Prevent overwriting events when persisting

[Fours]

Could use a condition expression to stop an overwrite and instead return a rejection: http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.ConditionExpressions.html

This would be helpful in the exceptional scenario of two persistent actors running with the same persistence id, trying to save an event with the same seqNum - I would rather one fail to persist than overwrite an existing item.

Thoughts?

[mallize]

Agreed. I would never want to overwrite an event. Should this end up with a call to onPersistFailure which would allow the developer to decide how to handle this?

[calvinlfer]

+1
This would help detect split brain scenarios and prevent corruption of your data where you use Akka Cluster Sharding and the constraint of having only a single Persistent Actor with a given ID X within a cluster is violated