Couple of other things. A. In the discussion, we talked about the usage of getting the latest high watermark from the broker. Currently, the high watermark in a partition can go back a bit for a short period of time during leader change. So, the high watermark returned in the getOffset api is not 100% accurate. There is a jira (KAFKA-2334) to track this issue.
B. The proposal in the wiki is to put the expected offset in every message, even when the messages are compressed. With Jiangjie's proposal of relative offset, the expected offset probably can only be set at the shallow compressed message level. We will need to think this through. Thanks, Jun On Tue, Aug 4, 2015 at 3:05 PM, Jiangjie Qin <j...@linkedin.com.invalid> wrote: > Jun, I see. So this only applies to uncompressed messages. Maybe that is > fine given most user will probably turn on compression? > I think the first approach is a more general approach but from application > point of view might harder to implement. I am thinking is it easier for the > application simply have one producer for a partition and hash the message > to producer. In that case, we can use the second approach but still have > multiple producers. The downside might be potentially more memory > footprint? We might also need to think about the fault tolerance a little > bit more. > > Ben, I agree when everything goes fine, having pipeline turned on is > probably fine. But if we take leader migration, broker down, message > appended to leader but not follower, etc, etc into consideration, it is not > clear to me how the conditional publish will still provide its guarantee > without enforcing those strict settings. > > Thanks, > > Jiangjie (Becket) Qin > > > > On Mon, Aug 3, 2015 at 9:55 PM, Jun Rao <j...@confluent.io> wrote: > > > A couple of thoughts on the commit log use case. Suppose that we want to > > maintain multiple replicas of a K/V store backed by a shared Kafka > > topic/partition as a commit log. There are two possible ways to use Kafka > > as a commit log. > > > > 1. The first approach allows multiple producers to publish to Kafka. Each > > replica of the data store keeps reading from a Kafka topic/partition to > > refresh the replica's view. Every time a replica gets an update to a key > > from a client, it combines the update and the current value of the key in > > its view and generates a post-value. It then does a conditional publish > to > > Kafka with the post-value. The update is successful if the conditional > > publish succeeds. Otherwise, the replica has to recompute the post-value > > (potentially after the replica's view is refreshed) and retry the > > conditional publish. A potential issue with this approach is when there > is > > a transient failure during publishing to Kafka (e.g., the leader of the > > partition changes). When this happens, the conditional publish will get > an > > error. The replica doesn't know whether the publish actually succeeded or > > not. If we just blindly retry, it may not give the correct behavior > (e.g., > > we could be applying +1 twice). So, not sure if conditional publish > itself > > is enough for this approach. > > > > 2. The second approach allows only a single producer to publish to Kafka. > > We somehow elect one of the replicas to be the master that handles all > > updates. Normally, we don't need conditional publish since there is a > > single producer. Conditional publish can potentially be used to deal with > > duplicates. If the master encounters the same transient failure as the > > above, it can get the latest offset from the Kafka topic/partition to see > > if the publish actually succeeded or not since it's the only producer. A > > potential issue here is to handle the zombie master problem: if the > master > > has a soft failure and another master is elected, we need to prevent the > > old master from publishing new data to Kafka. So, for this approach to > work > > properly, we need some kind of support of single writer in addition to > > conditional publish. > > > > > > Jiangjie, > > > > The issue with partial commit is the following. Say we have a batch of 10 > > uncompressed messages sent to the leader. The followers only fetched the > > first 5 messages and then the leader dies. In this case, we only > committed > > 5 out of the 10 messages. > > > > Thanks, > > > > Jun > > > > > > On Tue, Jul 28, 2015 at 1:16 AM, Daniel Schierbeck < > > daniel.schierb...@gmail.com> wrote: > > > > > Jiangjie: I think giving users the possibility of defining a custom > > policy > > > for handling rejections is a good idea. For instance, this will allow > > Kafka > > > to act as an event store in an Event Sourcing application. If the > > event(s) > > > are rejected by the store, the original command may need to be > > re-validated > > > against the new state. > > > > > > On Tue, Jul 28, 2015 at 1:27 AM Jiangjie Qin <j...@linkedin.com.invalid > > > > > wrote: > > > > > > > @Ewen, good point about batching. Yes, it would be tricky if we want > to > > > do > > > > a per-key conditional produce. My understanding is that the > > prerequisite > > > of > > > > this KIP is: > > > > 1. Single producer for each partition. > > > > 2. Acks=-1, max.in.flight.request.per.connection=1, > > > retries=SOME_BIG_NUMBER > > > > > > > > The major problem it tries to solve is exact once produce, i.e. solve > > the > > > > duplicates from producer side. In that case, a batch will be > considered > > > as > > > > atomic. The only possibility of a batch got rejected should be it is > > > > already appended. So the producer should just move on. > > > > > > > > It looks to me even a transient multiple producer scenario will cause > > > issue > > > > because user need to think about what should do if a request got > > rejected > > > > and the answer varies for different use cases. > > > > > > > > Thanks, > > > > > > > > Jiangjie (Becket) Qin > > > > > > > > On Sun, Jul 26, 2015 at 11:54 AM, Ben Kirwin <b...@kirw.in> wrote: > > > > > > > > > So I had another look at the 'Idempotent Producer' proposal this > > > > > afternoon, and made a few notes on how I think they compare; if > I've > > > > > made any mistakes, I'd be delighted if someone with more context on > > > > > the idempotent producer design would correct me. > > > > > > > > > > As a first intuition, you can think of the 'conditional publish' > > > > > proposal as the special case of the 'idempotent producer' idea, > where > > > > > there's just a single producer per-partition. The key observation > > here > > > > > is: if there's only one producer, you can conflate the 'sequence > > > > > number' and the expected offset. The conditional publish proposal > > uses > > > > > existing Kafka offset APIs for roughly the same things as the > > > > > idempotent producer proposal uses sequence numbers for -- eg. > instead > > > > > of having a "lease PID" API that returns the current sequence > number, > > > > > we can use the existing 'offset API' to retrieve the upcoming > offset. > > > > > > > > > > Both proposals attempt to deal with the situation where there are > > > > > transiently multiple publishers for the same partition (and PID). > The > > > > > idempotent producer setup tracks a generation id for each pid, and > > > > > discards any writes with a generation id smaller than the latest > > > > > value. Conditional publish is 'first write wins' -- and instead of > > > > > dropping duplicates on the server, it returns an error to the > client. > > > > > The duplicate-handling behaviour (dropping vs. erroring) has some > > > > > interesting consequences: > > > > > > > > > > - If all producers are producing the same stream of messages, > > silently > > > > > dropping duplicates on the server is more convenient. (Suppose we > > have > > > > > a batch of messages 0-9, and the high-water mark on the server is > 7. > > > > > Idempotent producer, as I read it, would append 7-9 to the > partition > > > > > and return success; meanwhile, conditional publish would fail the > > > > > entire batch.) > > > > > > > > > > - If producers might be writing different streams of messages, the > > > > > proposed behaviour of the idempotent producer is probably worse -- > > > > > since it can silently interleave messages from two different > > > > > producers. This can be a problem for some commit-log style > use-cases, > > > > > since it can transform a valid series of operations into an invalid > > > > > one. > > > > > > > > > > - Given the error-on-duplicate behaviour, it's possible to > implement > > > > > deduplication on the client. (Sketch: if a publish returns an error > > > > > for some partition, fetch the upcoming offset / sequence number for > > > > > that partition, and discard all messages with a smaller offset on > the > > > > > client before republishing.) > > > > > > > > > > I think this makes the erroring behaviour more general, though > > > > > deduplicating saves a roundtrip or two at conflict time. > > > > > > > > > > I'm less clear about the behaviour of the generation id, or what > > > > > happens when (say) two producers with the same generation id are > spun > > > > > up at the same time. I'd be interested in hearing other folks' > > > > > comments on this. > > > > > > > > > > Ewen: I'm not sure I understand the questions well enough to answer > > > > > properly, but some quick notes: > > > > > - I don't think it makes sense to assign an expected offset without > > > > > already having assigned a partition. If the producer code is doing > > the > > > > > partition assignment, it should probably do the offset assignment > > > > > too... or we could just let application code handle both. > > > > > - I'm not aware of any case where reassigning offsets to messages > > > > > automatically after an offset mismatch makes sense: in the cases > > we've > > > > > discussed, it seems like either it's safe to drop duplicates, or we > > > > > want to handle the error at the application level. > > > > > > > > > > I'm going to try and come with an idempotent-producer-type example > > > > > that works with the draft patch in the next few days, so hopefully > > > > > we'll have something more concrete to discuss. Otherwise -- if you > > > > > have a clear idea of how eg. sequence number assignment would work > in > > > > > the idempotent-producer proposal, we could probably translate that > > > > > over to get the equivalent for the conditional publish API. > > > > > > > > > > > > > > > > > > > > On Fri, Jul 24, 2015 at 2:16 AM, Ewen Cheslack-Postava > > > > > <e...@confluent.io> wrote: > > > > > > @Becket - for compressed batches, I think this just works out > given > > > the > > > > > KIP > > > > > > as described. Without the change you're referring to, it still > only > > > > makes > > > > > > sense to batch messages with this KIP if all the expected offsets > > are > > > > > > sequential (else some messages are guaranteed to fail). I think > > that > > > > > > probably just works out, but raises an issue I brought up on the > > KIP > > > > > call. > > > > > > > > > > > > Batching can be a bit weird with this proposal. If you try to > write > > > > key A > > > > > > and key B, the second operation is dependent on the first. Which > > > means > > > > to > > > > > > make an effective client for this, we need to keep track of > > > > per-partition > > > > > > offsets so we can set expected offsets properly. For example, if > A > > > was > > > > > > expected to publish at offset 10, then if B was published to the > > same > > > > > > partition, we need to make sure it's marked as expected offset 11 > > > > > (assuming > > > > > > no subpartition high water marks). We either need to have the > > > > application > > > > > > keep track of this itself and set the offsets, which requires > that > > it > > > > > know > > > > > > about how keys map to partitions, or the client needs to manage > > this > > > > > > process. But if the client manages it, I think the client gets > > quite > > > a > > > > > bit > > > > > > more complicated. If the produce request containing A fails, what > > > > happens > > > > > > to B? Are there retries that somehow update the expected offset, > or > > > do > > > > we > > > > > > just give up since we know it's always going to fail with the > > > expected > > > > > > offset that was automatically assigned to it? > > > > > > > > > > > > One way to handle this is to use Yasuhiro's idea of increasing > the > > > > > > granularity of high watermarks using subpartitions. But I guess > my > > > > > question > > > > > > is: if one producer client is writing many keys, and some of > those > > > keys > > > > > are > > > > > > produced to the same partition, and those messages are batched, > > what > > > > > > happens? Do we end up with lots of failed messages? Or do we have > > > > > > complicated logic in the producer to figure out what the right > > > expected > > > > > > offset for each message is? Or do they all share the same base > > > expected > > > > > > offset as in the compressed case, in which case they all share > the > > > same > > > > > > fate and subpartitioning doesn't help? Or is there a simpler > > solution > > > > I'm > > > > > > just not seeing? Maybe this just disables batching entirely and > > > > > throughput > > > > > > isn't an issue in these cases? > > > > > > > > > > > > Sorry, I know that's probably not entirely clear, but that's > > because > > > > I'm > > > > > > very uncertain of how batching works with this KIP. > > > > > > > > > > > > > > > > > > On how this relates to other proposals: I think it might also be > > > > helpful > > > > > to > > > > > > get an overview of all the proposals for relevant modifications > to > > > > > > producers/produce requests since many of these proposals are > > possibly > > > > > > alternatives (though some may not be mutually exclusive). Many > > people > > > > > don't > > > > > > have all the context from the past couple of years of the > project. > > > Are > > > > > > there any other relevant wikis or docs besides the following? > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/Idempotent+Producer > > > > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/Transactional+Messaging+in+Kafka > > > > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Enriched+Message+Metadata > > > > > > > > > > > > -Ewen > > > > > > > > > > > > On Wed, Jul 22, 2015 at 11:18 AM, Gwen Shapira < > > > gshap...@cloudera.com> > > > > > > wrote: > > > > > > > > > > > >> Tangent: I think we should complete the move of Produce / Fetch > > RPC > > > to > > > > > >> the client libraries before we add more revisions to this > > protocol. > > > > > >> > > > > > >> On Wed, Jul 22, 2015 at 11:02 AM, Jiangjie Qin > > > > > >> <j...@linkedin.com.invalid> wrote: > > > > > >> > I missed yesterday's KIP hangout. I'm currently working on > > another > > > > KIP > > > > > >> for > > > > > >> > enriched metadata of messages. Guozhang has already created a > > wiki > > > > > page > > > > > >> > before ( > > > > > >> > > > > > > >> > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Enriched+Message+Metadata > > > > > >> ). > > > > > >> > We plan to fill the relative offset to the offset field in the > > > batch > > > > > sent > > > > > >> > by producer to avoid broker side re-compression. The message > > > offset > > > > > would > > > > > >> > become batch base offset + relative offset. I guess maybe the > > > > expected > > > > > >> > offset in KIP-27 can be only set for base offset? Would that > > > affect > > > > > >> certain > > > > > >> > use cases? > > > > > >> > > > > > > >> > For Jun's comments, I am not sure I completely get it. I think > > the > > > > > >> producer > > > > > >> > only sends one batch per partition in a request. So either > that > > > > batch > > > > > is > > > > > >> > appended or not. Why a batch would be partially committed? > > > > > >> > > > > > > >> > Thanks, > > > > > >> > > > > > > >> > Jiangjie (Becket) Qin > > > > > >> > > > > > > >> > On Tue, Jul 21, 2015 at 10:42 AM, Ben Kirwin <b...@kirw.in> > > wrote: > > > > > >> > > > > > > >> >> That's a fair point. I've added some imagined job logic to > the > > > KIP, > > > > > so > > > > > >> >> we can make sure the proposal stays in sync with the usages > > we're > > > > > >> >> discussing. (The logic is just a quick sketch for now -- I > > expect > > > > > I'll > > > > > >> >> need to elaborate it as we get into more detail, or to > address > > > > other > > > > > >> >> concerns...) > > > > > >> >> > > > > > >> >> On Tue, Jul 21, 2015 at 11:45 AM, Jun Rao <j...@confluent.io> > > > > wrote: > > > > > >> >> > For 1, yes, when there is a transient leader change, it's > > > > > guaranteed > > > > > >> >> that a > > > > > >> >> > prefix of the messages in a request will be committed. > > However, > > > > it > > > > > >> seems > > > > > >> >> > that the client needs to know what subset of messages are > > > > > committed in > > > > > >> >> > order to resume the sending. Then the question is how. > > > > > >> >> > > > > > > >> >> > As Flavio indicated, for the use cases that you listed, it > > > would > > > > be > > > > > >> >> useful > > > > > >> >> > to figure out the exact logic by using this feature. For > > > example, > > > > > in > > > > > >> the > > > > > >> >> > partition K/V store example, when we fail over to a new > > writer > > > to > > > > > the > > > > > >> >> > commit log, the zombie writer can publish new messages to > the > > > log > > > > > >> after > > > > > >> >> the > > > > > >> >> > new writer takes over, but before it publishes any message. > > We > > > > > >> probably > > > > > >> >> > need to outline how this case can be handled properly. > > > > > >> >> > > > > > > >> >> > Thanks, > > > > > >> >> > > > > > > >> >> > Jun > > > > > >> >> > > > > > > >> >> > On Mon, Jul 20, 2015 at 12:05 PM, Ben Kirwin <b...@kirw.in> > > > > wrote: > > > > > >> >> > > > > > > >> >> >> Hi Jun, > > > > > >> >> >> > > > > > >> >> >> Thanks for the close reading! Responses inline. > > > > > >> >> >> > > > > > >> >> >> > Thanks for the write-up. The single producer use case > you > > > > > mentioned > > > > > >> >> makes > > > > > >> >> >> > sense. It would be useful to include that in the KIP > wiki. > > > > > >> >> >> > > > > > >> >> >> Great -- I'll make sure that the wiki is clear about this. > > > > > >> >> >> > > > > > >> >> >> > 1. What happens when the leader of the partition changes > > in > > > > the > > > > > >> middle > > > > > >> >> >> of a > > > > > >> >> >> > produce request? In this case, the producer client is > not > > > sure > > > > > >> whether > > > > > >> >> >> the > > > > > >> >> >> > request succeeds or not. If there is only a single > message > > > in > > > > > the > > > > > >> >> >> request, > > > > > >> >> >> > the producer can just resend the request. If it sees an > > > > > >> OffsetMismatch > > > > > >> >> >> > error, it knows that the previous send actually > succeeded > > > and > > > > > can > > > > > >> >> proceed > > > > > >> >> >> > with the next write. This is nice since it not only > allows > > > the > > > > > >> >> producer > > > > > >> >> >> to > > > > > >> >> >> > proceed during transient failures in the broker, it also > > > > avoids > > > > > >> >> >> duplicates > > > > > >> >> >> > during producer resend. One caveat is when there are > > > multiple > > > > > >> >> messages in > > > > > >> >> >> > the same partition in a produce request. The issue is > that > > > in > > > > > our > > > > > >> >> current > > > > > >> >> >> > replication protocol, it's possible for some, but not > all > > > > > messages > > > > > >> in > > > > > >> >> the > > > > > >> >> >> > request to be committed. This makes resend a bit harder > to > > > > deal > > > > > >> with > > > > > >> >> >> since > > > > > >> >> >> > on receiving an OffsetMismatch error, it's not clear > which > > > > > messages > > > > > >> >> have > > > > > >> >> >> > been committed. One possibility is to expect that > > > compression > > > > is > > > > > >> >> enabled, > > > > > >> >> >> > in which case multiple messages are compressed into a > > single > > > > > >> message. > > > > > >> >> I > > > > > >> >> >> was > > > > > >> >> >> > thinking that another possibility is for the broker to > > > return > > > > > the > > > > > >> >> current > > > > > >> >> >> > high watermark when sending an OffsetMismatch error. > Based > > > on > > > > > this > > > > > >> >> info, > > > > > >> >> >> > the producer can resend the subset of messages that have > > not > > > > > been > > > > > >> >> >> > committed. However, this may not work in a compacted > topic > > > > since > > > > > >> there > > > > > >> >> >> can > > > > > >> >> >> > be holes in the offset. > > > > > >> >> >> > > > > > >> >> >> This is a excellent question. It's my understanding that > at > > > > least > > > > > a > > > > > >> >> >> *prefix* of messages will be committed (right?) -- which > > seems > > > > to > > > > > be > > > > > >> >> >> enough for many cases. I'll try and come up with a more > > > concrete > > > > > >> >> >> answer here. > > > > > >> >> >> > > > > > >> >> >> > 2. Is this feature only intended to be used with ack = > > all? > > > > The > > > > > >> client > > > > > >> >> >> > doesn't get the offset with ack = 0. With ack = 1, it's > > > > possible > > > > > >> for a > > > > > >> >> >> > previously acked message to be lost during leader > > > transition, > > > > > which > > > > > >> >> will > > > > > >> >> >> > make the client logic more complicated. > > > > > >> >> >> > > > > > >> >> >> It's true that acks = 0 doesn't seem to be particularly > > > useful; > > > > in > > > > > >> all > > > > > >> >> >> the cases I've come across, the client eventually wants to > > > know > > > > > about > > > > > >> >> >> the mismatch error. However, it seems like there are some > > > cases > > > > > where > > > > > >> >> >> acks = 1 would be fine -- eg. in a bulk load of a fixed > > > dataset, > > > > > >> >> >> losing messages during a leader transition just means you > > need > > > > to > > > > > >> >> >> rewind / restart the load, which is not especially > > > catastrophic. > > > > > For > > > > > >> >> >> many other interesting cases, acks = all is probably > > > preferable. > > > > > >> >> >> > > > > > >> >> >> > 3. How does the producer client know the offset to send > > the > > > > > first > > > > > >> >> >> message? > > > > > >> >> >> > Do we need to expose an API in producer to get the > current > > > > high > > > > > >> >> >> watermark? > > > > > >> >> >> > > > > > >> >> >> You're right, it might be irritating to have to go through > > the > > > > > >> >> >> consumer API just for this. There are some cases where the > > > > offsets > > > > > >> are > > > > > >> >> >> already available -- like the commit-log-for-KV-store > > example > > > -- > > > > > but > > > > > >> >> >> in general, being able to get the offsets from the > producer > > > > > interface > > > > > >> >> >> does sound convenient. > > > > > >> >> >> > > > > > >> >> >> > We plan to have a KIP discussion meeting tomorrow at > 11am > > > PST. > > > > > >> Perhaps > > > > > >> >> >> you > > > > > >> >> >> > can describe this KIP a bit then? > > > > > >> >> >> > > > > > >> >> >> Sure, happy to join. > > > > > >> >> >> > > > > > >> >> >> > Thanks, > > > > > >> >> >> > > > > > > >> >> >> > Jun > > > > > >> >> >> > > > > > > >> >> >> > > > > > > >> >> >> > > > > > > >> >> >> > On Sat, Jul 18, 2015 at 10:37 AM, Ben Kirwin < > b...@kirw.in > > > > > > > > wrote: > > > > > >> >> >> > > > > > > >> >> >> >> Just wanted to flag a little discussion that happened > on > > > the > > > > > >> ticket: > > > > > >> >> >> >> > > > > > >> >> >> >> > > > > > >> >> >> > > > > > >> >> > > > > > >> > > > > > > > > > > > > > > > https://issues.apache.org/jira/browse/KAFKA-2260?focusedCommentId=14632259&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14632259 > > > > > >> >> >> >> > > > > > >> >> >> >> In particular, Yasuhiro Matsuda proposed an interesting > > > > > variant on > > > > > >> >> >> >> this that performs the offset check on the message key > > > > > (instead of > > > > > >> >> >> >> just the partition), with bounded space requirements, > at > > > the > > > > > cost > > > > > >> of > > > > > >> >> >> >> potentially some spurious failures. (ie. the produce > > > request > > > > > may > > > > > >> fail > > > > > >> >> >> >> even if that particular key hasn't been updated > > recently.) > > > > This > > > > > >> >> >> >> addresses a couple of the drawbacks of the per-key > > approach > > > > > >> mentioned > > > > > >> >> >> >> at the bottom of the KIP. > > > > > >> >> >> >> > > > > > >> >> >> >> On Fri, Jul 17, 2015 at 6:47 PM, Ben Kirwin < > b...@kirw.in > > > > > > > > wrote: > > > > > >> >> >> >> > Hi all, > > > > > >> >> >> >> > > > > > > >> >> >> >> > So, perhaps it's worth adding a couple specific > > examples > > > of > > > > > >> where > > > > > >> >> this > > > > > >> >> >> >> > feature is useful, to make this a bit more concrete: > > > > > >> >> >> >> > > > > > > >> >> >> >> > - Suppose I'm using Kafka as a commit log for a > > > partitioned > > > > > KV > > > > > >> >> store, > > > > > >> >> >> >> > like Samza or Pistachio (?) do. We bootstrap the > > process > > > > > state > > > > > >> by > > > > > >> >> >> >> > reading from that partition, and log all state > updates > > to > > > > > that > > > > > >> >> >> >> > partition when we're running. Now imagine that one of > > my > > > > > >> processes > > > > > >> >> >> >> > locks up -- GC or similar -- and the system > transitions > > > > that > > > > > >> >> partition > > > > > >> >> >> >> > over to another node. When the GC is finished, the > old > > > > > 'owner' > > > > > >> of > > > > > >> >> that > > > > > >> >> >> >> > partition might still be trying to write to the > commit > > > log > > > > at > > > > > >> the > > > > > >> >> same > > > > > >> >> >> >> > as the new one is. A process might detect this by > > > noticing > > > > > that > > > > > >> the > > > > > >> >> >> >> > offset of the published message is bigger than it > > thought > > > > the > > > > > >> >> upcoming > > > > > >> >> >> >> > offset was, which implies someone else has been > writing > > > to > > > > > the > > > > > >> >> log... > > > > > >> >> >> >> > but by then it's too late, and the commit log is > > already > > > > > >> corrupt. > > > > > >> >> With > > > > > >> >> >> >> > a 'conditional produce', one of those processes will > > have > > > > > it's > > > > > >> >> publish > > > > > >> >> >> >> > request refused -- so we've avoided corrupting the > > state. > > > > > >> >> >> >> > > > > > > >> >> >> >> > - Envision some copycat-like system, where we have > some > > > > > sharded > > > > > >> >> >> >> > postgres setup and we're tailing each shard into its > > own > > > > > >> partition. > > > > > >> >> >> >> > Normally, it's fairly easy to avoid duplicates here: > we > > > can > > > > > >> track > > > > > >> >> >> >> > which offset in the WAL corresponds to which offset > in > > > > Kafka, > > > > > >> and > > > > > >> >> we > > > > > >> >> >> >> > know how many messages we've written to Kafka > already, > > so > > > > the > > > > > >> >> state is > > > > > >> >> >> >> > very simple. However, it is possible that for a > moment > > -- > > > > > due to > > > > > >> >> >> >> > rebalancing or operator error or some other thing -- > > two > > > > > >> different > > > > > >> >> >> >> > nodes are tailing the same postgres shard at once! > > > Normally > > > > > this > > > > > >> >> would > > > > > >> >> >> >> > introduce duplicate messages, but by specifying the > > > > expected > > > > > >> >> offset, > > > > > >> >> >> >> > we can avoid this. > > > > > >> >> >> >> > > > > > > >> >> >> >> > So perhaps it's better to say that this is useful > when > > a > > > > > single > > > > > >> >> >> >> > producer is *expected*, but multiple producers are > > > > > *possible*? > > > > > >> (In > > > > > >> >> the > > > > > >> >> >> >> > same way that the high-level consumer normally has 1 > > > > consumer > > > > > >> in a > > > > > >> >> >> >> > group reading from a partition, but there are small > > > windows > > > > > >> where > > > > > >> >> more > > > > > >> >> >> >> > than one might be reading at the same time.) This is > > also > > > > the > > > > > >> >> spirit > > > > > >> >> >> >> > of the 'runtime cost' comment -- in the common case, > > > where > > > > > >> there is > > > > > >> >> >> >> > little to no contention, there's no performance > > overhead > > > > > >> either. I > > > > > >> >> >> >> > mentioned this a little in the Motivation section -- > > > maybe > > > > I > > > > > >> should > > > > > >> >> >> >> > flesh that out a little bit? > > > > > >> >> >> >> > > > > > > >> >> >> >> > For me, the motivation to work this up was that I > kept > > > > > running > > > > > >> into > > > > > >> >> >> >> > cases, like the above, where the existing API was > > > > > >> >> almost-but-not-quite > > > > > >> >> >> >> > enough to give the guarantees I was looking for -- > and > > > the > > > > > >> >> extension > > > > > >> >> >> >> > needed to handle those cases too was pretty small and > > > > > >> >> natural-feeling. > > > > > >> >> >> >> > > > > > > >> >> >> >> > On Fri, Jul 17, 2015 at 4:49 PM, Ashish Singh < > > > > > >> asi...@cloudera.com > > > > > >> >> > > > > > > >> >> >> >> wrote: > > > > > >> >> >> >> >> Good concept. I have a question though. > > > > > >> >> >> >> >> > > > > > >> >> >> >> >> Say there are two producers A and B. Both producers > > are > > > > > >> producing > > > > > >> >> to > > > > > >> >> >> >> same > > > > > >> >> >> >> >> partition. > > > > > >> >> >> >> >> - A sends a message with expected offset, x1 > > > > > >> >> >> >> >> - Broker accepts is and sends an Ack > > > > > >> >> >> >> >> - B sends a message with expected offset, x1 > > > > > >> >> >> >> >> - Broker rejects it, sends nack > > > > > >> >> >> >> >> - B sends message again with expected offset, x1+1 > > > > > >> >> >> >> >> - Broker accepts it and sends Ack > > > > > >> >> >> >> >> I guess this is what this KIP suggests, right? If > yes, > > > > then > > > > > how > > > > > >> >> does > > > > > >> >> >> >> this > > > > > >> >> >> >> >> ensure that same message will not be written twice > > when > > > > two > > > > > >> >> producers > > > > > >> >> >> >> are > > > > > >> >> >> >> >> producing to same partition? Producer on receiving a > > > nack > > > > > will > > > > > >> try > > > > > >> >> >> again > > > > > >> >> >> >> >> with next offset and will keep doing so till the > > message > > > > is > > > > > >> >> accepted. > > > > > >> >> >> >> Am I > > > > > >> >> >> >> >> missing something? > > > > > >> >> >> >> >> > > > > > >> >> >> >> >> Also, you have mentioned on KIP, "it imposes little > to > > > no > > > > > >> runtime > > > > > >> >> >> cost > > > > > >> >> >> >> in > > > > > >> >> >> >> >> memory or time", I think that is not true for time. > > With > > > > > this > > > > > >> >> >> approach > > > > > >> >> >> >> >> producers' performance will reduce proportionally to > > > > number > > > > > of > > > > > >> >> >> producers > > > > > >> >> >> >> >> writing to same partition. Please correct me if I am > > > > missing > > > > > >> out > > > > > >> >> >> >> something. > > > > > >> >> >> >> >> > > > > > >> >> >> >> >> > > > > > >> >> >> >> >> On Fri, Jul 17, 2015 at 11:32 AM, Mayuresh Gharat < > > > > > >> >> >> >> >> gharatmayures...@gmail.com> wrote: > > > > > >> >> >> >> >> > > > > > >> >> >> >> >>> If we have 2 producers producing to a partition, > they > > > can > > > > > be > > > > > >> out > > > > > >> >> of > > > > > >> >> >> >> order, > > > > > >> >> >> >> >>> then how does one producer know what offset to > expect > > > as > > > > it > > > > > >> does > > > > > >> >> not > > > > > >> >> >> >> >>> interact with other producer? > > > > > >> >> >> >> >>> > > > > > >> >> >> >> >>> Can you give an example flow that explains how it > > works > > > > > with > > > > > >> >> single > > > > > >> >> >> >> >>> producer and with multiple producers? > > > > > >> >> >> >> >>> > > > > > >> >> >> >> >>> > > > > > >> >> >> >> >>> Thanks, > > > > > >> >> >> >> >>> > > > > > >> >> >> >> >>> Mayuresh > > > > > >> >> >> >> >>> > > > > > >> >> >> >> >>> On Fri, Jul 17, 2015 at 10:28 AM, Flavio Junqueira > < > > > > > >> >> >> >> >>> fpjunque...@yahoo.com.invalid> wrote: > > > > > >> >> >> >> >>> > > > > > >> >> >> >> >>> > I like this feature, it reminds me of conditional > > > > > updates in > > > > > >> >> >> >> zookeeper. > > > > > >> >> >> >> >>> > I'm not sure if it'd be best to have some > mechanism > > > for > > > > > >> fencing > > > > > >> >> >> >> rather > > > > > >> >> >> >> >>> than > > > > > >> >> >> >> >>> > a conditional write like you're proposing. The > > reason > > > > I'm > > > > > >> >> saying > > > > > >> >> >> >> this is > > > > > >> >> >> >> >>> > that the conditional write applies to requests > > > > > individually, > > > > > >> >> >> while it > > > > > >> >> >> >> >>> > sounds like you want to make sure that there is a > > > > single > > > > > >> client > > > > > >> >> >> >> writing > > > > > >> >> >> >> >>> so > > > > > >> >> >> >> >>> > over multiple requests. > > > > > >> >> >> >> >>> > > > > > > >> >> >> >> >>> > -Flavio > > > > > >> >> >> >> >>> > > > > > > >> >> >> >> >>> > > On 17 Jul 2015, at 07:30, Ben Kirwin < > > b...@kirw.in> > > > > > wrote: > > > > > >> >> >> >> >>> > > > > > > > >> >> >> >> >>> > > Hi there, > > > > > >> >> >> >> >>> > > > > > > > >> >> >> >> >>> > > I just added a KIP for a 'conditional publish' > > > > > operation: > > > > > >> a > > > > > >> >> >> simple > > > > > >> >> >> >> >>> > > CAS-like mechanism for the Kafka producer. The > > wiki > > > > > page > > > > > >> is > > > > > >> >> >> here: > > > > > >> >> >> >> >>> > > > > > > > >> >> >> >> >>> > > > > > > > >> >> >> >> >>> > > > > > > >> >> >> >> >>> > > > > > >> >> >> >> > > > > > >> >> >> > > > > > >> >> > > > > > >> > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-27+-+Conditional+Publish > > > > > >> >> >> >> >>> > > > > > > > >> >> >> >> >>> > > And there's some previous discussion on the > > ticket > > > > and > > > > > the > > > > > >> >> users > > > > > >> >> >> >> list: > > > > > >> >> >> >> >>> > > > > > > > >> >> >> >> >>> > > > https://issues.apache.org/jira/browse/KAFKA-2260 > > > > > >> >> >> >> >>> > > > > > > > >> >> >> >> >>> > > > > > > > >> >> >> >> >>> > > > > > > >> >> >> >> >>> > > > > > >> >> >> >> > > > > > >> >> >> > > > > > >> >> > > > > > >> > > > > > > > > > > > > > > > https://mail-archives.apache.org/mod_mbox/kafka-users/201506.mbox/%3CCAAeOB6ccyAA13YNPqVQv2o-mT5r=c9v7a+55sf2wp93qg7+...@mail.gmail.com%3E > > > > > >> >> >> >> >>> > > > > > > > >> >> >> >> >>> > > As always, comments and suggestions are very > > > welcome. > > > > > >> >> >> >> >>> > > > > > > > >> >> >> >> >>> > > Thanks, > > > > > >> >> >> >> >>> > > Ben > > > > > >> >> >> >> >>> > > > > > > >> >> >> >> >>> > > > > > > >> >> >> >> >>> > > > > > >> >> >> >> >>> > > > > > >> >> >> >> >>> -- > > > > > >> >> >> >> >>> -Regards, > > > > > >> >> >> >> >>> Mayuresh R. Gharat > > > > > >> >> >> >> >>> (862) 250-7125 > > > > > >> >> >> >> >>> > > > > > >> >> >> >> >> > > > > > >> >> >> >> >> > > > > > >> >> >> >> >> > > > > > >> >> >> >> >> -- > > > > > >> >> >> >> >> > > > > > >> >> >> >> >> Regards, > > > > > >> >> >> >> >> Ashish > > > > > >> >> >> >> > > > > > >> >> >> > > > > > >> >> > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > Thanks, > > > > > > Ewen > > > > > > > > > > > > > > >
