Hi Radai, Thanks for your email. You raise some good points.
Your larger point centers on your --correct!-- observation that persisting messages belonging to aborted transactions causes complications for consumers. You are right in observing that the current proposal does require consumers to buffer, though we can make that efficient by storing only the 'start offsets' of transactions and then replaying the log once we hit the corresponding commit/abort markers. And yes, it does mean that in some consumer modes, we will deliver messages belonging to aborted transactions. As such, I would really like to understand the details of your alternative to see if we can address these issues efficiently. When you say that you will write to the main partition from the 'sideways' transaction partition atomically, do you literally mean that a sequence of individual messages from the 'sideways' partition will be written to the main partition atomically? If the 'sideways partition' is written to disk, you have two copies of data. I would like to know how you would ensure that you do not create duplicates in the event of crashes. And if it is in-mem, you are now buffering anyway, on all replicas. And, in the absence of any markers in the log, how would you ensure that the batch of messages in a committed transaction is delivered to the consumer all together? In the current proposal, the return of a 'poll' would include all the messages of a transaction, or none of them. * * Re: 3:: you ask about the migration plan. This has been detailed in the design doc <https://docs.google.com/document/d/11Jqy_GjUGtdXJK94XGsEIK7CP1SnQGdp2eF0wSw9ra8/edit#bookmark=id.3s6zgsaq9cep>. In particular, the recommended sequence is to bump all your clients before bumping up the message format. Further, since transactions are entirely opt-in, it is not unreasonable to ensure that the ecosystem is ready for them before starting to use them. * * Thanks, Apurva On Thu, Dec 15, 2016 at 3:07 PM, radai <radai.rosenbl...@gmail.com> wrote: > some clarifications on my alternative proposal: > > TX msgs are written "sideways" to a transaction (ad-hoc) partition. this > partition can be replicated to followers, or can be an in-mem buffer - > depends on the resilience guarantees you want to provide for TXs in case of > broker crash. > on "commit" the partition leader broker (being the single point of > synchronization for the partition anyway) can atomically append the > contents of this TX "partition" onto the real target partition. this is the > point where the msgs get "real" offsets. there's some trickiness around how > not to expose these offsets to any consumers until everything's been > replicated to followers, but we believe its possible. > > > > On Thu, Dec 15, 2016 at 2:31 PM, radai <radai.rosenbl...@gmail.com> wrote: > > > I can see several issues with the current proposal. > > > > messages, even if sent under a TX, are delivered directly to their > > destination partitions, downstream consumers need to be TX-aware. they > can > > either: > > 1. be oblivious to TXs. that means they will deliver "garbage" - msgs > > sent during eventually-aborted TXs. > > 2. "opt-in" - they would have to not deliver _ANY_ msg until they know > > the fate of all outstanding overlapping TXs - if i see msg A1 (in a TX), > > followed by B, which is not under any TX, i cannot deliver B until i know > > if A1 was committed or not (or I violate ordering). this would require > some > > sort of buffering on consumers. with a naive buffering impl i could DOS > > everyone on a topic - just start a TX on a very busy topic and keep it > open > > as long as I can .... > > 3. explode if youre an old consumer that sees a control msg (whats > your > > migration plan?) > > 4. cross-cluster replication mechanisms either replicate the garbage > or > > need to clean it up. there are >1 such different mechanism (almost one > per > > company really :-) ) so lots of adjustments. > > > > I think the end result could be better if ongoing TXs are treated as > > logically separate topic partitions, and only atomically appended onto > the > > target partitions on commit (meaning they are written to separate journal > > file(s) on the broker). > > > > such a design would present a "clean" view to any downstream consumers - > > anything not committed wont even show up. old consumers wont need to know > > about control msgs, no issues with unbounded msg buffering, generally > > cleaner overall? > > > > there would need to be adjustments made to watermark and follower fetch > > logic but some of us here have discussed this over lunch and we think its > > doable. > > > > > > On Thu, Dec 15, 2016 at 1:08 AM, Rajini Sivaram <rsiva...@pivotal.io> > > wrote: > > > >> Hi Apurva, > >> > >> Thank you, makes sense. > >> > >> Rajini > >> > >> On Wed, Dec 14, 2016 at 7:36 PM, Apurva Mehta <apu...@confluent.io> > >> wrote: > >> > >> > Hi Rajini, > >> > > >> > I think my original response to your point 15 was not accurate. The > >> regular > >> > definition of durability is that data once committed would never be > >> lost. > >> > So it is not enough for only the control messages to be flushed before > >> > being acknowledged -- all the messages (and offset commits) which are > >> part > >> > of the transaction would need to be flushed before being acknowledged > as > >> > well. > >> > > >> > Otherwise, it is possible that if all replicas of a topic partition > >> crash > >> > before the transactional messages are flushed, those messages will be > >> lost > >> > even if the commit marker exists in the log. In this case, the > >> transaction > >> > would be 'committed' with incomplete data. > >> > > >> > Right now, there isn't any config which will ensure that the flush to > >> disk > >> > happens before the acknowledgement. We could add it in the future, and > >> get > >> > durability guarantees for kafka transactions. > >> > > >> > I hope this clarifies the situation. The present KIP does not intend > to > >> add > >> > the aforementioned config, so even the control messages are > susceptible > >> to > >> > being lost if there is a simultaneous crash across all replicas. So > >> > transactions are only as durable as existing Kafka messages. We don't > >> > strengthen any durability guarantees as part of this KIP. > >> > > >> > Thanks, > >> > Apurva > >> > > >> > > >> > On Wed, Dec 14, 2016 at 1:52 AM, Rajini Sivaram <rsiva...@pivotal.io> > >> > wrote: > >> > > >> > > Hi Apurva, > >> > > > >> > > Thank you for the answers. Just one follow-on. > >> > > > >> > > 15. Let me rephrase my original question. If all control messages > >> > (messages > >> > > to transaction logs and markers on user logs) were acknowledged only > >> > after > >> > > flushing the log segment, will transactions become durable in the > >> > > traditional sense (i.e. not restricted to min.insync.replicas > >> failures) ? > >> > > This is not a suggestion to update the KIP. It seems to me that the > >> > design > >> > > enables full durability if required in the future with a rather > >> > > non-intrusive change. I just wanted to make sure I haven't missed > >> > anything > >> > > fundamental that prevents Kafka from doing this. > >> > > > >> > > > >> > > > >> > > On Wed, Dec 14, 2016 at 5:30 AM, Ben Kirwin <b...@kirw.in> wrote: > >> > > > >> > > > Hi Apurva, > >> > > > > >> > > > Thanks for the detailed answers... and sorry for the late reply! > >> > > > > >> > > > It does sound like, if the input-partitions-to-app-id mapping > never > >> > > > changes, the existing fencing mechanisms should prevent > duplicates. > >> > > Great! > >> > > > I'm a bit concerned the proposed API will be delicate to program > >> > against > >> > > > successfully -- even in the simple case, we need to create a new > >> > producer > >> > > > instance per input partition, and anything fancier is going to > need > >> its > >> > > own > >> > > > implementation of the Streams/Samza-style 'task' idea -- but that > >> may > >> > be > >> > > > fine for this sort of advanced feature. > >> > > > > >> > > > For the second question, I notice that Jason also elaborated on > this > >> > > > downthread: > >> > > > > >> > > > > We also looked at removing the producer ID. > >> > > > > This was discussed somewhere above, but basically the idea is to > >> > store > >> > > > the > >> > > > > AppID in the message set header directly and avoid the mapping > to > >> > > > producer > >> > > > > ID altogether. As long as batching isn't too bad, the impact on > >> total > >> > > > size > >> > > > > may not be too bad, but we were ultimately more comfortable > with a > >> > > fixed > >> > > > > size ID. > >> > > > > >> > > > ...which suggests that the distinction is useful for performance, > >> but > >> > not > >> > > > necessary for correctness, which makes good sense to me. (Would a > >> > 128-bid > >> > > > ID be a reasonable compromise? That's enough room for a UUID, or a > >> > > > reasonable hash of an arbitrary string, and has only a marginal > >> > increase > >> > > on > >> > > > the message size.) > >> > > > > >> > > > On Tue, Dec 6, 2016 at 11:44 PM, Apurva Mehta < > apu...@confluent.io> > >> > > wrote: > >> > > > > >> > > > > Hi Ben, > >> > > > > > >> > > > > Now, on to your first question of how deal with consumer > >> rebalances. > >> > > The > >> > > > > short answer is that the application needs to ensure that the > the > >> > > > > assignment of input partitions to appId is consistent across > >> > > rebalances. > >> > > > > > >> > > > > For Kafka streams, they already ensure that the mapping of input > >> > > > partitions > >> > > > > to task Id is invariant across rebalances by implementing a > custom > >> > > sticky > >> > > > > assignor. Other non-streams apps can trivially have one producer > >> per > >> > > > input > >> > > > > partition and have the appId be the same as the partition number > >> to > >> > > > achieve > >> > > > > the same effect. > >> > > > > > >> > > > > With this precondition in place, we can maintain transactions > >> across > >> > > > > rebalances. > >> > > > > > >> > > > > Hope this answers your question. > >> > > > > > >> > > > > Thanks, > >> > > > > Apurva > >> > > > > > >> > > > > On Tue, Dec 6, 2016 at 3:22 PM, Ben Kirwin <b...@kirw.in> wrote: > >> > > > > > >> > > > > > Thanks for this! I'm looking forward to going through the full > >> > > proposal > >> > > > > in > >> > > > > > detail soon; a few early questions: > >> > > > > > > >> > > > > > First: what happens when a consumer rebalances in the middle > of > >> a > >> > > > > > transaction? The full documentation suggests that such a > >> > transaction > >> > > > > ought > >> > > > > > to be rejected: > >> > > > > > > >> > > > > > > [...] if a rebalance has happened and this consumer > >> > > > > > > instance becomes a zombie, even if this offset message is > >> > appended > >> > > in > >> > > > > the > >> > > > > > > offset topic, the transaction will be rejected later on when > >> it > >> > > tries > >> > > > > to > >> > > > > > > commit the transaction via the EndTxnRequest. > >> > > > > > > >> > > > > > ...but it's unclear to me how we ensure that a transaction > can't > >> > > > complete > >> > > > > > if a rebalance has happened. (It's quite possible I'm missing > >> > > something > >> > > > > > obvious!) > >> > > > > > > >> > > > > > As a concrete example: suppose a process with PID 1 adds > offsets > >> > for > >> > > > some > >> > > > > > partition to a transaction; a consumer rebalance happens that > >> > assigns > >> > > > the > >> > > > > > partition to a process with PID 2, which adds some offsets to > >> its > >> > > > current > >> > > > > > transaction; both processes try and commit. Allowing both > >> commits > >> > > would > >> > > > > > cause the messages to be processed twice -- how is that > avoided? > >> > > > > > > >> > > > > > Second: App IDs normally map to a single PID. It seems like > one > >> > could > >> > > > do > >> > > > > > away with the PID concept entirely, and just use App IDs in > most > >> > > places > >> > > > > > that require a PID. This feels like it would be significantly > >> > > simpler, > >> > > > > > though it does increase the message size. Are there other > >> reasons > >> > why > >> > > > the > >> > > > > > App ID / PID split is necessary? > >> > > > > > > >> > > > > > On Wed, Nov 30, 2016 at 2:19 PM, Guozhang Wang < > >> wangg...@gmail.com > >> > > > >> > > > > wrote: > >> > > > > > > >> > > > > > > Hi all, > >> > > > > > > > >> > > > > > > I have just created KIP-98 to enhance Kafka with exactly > once > >> > > > delivery > >> > > > > > > semantics: > >> > > > > > > > >> > > > > > > *https://cwiki.apache.org/confluence/display/KAFKA/KIP- > >> > > > > > > 98+-+Exactly+Once+Delivery+and+Transactional+Messaging > >> > > > > > > <https://cwiki.apache.org/confluence/display/KAFKA/KIP- > >> > > > > > > 98+-+Exactly+Once+Delivery+and+Transactional+Messaging>* > >> > > > > > > > >> > > > > > > This KIP adds a transactional messaging mechanism along with > >> an > >> > > > > > idempotent > >> > > > > > > producer implementation to make sure that 1) duplicated > >> messages > >> > > sent > >> > > > > > from > >> > > > > > > the same identified producer can be detected on the broker > >> side, > >> > > and > >> > > > > 2) a > >> > > > > > > group of messages sent within a transaction will atomically > be > >> > > either > >> > > > > > > reflected and fetchable to consumers or not as a whole. > >> > > > > > > > >> > > > > > > The above wiki page provides a high-level view of the > proposed > >> > > > changes > >> > > > > as > >> > > > > > > well as summarized guarantees. Initial draft of the detailed > >> > > > > > implementation > >> > > > > > > design is described in this Google doc: > >> > > > > > > > >> > > > > > > https://docs.google.com/document/d/11Jqy_ > >> > > > > GjUGtdXJK94XGsEIK7CP1SnQGdp2eF > >> > > > > > > 0wSw9ra8 > >> > > > > > > > >> > > > > > > > >> > > > > > > We would love to hear your comments and suggestions. > >> > > > > > > > >> > > > > > > Thanks, > >> > > > > > > > >> > > > > > > -- Guozhang > >> > > > > > > > >> > > > > > > >> > > > > > >> > > > > >> > > > >> > > >> > > > > >
