Akka-persistence, event propagation from one actor to another

Question

I have one question about akka-persistence and event migration. I do have read the "Schema Evolution for Event Sourced Actors" chapter. However, this does not give an answer to my question.

Given I have one persistent actor ChildActor that produce Created event. But, later we discover that ChildActor should be a child of ParentActor. And ParentActor has to update his state based on the creation of ChildActor (to maintains a collection of childs).

We can add a new command CreateChild for ParentActor that will create the ChildActor. However, the parent will never receive the Created event emitted by his child. Thus it will not be able to update his state. Of course, ParentActor can create a ChildCreated event for himself.

But, what about the Created events already persisted by FirstActor?

How can we "send" (and, ideally adapt) them to the ParentActor?

So, my question is:

Can we "route" persisted events from one actor to another?

Thanks

Answer 1

It is possible to watch the events persisted by a given persistence ID with the events by persistence ID query. Since this query is very much like what Akka Persistence must do in replaying events to rebuild a persistent actor's state, it's available in all the commonly used plugins: you'll need to check the documentation for your plugin for how to summon a ReadJournal. Once summoned, assuming that the ReadJournal is further an instance of EventsByPersistenceIdQuery, you would use (Scala):

readJournal.eventsByPersistenceId(childActorPersistenceId, fromOffset, Long.MaxValue)

which would give you an Akka Streams Source of events in order starting at fromOffset. Your subscribing actor may (probably will) want to save in its state the last-seen sequence number as part of its state so if it resumes it doesn't see the event it processed (ideally the event updating the sequence number would be in the same batch or otherwise atomically part of the state update).

Note that there will be an observable delay from persisting the event to when ParentActor sees the event, though many of the recent iterations of plugins (e.g. Cassandra or R2DBC) can directly propagate the event or at least the notification that there's an event for the persistence ID to the query.