Add Sink to extension as a writer endpoint

[scullxbones]

Closes flow for ReadJournal queries to write to journal

[matheuslima]

Can you explain better this issue?

[scullxbones]

So this would be something in the extension (I guess?) that would provide a Sink to stream journal events into. It would be useful if you were running two of these journals (for example) and wanted to transfer one journal into the other - just run an allEvents read journal query and tie it to the sink of the second journal to transfer the data. I guess just a Sink[EventEnvelope] that ran through the regular journal append path would be good for now. Ideally the journal append path should probably be an adapter on top of a Sink, to push everything towards streams and away from ad-hoc.

[matheuslima]

" It would be useful if you were running two of these journals (for example) and wanted to transfer one journal into the other"
This is for migration journal proposals? Or migration some events from a journal to another.

[scullxbones]

That's the use case I have in my head, but it's also deeper than that. I think that it would be good to rebuild the write side of the journal on top of Akka streams for the advantages (like backpressure).

If we ignore deletes for now, there are two parts to the write side:

• Append - fufilled by a Sink
• Replay - fulfilled by the EventsByPersistenceId query

Exposing a Sink could be a good start towards re-building the base layer. I think this could end up cleaning up the code as well, moving more into the common project.

[matheuslima]

Wow, it's awesome! The plugin would become fully async, non-blocking and backpressured.