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 <[email protected]>: > 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 <[email protected] > >: > > > > 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 >
