Add EventLog abstraction (implemented by AkkaPersistenceEventLog and KafkaEventLog).

[odd]

No description provided.

[krasserm]

Hi Odd, thanks for your contribution. I'm not sure if this is premature abstraction because of the semantic differences explained in the other comments. Do you have a specific use case for adding this abstraction?

[krasserm]

This is an implementation detail of the Akka Persistence event log. Multiple materialization of a Kafka event log for the same topic partition can be made. Also, persistenceId refers to an implementation detail here.

[krasserm]

A source from a Kafka event log continues to emit Delivered messages.

[odd]

Hi Martin, during work on a sample application designed to test ways of caching running event sourced flows by identifier (to get functionality that mimics the behavior of persistent actors with auto passivation), the EventLog-abstraction seemed useful to abstract over this specific case:

EventSourcing(..).join(eventLog.flow[E](identifier))

(i.e. to be able to get an EventSourced flow over an EventLog without knowing the exact EventLog implementation). I have only used it with Akka Persistence event logs yet though, so it might not hold its ground.

Of course, documentation that is specific for Akka Persistence event logs (or Kafka event logs) should not be on the abstract event log but rather on each specific implementation. But if the semantic differences between the different implementations are too great as you say, I agree that the abstraction does not carry its weight.

Meanwhile, I'll keep the same abstraction in my cached-event-flow PR (yet to be pushed) and you can decide on it together with the rest at a later time (not too far off in the future I hope).

[krasserm]

I see, thanks for clarifying. I'm fine with updating the API docs of EventLog describing the degrees of freedom for implementations and then merge your PR.

[odd]

Just to clarify, do you want me to clarify the degrees of freedom for implementations in this PR or do you want to add the clarifications yourself?

[krasserm]

@odd it would be great if you could do that. I actually meant: I'm fine if you update the API docs .. 😃. Thanks in advance!