Support for asynchronous, incremental snapshot capturing

[volkerstampa]

Imagine the following scenario:

• 2 actors: A, B
• A is eventsourced and
  • gets message from "outside"
  • updates its internal state
  • computes new messages and sends them to B
• B is not eventsourced
  • gets message only from A
  • updates its internal state

In this case taking snapshots in a straightforward manner does not lead to consistent recovery as message sent from A to B could be either applied twice or get lost. For consistent recovery the following trick can be applied:

• Send SnapshotRequest to A
• A creates A-snapshot and does not call SnapshotRequest.process, but forwards its snapshot in custom message to B
• B creates B-snapshot and combines it with received A-snapshot to AB-snapshot
• B calls SnapshotRequest.process(AB-snapshot)

Having library support for this kind of consistent snapshots could relieve a developer from implementing the described process and probably saves a couple of process related custom types.

[krasserm]

Volker, thanks for bringing this up. This is definitely something the snapshotting API should support. I renamed the ticket title a bit because your issue is more related to asynchronous, incremental snapshot capturing. Whether the aggregated snapshot is a consistent snapshot depends on application logic.