fromJournal method in Typed Event Adapter should return a list of events

[michaltomanski]

Currently, the EventAdapter in Akka Typed is:

abstract class EventAdapter[Event, PersistedEvent] {
  def toJournal(e: Event): PersistedEvent
  def fromJournal(p: PersistedEvent): Event
}

I suggest changing the fromJournal signature to return a sequence of events, which would give us something like

abstract class EventAdapter[Event, PersistedEvent] {
  def toJournal(e: Event): PersistedEvent
  def fromJournal(p: PersistedEvent): Seq[Event]
}

Alternatively, we can use a specialized structure for the return type

sealed abstract class EventSeq[E] {
  def events: immutable.Seq[E]
}

This modification has several advantages:

• compatibility with untyped version of EventAdapter (where an EventSeq is returned by fromJournalmethod)
• possibility to handle schema evolution case where one event has to be split into two, as described in Akka docs
• possibility to ignore events (by returning an empty list), as described in Akka docs

This request is a follow up to the discussion:
https://discuss.lightbend.com/t/recommended-approach-to-unused-events-with-akka-typed-persistence/4159

[patriknw]

It was added in #25050 but I can't see that we discussed that matter. Maybe we were looking for simplest possible API, but went too far in that direction.

Any thoughts @chbatey ?

[chbatey]

I recall us discussing simplifying it but nothing specific, I have nothing against making it a Seq

[michaltomanski]

Thanks @patriknw !

[patriknw]

thanks for reporting, will be included in 2.6.0-M4 that is to be released today