I am also quite interested in this thread, and I have another question here to ask about committing consumed messages. For example, if I need a program which acts both as a consumer and a producer, and the actions are wrapped in a "transaction":
Transaction start: Get next message from broker A; Do something; Send a message to broker B; Commit. If the transaction aborts after reading the message from broker A, is it possible to logically "put the message back" to brokers? I remember that Amazon Queue Service use some sort of lease mechanism, which might work for this case. But I am afraid that will affect the throughput a lot.. --Guozhang -----Original Message----- From: Jay Kreps [mailto:jay.kr...@gmail.com] Sent: Friday, October 26, 2012 2:08 PM To: kafka-users@incubator.apache.org Subject: Re: Transactional writing This is an important feature and I am interested in helping out in the design and implementation, though I am working on 0.8 features for the next month so I may not be of too much use. I have thought a little bit about this, but I am not yet sure of the best approach. Here is a specific use case I think is important to address: consider a case where you are doing processing of one or more streams and producing an output stream. This processing may involve some kind of local state (say counters or other local aggregation intermediate state). This is a common scenario. The problem is to give reasonable semantics to this computation in the presence of failures. The processor effectively has a position/offset in each of its input streams as well as whatever local state. The problem is that if this process fails it needs to restore to a state that matches the last produced messages. There are several solutions to this problem. One is to make the output somehow idempotent, this will solve some cases but is not a general solution as many things cannot be made idempotent easily. I think the two proposals you give outline a couple of basic approaches: 1. Store the messages on the server somewhere but don't add them to the log until the commit call 2. Store the messages in the log but don't make them available to the consumer until the commit call Another option you didn't mention: I can give several subtleties to these approaches. One advantage of the second approach is that messages are in the log and can be available for reading or not. This makes it possible to support a kind of "dirty read" that allows the consumer to specify whether they want to immediately see all messages with low latency but potentially see uncommitted messages or only see committed messages. The problem with the second approach at least in the way you describe it is that you have to lock the log until the commit occurs otherwise you can't roll back (because otherwise someone else may have appended their own messages and you can't truncate the log). This would have all the problems of remote locks. I think this might be a deal-breaker. Another variation on the second approach would be the following: have each producer maintain an id and generation number. Keep a schedule of valid offset/id/generation numbers on the broker and only hand these out. This solution would support non-blocking multi-writer appends but requires more participation from the producer (i.e. getting a generation number and id). Cheers, -Jay On Thu, Oct 25, 2012 at 7:04 PM, Tom Brown <tombrow...@gmail.com> wrote: > I have come up with two different possibilities, both with different > trade-offs. > > The first would be to support "true" transactions by writing > transactional data into a temporary file and then copy it directly to > the end of the partition when the commit command is created. The > upside to this approach is that individual transactions can be larger > than a single batch, and more than one producer could conduct > transactions at once. The downside is the extra IO involved in writing > it and reading it from disk an extra time. > > The second would be to allow any number of messages to be appended to > a topic, but not move the "end of topic" offset until the commit was > received. If a rollback was received, or the producer timed out, the > partition could be truncated at the most recently recognized "end of > topic" offset. The upside is that there is very little extra IO (only > to store the official "end of topic" metadata), and it seems like it > should be easy to implement. The downside is that this the > "transaction" feature is incompatible with anything but a single > producer per partition. > > I am interested in your thoughts on these. > > --Tom > > On Thu, Oct 25, 2012 at 9:31 PM, Philip O'Toole <phi...@loggly.com> wrote: > > On Thu, Oct 25, 2012 at 06:19:04PM -0700, Neha Narkhede wrote: > >> The closest concept of transaction on the publisher side, that I can > >> think of, is using batch of messages in a single call to the > >> synchronous producer. > >> > >> Precisely, you can configure a Kafka producer to use the "sync" mode > >> and batch messages that require transactional guarantees in a > >> single send() call. That will ensure that either all the messages in > >> the batch are sent or none. > > > > This is an interesting feature -- something I wasn't aware of. Still it > > doesn't solve the problem *completely*. As many people realise, it's > still > > possible for the batch of messages to get into Kafka fine, but the ack > from > > Kafka to be lost on its way back to the Producer. In that case the > Producer > > erroneously believes the messages didn't get in, and might re-send them. > > > > You guys *haven't* solved that issue, right? I believe you write about > it on > > the Kafka site. > > > >> > >> Thanks, > >> Neha > >> > >> On Thu, Oct 25, 2012 at 2:44 PM, Tom Brown <tombrow...@gmail.com> > wrote: > >> > Is there an accepted, or recommended way to make writes to a Kafka > >> > queue idempotent, or within a transaction? > >> > > >> > I can configure my system such that each queue has exactly one > producer. > >> > > >> > (If there are no accepted/recommended ways, I have a few ideas I would > >> > like to propose. I would also be willing to implement them if needed) > >> > > >> > Thanks in advance! > >> > > >> > --Tom > > > > -- > > Philip O'Toole > > > > Senior Developer > > Loggly, Inc. > > San Francisco, Calif. > > www.loggly.com > > > > Come join us! > > http://loggly.com/company/careers/ >
