user will not know whether that the offset which is stored before
the topic deletion is no longer valid. This motivates the idea to include
per-partition partitionEpoch. Does this sound reasonable?
Then the next question maybe, should we use a global metadataEpoch +
per-partition partitionEpoch, instead of using per-partition leaderEpoch +
per-partition leaderEpoch. The former solution using metadataEpoch would
not work due to the following scenario (provided by Jun):
"Consider the following scenario. In metadata v1, the leader for a
partition is at broker 1. In metadata v2, leader is at broker 2. In
metadata v3, leader is at broker 1 again. The last committed offset in v1,
v2 and v3 are 10, 20 and 30, respectively. A consumer is started and reads
metadata v1 and reads messages from offset 0 to 25 from broker 1. My
understanding is that in the current proposal, the metadata version
associated with offset 25 is v1. The consumer is then restarted and fetches
metadata v2. The consumer tries to read from broker 2, which is the old
leader with the last offset at 20. In this case, the consumer will still
get OffsetOutOfRangeException incorrectly."
Regarding your comment "For the second purpose, this is "soft state"
anyway. If the client thinks X is the leader but Y is really the leader,
the client will talk to X, and X will point out its mistake by sending back
a NOT_LEADER_FOR_PARTITION.", it is probably no true. The problem here is
that the old leader X may still think it is the leader of the partition and
thus it will not send back NOT_LEADER_FOR_PARTITION. The reason is provided
in KAFKA-6262. Can you check if that makes sense?
Regards,
Dong
On Wed, Jan 24, 2018 at 10:39 AM, Colin McCabe <cmcc...@apache.org> wrote:
> Hi Dong,
>
> Thanks for proposing this KIP. I think a metadata epoch is a really good
> idea.
>
> I read through the DISCUSS thread, but I still don't have a clear picture
> of why the proposal uses a metadata epoch per partition rather than a
> global metadata epoch. A metadata epoch per partition is kind of
> unpleasant-- it's at least 4 extra bytes per partition that we have to send
> over the wire in every full metadata request, which could become extra
> kilobytes on the wire when the number of partitions becomes large. Plus,
> we have to update all the auxillary classes to include an epoch.
>
> We need to have a global metadata epoch anyway to handle partition
> addition and deletion. For example, if I give you
> MetadataResponse{part1,epoch 1, part2, epoch 1} and {part1, epoch1}, which
> MetadataResponse is newer? You have no way of knowing. It could be that
> part2 has just been created, and the response with 2 partitions is newer.
> Or it coudl be that part2 has just been deleted, and therefore the response
> with 1 partition is newer. You must have a global epoch to disambiguate
> these two cases.
>
> Previously, I worked on the Ceph distributed filesystem. Ceph had the
> concept of a map of the whole cluster, maintained by a few servers doing
> paxos. This map was versioned by a single 64-bit epoch number which
> increased on every change. It was propagated to clients through gossip. I
> wonder if something similar could work here?
>
> It seems like the the Kafka MetadataResponse serves two somewhat unrelated
> purposes. Firstly, it lets clients know what partitions exist in the
> system and where they live. Secondly, it lets clients know which nodes
> within the partition are in-sync (in the ISR) and which node is the leader.
>
> The first purpose is what you really need a metadata epoch for, I think.
> You want to know whether a partition exists or not, or you want to know
> which nodes you should talk to in order to write to a given partition. A
> single metadata epoch for the whole response should be adequate here. We
> should not change the partition assignment without going through zookeeper
> (or a similar system), and this inherently serializes updates into a
> numbered stream. Brokers should also stop responding to requests when they
> are unable to contact ZK for a certain time period. This prevents the case
> where a given partition has been moved off some set of nodes, but a client
> still ends up talking to those nodes and writing data there.
>
> For the second purpose, this is "soft state" anyway. If the client thinks
> X is the leader but Y is really the leader, the client will talk to X, and
> X will point out its mistake by sending back a NOT_LEADER_FOR_PARTITION.
> Then the client can update its metadata again and find the new leader, if
> there is one. There is no need for an epoch to handle this. Similarly, I
> can't think of a reason why changing the in-sync replica set needs to bump
> the epoch.
>
> best,
> Colin
>
>
> On Wed, Jan 24, 2018, at 09:45, Dong Lin wrote:
> > Thanks much for reviewing the KIP!
> >
> > Dong
> >
> > On Wed, Jan 24, 2018 at 7:10 AM, Guozhang Wang <wangg...@gmail.com>
> wrote:
> >
> > > Yeah that makes sense, again I'm just making sure we understand all the
> > > scenarios and what to expect.
> > >
> > > I agree that if, more generally speaking, say users have only consumed
> to
> > > offset 8, and then call seek(16) to "jump" to a further position, then
> she
> > > needs to be aware that OORE maybe thrown and she needs to handle it or
> rely
> > > on reset policy which should not surprise her.
> > >
> > >
> > > I'm +1 on the KIP.
> > >
> > > Guozhang
> > >
> > >
> > > On Wed, Jan 24, 2018 at 12:31 AM, Dong Lin <lindon...@gmail.com>
> wrote:
> > >
> > > > Yes, in general we can not prevent OffsetOutOfRangeException if user
> > > seeks
> > > > to a wrong offset. The main goal is to prevent
> OffsetOutOfRangeException
> > > if
> > > > user has done things in the right way, e.g. user should know that
> there
> > > is
> > > > message with this offset.
> > > >
> > > > For example, if user calls seek(..) right after construction, the
> only
> > > > reason I can think of is that user stores offset externally. In this
> > > case,
> > > > user currently needs to use the offset which is obtained using
> > > position(..)
> > > > from the last run. With this KIP, user needs to get the offset and
> the
> > > > offsetEpoch using positionAndOffsetEpoch(...) and stores these
> > > information
> > > > externally. The next time user starts consumer, he/she needs to call
> > > > seek(..., offset, offsetEpoch) right after construction. Then KIP
> should
> > > be
> > > > able to ensure that we don't throw OffsetOutOfRangeException if
> there is
> > > no
> > > > unclean leader election.
> > > >
> > > > Does this sound OK?
> > > >
> > > > Regards,
> > > > Dong
> > > >
> > > >
> > > > On Tue, Jan 23, 2018 at 11:44 PM, Guozhang Wang <wangg...@gmail.com>
> > > > wrote:
> > > >
> > > > > "If consumer wants to consume message with offset 16, then consumer
> > > must
> > > > > have
> > > > > already fetched message with offset 15"
> > > > >
> > > > > --> this may not be always true right? What if consumer just call
> > > > seek(16)
> > > > > after construction and then poll without committed offset ever
> stored
> > > > > before? Admittedly it is rare but we do not programmably disallow
> it.
> > > > >
> > > > >
> > > > > Guozhang
> > > > >
> > > > > On Tue, Jan 23, 2018 at 10:42 PM, Dong Lin <lindon...@gmail.com>
> > > wrote:
> > > > >
> > > > > > Hey Guozhang,
> > > > > >
> > > > > > Thanks much for reviewing the KIP!
> > > > > >
> > > > > > In the scenario you described, let's assume that broker A has
> > > messages
> > > > > with
> > > > > > offset up to 10, and broker B has messages with offset up to 20.
> If
> > > > > > consumer wants to consume message with offset 9, it will not
> receive
> > > > > > OffsetOutOfRangeException
> > > > > > from broker A.
> > > > > >
> > > > > > If consumer wants to consume message with offset 16, then
> consumer
> > > must
> > > > > > have already fetched message with offset 15, which can only come
> from
> > > > > > broker B. Because consumer will fetch from broker B only if
> > > leaderEpoch
> > > > > >=
> > > > > > 2, then the current consumer leaderEpoch can not be 1 since this
> KIP
> > > > > > prevents leaderEpoch rewind. Thus we will not have
> > > > > > OffsetOutOfRangeException
> > > > > > in this case.
> > > > > >
> > > > > > Does this address your question, or maybe there is more advanced
> > > > scenario
> > > > > > that the KIP does not handle?
> > > > > >
> > > > > > Thanks,
> > > > > > Dong
> > > > > >
> > > > > > On Tue, Jan 23, 2018 at 9:43 PM, Guozhang Wang <
> wangg...@gmail.com>
> > > > > wrote:
> > > > > >
> > > > > > > Thanks Dong, I made a pass over the wiki and it lgtm.
> > > > > > >
> > > > > > > Just a quick question: can we completely eliminate the
> > > > > > > OffsetOutOfRangeException with this approach? Say if there is
> > > > > consecutive
> > > > > > > leader changes such that the cached metadata's partition epoch
> is
> > > 1,
> > > > > and
> > > > > > > the metadata fetch response returns with partition epoch 2
> > > pointing
> > > > to
> > > > > > > leader broker A, while the actual up-to-date metadata has
> partition
> > > > > > epoch 3
> > > > > > > whose leader is now broker B, the metadata refresh will still
> > > succeed
> > > > > and
> > > > > > > the follow-up fetch request may still see OORE?
> > > > > > >
> > > > > > >
> > > > > > > Guozhang
> > > > > > >
> > > > > > >
> > > > > > > On Tue, Jan 23, 2018 at 3:47 PM, Dong Lin <lindon...@gmail.com
> >
> > > > wrote:
> > > > > > >
> > > > > > > > Hi all,
> > > > > > > >
> > > > > > > > I would like to start the voting process for KIP-232:
> > > > > > > >
> > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-
> > > > > > > > 232%3A+Detect+outdated+metadata+using+leaderEpoch+
> > > > and+partitionEpoch
> > > > > > > >
> > > > > > > > The KIP will help fix a concurrency issue in Kafka which
> > > currently
> > > > > can
> > > > > > > > cause message loss or message duplication in consumer.
> > > > > > > >
> > > > > > > > Regards,
> > > > > > > > Dong
> > > > > > > >
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > --
> > > > > > > -- Guozhang
> > > > > > >
> > > > > >
> > > > >
> > > > >
> > > > >
> > > > > --
> > > > > -- Guozhang
> > > > >
> > > >
> > >
> > >
> > >
> > > --
> > > -- Guozhang
> > >
>
