Placeholder/Discussion: Durable State support

[leviramsey]

Short description

Akka Persistence has added a Durable State persistence mode; should Cassandra support it?

Details

The JDBC/R2DBC plugins support Durable State, which suggests at least some investigation should be performed around supporting it in this plugin.

One possibility, matching the approximate usage pattern of Durable State with relational stores would be to write with ALL consistency and read with ONE consistency: this somewhat matches up with the general contrast in Durable State from Event Sourcing of optimizing for the read (typically smaller and faster with Durable State) over the write (typically larger and slower with Durable State). Doing it this way would also make it more likely to be "eventually update-in-place" and reduce storage requirements: which also seems to be one of the main selling points for Durable State.

Alternatively, Durable State could be implemented along the lines of a generic StateUpdated event, but this seems to not provide any Durable State benefits beyond the simplicity of the developer API. Alternatively, this could be a generic "turn a generic event sourcing journal into a durable state store" that would also extend crude Durable State support to the other plugins in the community.

[patriknw]

The difficult part would be to implement the eventsByTag (or eventsBySlices) for Durable State.

[leviramsey]

Wouldn't durable state only really be expected to expose the durable state queries? The JDBC plugin doesn't seem to implement eventsBy* for durable state.

[patriknw]

It's called changes: https://github.com/akka/akka-persistence-jdbc/blob/master/core/src/main/scala/akka/persistence/jdbc/state/scaladsl/JdbcDurableStateStore.scala#L115

Yeah, not implementing the queries would be much easier.