Hi Artem, It has been a while, but I have gotten back to this. I understand that when 2PC is used, the transaction timeout will be effectively infinite. I don't think that this behavior is desirable. A long running transaction can be extremely disruptive since it blocks consumers on any partitions written to within the pending transaction. The primary reason for a long running transaction is a failure of the client, or the network connecting the client to the broker. If such a failure occurs before the client calls the new prepareTransaction() method, it should be OK to abort the transaction after a relatively short timeout period. This approach would minimize the inconvenience and disruption of a long running transaction blocking consumers, and provide higher availability for a system using Kafka.
In order to achieve this behavior, I think we would need a 'prepare' RPC call so that the server knows that a transaction has been prepared, and does not timeout and abort such transactions. There will be some cost to this extra RPC call, but there will also be a benefit of better system availability in case of failures. There is another reason why I would prefer this implementation. I am working on an XA KIP, and XA requires that Kafka brokers be able to provide a list of prepared transactions during recovery. The broker can only know that a transaction has been prepared if an RPC call is made., so my KIP will need this functionality. In the XA KIP, I would like to use as much of the KIP-939 solution as possible, so it would be helpful if prepareTransactions() sent a 'prepare' RPC, and the broker recorded the prepared transaction state. This could be made configurable behavior if we are concerned that the cost of the extra RPC call is too much, and that some users would prefer to have speed in exchange for less system availability in some cases of client or network failure. Let me know what you think. -Rowland On Fri, Jan 5, 2024 at 8:03 PM Artem Livshits <alivsh...@confluent.io.invalid> wrote: > Hi Rowland, > > Thank you for the feedback. For the 2PC cases, the expectation is that the > timeout on the client would be set to "effectively infinite", that would > exceed all practical 2PC delays. Now I think that this flexibility is > confusing and can be misused, I have updated the KIP to just say that if > 2PC is used, the transaction never expires. > > -Artem > > On Thu, Jan 4, 2024 at 6:14 PM Rowland Smith <rowl...@gmail.com> wrote: > > > It is probably me. I copied the original message subject into a new > email. > > Perhaps that is not enough to link them. > > > > It was not my understanding from reading KIP-939 that we are doing away > > with any transactional timeout in the Kafka broker. As I understand it, > we > > are allowing the application to set the transaction timeout to a value > that > > exceeds the *transaction.max.timeout.ms > > <http://transaction.max.timeout.ms>* setting > > on the broker, and having no timeout if the application does not set > > *transaction.timeout.ms > > <http://transaction.timeout.ms>* on the producer. The KIP says that the > > semantics of *transaction.timeout.ms <http://transaction.timeout.ms>* > are > > not being changed, so I take that to mean that the broker will continue > to > > enforce a timeout if provided, and abort transactions that exceed it. > From > > the KIP: > > > > Client Configuration Changes > > > > *transaction.two.phase.commit.enable* The default would be ‘false’. If > set > > to ‘true’, then the broker is informed that the client is participating > in > > two phase commit protocol and can set transaction timeout to values that > > exceed *transaction.max.timeout.ms <http://transaction.max.timeout.ms>* > > setting > > on the broker (if the timeout is not set explicitly on the client and the > > two phase commit is set to ‘true’ then the transaction never expires). > > > > *transaction.timeout.ms <http://transaction.timeout.ms>* The semantics > is > > not changed, but it can be set to values that exceed > > *transaction.max.timeout.ms > > <http://transaction.max.timeout.ms>* if two.phase.commit.enable is set > to > > ‘true’. > > > > > > Thinking about this more I believe we would also have a possible race > > condition if the broker is unaware that a transaction has been prepared. > > The application might call prepare and get a positive response, but the > > broker might have already aborted the transaction for exceeding the > > timeout. It is a general rule of 2PC that once a transaction has been > > prepared it must be possible for it to be committed or aborted. It seems > in > > this case a prepared transaction might already be aborted by the broker, > so > > it would be impossible to commit. > > > > I hope this is making sense and I am not misunderstanding the KIP. Please > > let me know if I am. > > > > - Rowland > > > > > > On Thu, Jan 4, 2024 at 12:56 PM Justine Olshan > > <jols...@confluent.io.invalid> > > wrote: > > > > > Hey Rowland, > > > > > > Not sure why this message showed up in a different thread from the > other > > > KIP-939 discussion (is it just me?) > > > > > > In KIP-939, we do away with having any transactional timeout on the > Kafka > > > side. The external coordinator is fully responsible for controlling > > whether > > > the transaction completes. > > > > > > While I think there is some use in having a prepare stage, I just > wanted > > to > > > clarify what the current KIP is proposing. > > > > > > Thanks, > > > Justine > > > > > > On Wed, Jan 3, 2024 at 7:49 PM Rowland Smith <rowl...@gmail.com> > wrote: > > > > > > > Hi Artem, > > > > > > > > I saw your response in the thread I started discussing Kafka > > distributed > > > > transaction support and the XA interface. I would like to work with > you > > > to > > > > add XA support to Kafka on top of the excellent foundational work > that > > > you > > > > have started with KIP-939. I agree that explicit XA support should > not > > be > > > > included in the Kafka codebase as long as the right set of basic > > > operations > > > > are provided. I will begin pulling together a KIP to follow KIP-939. > > > > > > > > I did have one comment on KIP-939 itself. I see that you considered > an > > > > explicit "prepare" RPC, but decided not to add it. If I understand > your > > > > design correctly, that would mean that a 2PC transaction would have a > > > > single timeout that would need to be long enough to ensure that > > prepared > > > > transactions are not aborted when an external coordinator fails. > > However, > > > > this also means that an unprepared transaction would not be aborted > > > without > > > > waiting for the same timeout. Since long running transactions block > > > > transactional consumers, having a long timeout for all transactions > > could > > > > be disruptive. An explicit "prepare " RPC would allow the server to > > abort > > > > unprepared transactions after a relatively short timeout, and apply a > > > much > > > > longer timeout only to prepared transactions. The explicit "prepare" > > RPC > > > > would make Kafka server more resilient to client failure at the cost > of > > > an > > > > extra synchronous RPC call. I think its worth reconsidering this. > > > > > > > > With an XA implementation this might become a more significant issue > > > since > > > > the transaction coordinator has no memory of unprepared transactions > > > across > > > > restarts. Such transactions would need to be cleared by hand through > > the > > > > admin client even when the transaction coordinator restarts > > successfully. > > > > > > > > - Rowland > > > > > > > > > > -- *Rowland E. Smith* P: (862) 260-4163 M: (201) 396-3842
