Implementing at-least-once ingestion pipeline with Akka streams

[Evan Chan]

Hey folks,

I would like to implement an at-least-once ingestion pipeline, say from a source like Kafka, to a datastore, using Akka streams. 
The method of at-least-once is that each incoming message has an increasing numeric offset, and the idea is that successful writes into the datastore would send back the last committed offset, which should increase over time.  If a failure occurs, then the ingestion can be restarted from the last committed offset, by replaying messages from the source (or a write-ahead log, for example).

How would I design such a system?

Let's start with the naive implementation:

kafkaSource.map(someTransformFunc).to(dataStoreSink)

This gives us backpressure, but no ack feedbacks.   I can think of two ways to add the ack feedback.

One is to make the writing to the datastore not a sink but a mapAsync stage that uses futures to write, and returns the latest completed offset. 

kafkaSource.map(someTransformFunc).mapAsync(writeToDataStoreFuture _)

This is incomplete, because there is no flow.  Ideally, the stream of completed offsets would be fed back to kafkaSource so it knows the latest committed offset, and if some error occurred (possibly also a result), then the source could replay messages.  How to do this?

A second approach, which seems hacky and not so idiomatic, is to make kafkaSource and dataStoreSink into ActorPublisher and ActorSubscribers, respectively.  Then, the dataStoreSink could send out of band ack messages to the kafkaSource actor directly.

It's not clear to me that backpressure is really needed for the acks, I think the most critical part is that writing to the datastore has backpressure.

Any thoughts?

Many thanks,
Evan

[Evan Chan]

Ping.... anybody?

[Martynas Mickevičius]

Just an idea: you can actually form cycles in the Flow graph, which you can use to feedback acknowledgments.

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Martynas Mickevičius

Typesafe – Reactive Apps on the JVM

[Evan Chan]

Thanks.  I know you can form graphs, but doing a custom merge is not quite what I'm looking for.

I need to change the state of a Source, using feedback from a Sink.

Example: Kafka source. Depending on Sink output / acknowledgements, reset Kafka offset to an earlier offset.

This can't be done by filtering, delaying, etc. the Kafka input source via another processing unit.  This requires changing the state of the Source to begin with, or sending a message to it (if it were Actor-based).

Another example: let's say you have an HTTP-based source.  Depending on the acknowledgements, we need to give a proper response code: 200, 500, etc.

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[Evan Chan]

I got feedback from @ktosopl that you can send feedback in a graph back to a Source.  Does anyone have an example of this?

Thanks!

[Björn Antonsson]

Hi Evan,

I don't think that it's easily possible to feed back things into a Source right now. You would need a special kind of Source with an input port as well to wire it up properly in the Graph. The only solution that I can think of at the moment is to have a PropsSource and a PropsSink that materialize into actors, and then you "connect" them on the side by sending normal actor messages.

B/

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Björn Antonsson

Typesafe – Reactive Apps on the JVM

twitter: @bantonsson

[Evan Chan]

Thanks Bjorn.

[Endre Varga]

Hi Evan,

As we progress we make more and more things possible, we will keep this use-case in mind.

-Endre