Re: Exactly once transactions

Ok, so what is the advice? Not to use Kafka transactions ever because they
are unusable in real life?
Can you please provide a recipe how to make it work in the simple scenario:
no databases, just two topics, no admin actions.

Sergi

ср, 30 окт. 2019 г. в 22:39, Jörn Franke <jornfranke@gmail.com>:

> Please note that for exactlyOnce it is not sufficient to set simply an
> option. The producer and consumer must individually make sure that they
> only process the message once. For instance, the consumer can crash and it
> may then resend already submitted messages or the producer might crash and
> might write the same message twice to a database etc.
> Or due to a backup and restore or through a manual admin action all these
> things might happen.
> Those are not "edge" scenarios. In operations they can happen quiet often,
> especially in a Containerized infrastructure.
> This you have to consider for all messaging solutions (not only Kafka) in
> your technical design.
>
> > Am 30.10.2019 um 20:30 schrieb Sergi Vladykin <sergi.vladykin@gmail.com
> >:
> >
> > Hi!
> >
> > I investigate possibilities of "exactly once" Kafka transactions for
> > consume-transform-produce pattern. As far as I understand, the logic must
> > be the following (in pseudo-code):
> >
> > var cons = createKafkaConsumer(MY_CONSUMER_GROUP_ID);
> > cons.subscribe(TOPIC_A);
> > for (;;) {
> > var recs = cons.poll();
> > for (var part : recs.partitions()) {
> > var partRecs = recs.records(part);
> > var prod = getOrCreateProducerForPart(MY_TX_ID_PREFIX + part);
> > prod.beginTransaction();
> > sendAllRecs(prod, TOPIC_B, partRecs);
> > prod.sendOffsetsToTransaction(singletonMap(part,
> > lastRecOffset(partRecs) + 1),
> >
> > MY_CONSUMER_GROUP_ID);
> > prod.commitTransaction();
> > }
> > }
> >
> > Is this right approach?
> >
> > Because it looks to me there is a possible race here and the same record
> > from topic A can be committed to topic B more than once: if rebalancing
> > happens after our thread polled a record and before creating a producer,
> > then another thread will read and commit the same record, after that our
> > thread will wake up, create a producer (and fence the other one) and
> > successfully commit the same record second time.
> >
> > Can anyone explain please how to do "exactly once" in Kafka right?
> > Examples would be very helpful.
> >
> > Sergi
>