Lagom: Allow programmatic control over persistent entity lifecycle

[TimMoore]

Some use cases would benefit from more direct control over the persistent entity lifecycle.

For example:

1. Manually trigger passivation, for example in readonly command handler
2. Define a hook to do some cleanup on passivation
3. Co-locate a persistent entity with a related actor on the same node
4. Manually trigger snapshotting (see #1014)

[TimMoore]

Related: #1014

[ignasi35]

(Added 4. on the description)

[aklikic]

+1

[kotdv]

1. Remove history (events and snapshots) up to the current (last) event. GDPR use case.

[ignasi35]

Hi @kotdv I think your suggestion 5. (in https://github.com/lagom/lagom/issues/1042#issuecomment-451772773) is out of scope here. All the items on the description above are related to extending Lagom's feature set closer to the featureset of Akka Persistence itself. Rewriting (or removing) history is not supported in Akka Persistence.

[michaelahlers]

I can't overstate how valuable this would be for a few problems I'm trying to solve.

[TimMoore]

I think most or all of the cases above are covered by the first class support for the Akka Persistence Typed API introduced in Lagom 1.6:

• Passivation: https://doc.akka.io/docs/akka/current/typed/cluster-sharding.html#passivation
• Co-location is supported by decoupling persistence from sharding, or using the external shard allocation strategy (or a custom strategy)
• Snapshot predicates: https://doc.akka.io/docs/akka/current/typed/persistence-snapshot.html#snapshots
• Deleting snapshots: https://doc.akka.io/docs/akka/current/typed/persistence-snapshot.html#snapshot-deletion
• Deleting events: https://doc.akka.io/docs/akka/current/typed/persistence-snapshot.html#event-deletion
• Using Akka Persistence Typed in Lagom: https://www.lagomframework.com/documentation/1.6.x/java/UsingAkkaPersistenceTyped.html

If there are additional specific controls needed, I think the best thing would be to start a thread in the forums to discuss them.