nwangtw commented on issue #3198: Feature/create kafka spout URL: https://github.com/apache/incubator-heron/pull/3198#issuecomment-471887499 > @worlvlhole > Yes, I think you've got the most part. The KafkaSpout may be operated in 2 different reliability mode, `ATMOST_ONCE` or `ATLEAST_ONCE`, (I haven't added `EFFECTIVE_ONCE` implementation yet, I'm working on it). > > ### `ATMOST_ONCE` mode > the whole topology will not turn the `acking` mechanism on. so, the KafkaSpout can afford to emit the tuple without any message id, and it also immediately commit the currently-read offset back to Kafka broker, and neither `ack()` nor `fail()` callback will be invoked. Therefore, "in-flight" tuple will just get lost in case the KafkaSpout instance is blown up or the topology is restarted. That's what `ATMOST_ONCE` offers. > > ### `ATLEAST_ONCE` mode > the `acking` mechanism is turned on topology-wise, so the KafkaSpout uses the `ack registry` to keep tracking all the **continuous** acknowledgement ranges for each partition, while the `failure registry` keeps tracking the **lowest** failed acknowledgement for each partition. When it comes to the time that the Kafka Consumer needs to poll the Kafka cluster for more records (because it's emitted everything it got from the previous poll), then the KafkaSpout reconciles as following for each partition that it is consuming: > > 1. if there's any failed tuple, seek back to the lowest corresponding offset > 2. discard all the acknowledgements that it's received but is greater than the lowest failed offset > 3. clear the lowest failed offset in `failure registry` > 4. commit the offset to be the upper boundary of the first range in the `ack registry` > > So, it guarantees each tuple emitted by the KafkaSpout must be successfully processed across the whole topology at least once. > > ### Not Implemented > What is missing in this Kafka Spout implementation now is to handle the `EFFECTIVE_ONCE` scenario, which should completely rely on the `checkpointing` mechanism to decide how far it needs to rewind back. I'm working on it right now. > > I know this is quite some information, I'm writing README to explain things in more details, will keep updating the pull request. This information might be included in the document?
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