This will likely break due to: * workers can have more then one thread and hence process the source in parallel * splitting a source allows for the source to be broken up into multiple restrictions and hence the runner can process those restrictions in any order they want. (lets say your kafka partition has unconsumed commit offset range [20, 100), this could be split into [20, 60), [60, 100) and the [60, 100) offset range could be processed first)
You're right that you need to sort the output however you want within your DoFn before you make external calls to Kafka (this prevents you from using the KafkaIO sink implementation as a transform). There is an annotation @RequiresTimeSortedInput which is a special case for this sorting if you want it to be sorted by the elements timestamp but still you'll need to write to Kafka directly yourself from your DoFn. On Mon, Jun 8, 2020 at 4:24 PM Hadi Zhang <[email protected]> wrote: > We are using the Beam 2.20 Python SDK on a Flink 1.9 runner. Our > messages originate from a custom source that consumes messages from a > Kafka topic and emits them in the order of their Kafka offsets to a > DoFn. After this DoFn processes the messages, they are emitted to a > custom sink that sends messages to a Kafka topic. > > We want to process those messages in the order in which we receive > them from Kafka and then emit them to the Kafka sink in the same > order, but based on our understanding Beam does not provide an > in-order transport. However, in practice we noticed that with a Python > SDK worker on Flink and a parallelism setting of 1 and one sdk_worker > instance, messages seem to be both processed and emitted in order. Is > that implementation-specific in-order behavior something that we can > rely on, or is it very likely that this will break at some future > point? > > In case it's not recommended to depend on that behavior what is the > best approach for in-order processing? > > https://stackoverflow.com/questions/45888719/processing-total-ordering-of-events-by-key-using-apache-beam > recommends to order events in a heap, but according to our > understanding this approach will only work when directly writing to an > external system. >
