Search code examples
persistenceakkaakka.net

AKKA.NET Journals and Snapshot Store

Since I have not seen any example of using AKKA.NET Journals and Snapshot store, I assume I have to use both type of actors to implement an Event Store and CQRS.

  1. Is the Snapshot store expected to be updated every time when the actor state is changed, or should be set on a scheduled update like every 10 seconds?

  2. Should the Snapshot store actors talk to the Journal actors only, so the actors having the state should not talk to Journals and Snapshot at the same time? I'm thinking in the line of SOC.

  3. Assume I have to shut down the server and back up. A user tries to access a product (like computers) through a Web UI. At that time, the product actor does not exist in the actor system. To retrieve the state of the product, shouldn't I go to the snapshot store instead of running all the journals to recreate the state?

Solution

  • In Akka.Persistence both Journal and SnapshotStore are in fact actors used to abstract your actors from particular persistent provider. You almost never will have to use them directly - PersistentView and PersistentActor use them automatically under the hood.

    1. Snapshot stores are only way to optimize speed of your actor recovery in case when your persistent actor has a loot of events to recover from. In distributed environment snapshotting without event sourcing is not a mean to achieve persistence . Good idea is to have counter which produces a snapshot after X events being processed by the persistent actor. Time-based updates have no sense - in many cases actor probably didn't changed over specified time. Performance is also bad (lot of unnecessary cycles).

    2. SnapshotStores and Journals are unaware of each other. Akka.Persistence persistent actors have built-in recovering mechanism which handles actor's state recovery from SnapshtoStores and Journals and exposes methods to communicate with them.

    3. As I said you'd probably don't want to communicate with snapshot stores and journals directly. This is what persistent actors/persistent views are for. Ofc you could probably just read actor state directly from backend storage, but the you should compare if there are no other events after latest saved snapshot etc. Recreation of persistent actor/view on different working node is IMO a better solution.