Just some clarification on the current fencing logic. Currently, if the
producer uses acks=-1, a write will only succeed if the write is received
by all in-sync replicas (i.e., committed). This is true even when min.isr
is set since we first wait for a message to be committed and then check the
min.isr requirement. KIP-250 may change that, but we can discuss the
implication there. In the case where you have 3 replicas A, B and C, and A
and B are partitioned off and C becomes the new leader, the old leader A
can't commit new messages with the current ISR of {A, B, C} since C won't
be fetching from A. A will then try to persist the reduced ISR of just A
and B to ZK. This will fail since the ZK version is outdated. Then, no new
message can be committed by A. This is how we fence off the writer.
Currently, there is no fencing for the readers. We can potentially fence
off the reads in the old leader A after a timeout. However, we still need
to decide what to do within the timeout, especially when A is presented
with an offset that's larger than it's last offset.
Thanks,
Jun
On Fri, Jan 26, 2018 at 12:17 PM, Dong Lin <lindon...@gmail.com> wrote:
> Hey Colin,
>
>
> On Fri, Jan 26, 2018 at 10:16 AM, Colin McCabe <cmcc...@apache.org> wrote:
>
> > On Thu, Jan 25, 2018, at 16:47, Dong Lin wrote:
> > > Hey Colin,
> > >
> > > Thanks for the comment.
> > >
> > > On Thu, Jan 25, 2018 at 4:15 PM, Colin McCabe <cmcc...@apache.org>
> > wrote:
> > >
> > > > On Wed, Jan 24, 2018, at 21:07, Dong Lin wrote:
> > > > > 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 the topic deletion is no longer valid.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?
> > > >
> > > > Hi Dong,
> > > >
> > > > Perhaps we can store the last valid offset of each deleted topic in
> > > > ZooKeeper. Then, when a topic with one of those names gets
> > re-created, we
> > > > can start the topic at the previous end offset rather than at 0.
> This
> > > > preserves immutability. It is no more burdensome than having to
> > preserve a
> > > > "last epoch" for the deleted partition somewhere, right?
> > > >
> > >
> > > My concern with this solution is that the number of zookeeper nodes get
> > > more and more over time if some users keep deleting and creating
> topics.
> > Do
> > > you think this can be a problem?
> >
> > Hi Dong,
> >
> > We could expire the "partition tombstones" after an hour or so. In
> > practice this would solve the issue for clients that like to destroy and
> > re-create topics all the time. In any case, doesn't the current proposal
> > add per-partition znodes as well that we have to track even after the
> > partition is deleted? Or did I misunderstand that?
> >
>
> Actually the current KIP does not add per-partition znodes. Could you
> double check? I can fix the KIP wiki if there is anything misleading.
>
> If we expire the "partition tomstones" after an hour, and the topic is
> re-created after more than an hour since the topic deletion, then we are
> back to the situation where user can not tell whether the topic has been
> re-created or not, right?
>
>
> >
> > It's not really clear to me what should happen when a topic is destroyed
> > and re-created with new data. Should consumers continue to be able to
> > consume? We don't know where they stopped consuming from the previous
> > incarnation of the topic, so messages may have been lost. Certainly
> > consuming data from offset X of the new incarnation of the topic may give
> > something totally different from what you would have gotten from offset X
> > of the previous incarnation of the topic.
> >
>
> With the current KIP, if a consumer consumes a topic based on the last
> remembered (offset, partitionEpoch, leaderEpoch), and if the topic is
> re-created, consume will throw InvalidPartitionEpochException because the
> previous partitionEpoch will be different from the current partitionEpoch.
> This is described in the Proposed Changes -> Consumption after topic
> deletion in the KIP. I can improve the KIP if there is anything not clear.
>
>
> > By choosing to reuse the same (topic, partition, offset) 3-tuple, we have
>
> chosen to give up immutability. That was a really bad decision. And now
> > we have to worry about time dependencies, stale cached data, and all the
> > rest. We can't completely fix this inside Kafka no matter what we do,
> > because not all that cached data is inside Kafka itself. Some of it may
> be
> > in systems that Kafka has sent data to, such as other daemons, SQL
> > databases, streams, and so forth.
> >
>
> The current KIP will uniquely identify a message using (topic, partition,
> offset, partitionEpoch) 4-tuple. This addresses the message immutability
> issue that you mentioned. Is there any corner case where the message
> immutability is still not preserved with the current KIP?
>
>
> >
> > I guess the idea here is that mirror maker should work as expected when
> > users destroy a topic and re-create it with the same name. That's kind
> of
> > tough, though, since in that scenario, mirror maker probably should
> destroy
> > and re-create the topic on the other end, too, right? Otherwise, what
> you
> > end up with on the other end could be half of one incarnation of the
> topic,
> > and half of another.
> >
> > What mirror maker really needs is to be able to follow a stream of events
> > about the kafka cluster itself. We could have some master topic which is
> > always present and which contains data about all topic deletions,
> > creations, etc. Then MM can simply follow this topic and do what is
> needed.
> >
> > >
> > >
> > > >
> > > > >
> > > > > 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?
> > > >
> > > > This is solvable with a timeout, right? If the leader can't
> > communicate
> > > > with the controller for a certain period of time, it should stop
> > acting as
> > > > the leader. We have to solve this problem, anyway, in order to fix
> > all the
> > > > corner cases.
> > > >
> > >
> > > Not sure if I fully understand your proposal. The proposal seems to
> > require
> > > non-trivial changes to our existing leadership election mechanism.
> Could
> > > you provide more detail regarding how it works? For example, how should
> > > user choose this timeout, how leader determines whether it can still
> > > communicate with controller, and how this triggers controller to elect
> > new
> > > leader?
> >
> > Before I come up with any proposal, let me make sure I understand the
> > problem correctly. My big question was, what prevents split-brain here?
> >
> > Let's say I have a partition which is on nodes A, B, and C, with min-ISR
> > 2. The controller is D. At some point, there is a network partition
> > between A and B and the rest of the cluster. The Controller re-assigns
> the
> > partition to nodes C, D, and E. But A and B keep chugging away, even
> > though they can no longer communicate with the controller.
> >
> > At some point, a client with stale metadata writes to the partition. It
> > still thinks the partition is on node A, B, and C, so that's where it
> sends
> > the data. It's unable to talk to C, but A and B reply back that all is
> > well.
> >
> > Is this not a case where we could lose data due to split brain? Or is
> > there a mechanism for preventing this that I missed? If it is, it seems
> > like a pretty serious failure case that we should be handling with our
> > metadata rework. And I think epoch numbers and timeouts might be part of
> > the solution.
> >
>
> Right, split brain can happen if RF=4 and minIsr=2. However, I am not sure
> it is a pretty serious issue which we need to address today. This can be
> prevented by configuring the Kafka topic so that minIsr > RF/2. Actually,
> if user sets minIsr=2, is there anything reason that user wants to set RF=4
> instead of 4?
>
> Introducing timeout in leader election mechanism is non-trivial. I think we
> probably want to do that only if there is good use-case that can not
> otherwise be addressed with the current mechanism.
>
>
> > best,
> > Colin
> >
> >
> > >
> > >
> > > > best,
> > > > Colin
> > > >
> > > > >
> > > > > 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
> > > > > > > >
> > > > > >
> > > >
> >
>
