Hi guys,
I'm building an event sourced application using akka persistence and I'm
using a 1:1 setup with one persistent actor per aggregate.
The next thing I want to implement is a notification service that consumes
the events generated by the aggregate and notifies the users when specific
state changes has occurred.
My first attempt at this was to create a PersistentView that receives the
events and update an in memory state based on some of these events. When a
new event arrives, it will update the local actor state and send a
notification to the user if a certain criteria has been met. This works,
but the problem occurs during recovery since all the events will be
replayed to the view which would cause the same notification to be
redelivered to the user. I don't see a way of differentiating between
recovery messages and normal messages received by the view.
So to solve this I definitely need to persist the events that has been
processed. I could of course implement this logic in the persistent
aggregate actor itself since it has all the state information needed, but
ideally I would like to decouple this logic from the aggregate.
Maybe I could still use a persistent view, but I would need to store the
events in some persistent storage so that I can keep track of which events
has been processed. Or should I just use another persistent actor for this
notification service? If so, how should I update this actor? Using a view?
:)
Essentially what I need is:
- After recovery is completed, initialize notification service (or other
services subscribing to events) with current state
- Subsequent events generated by aggregate is forwarded to this service
How about this solution?:
1) Notification service registers it self with the aggregate by sending a
message when it starts
aggregate ! RegisterSubscriber
2) Aggregate keeps track of all the listeners and persists these to the
journal
3) Aggregate: After RecoveryCompleted is received, send InitState(state:
State) or InitEvents(events: List[Event]) to listeners
listeners foreach { ref => ref ! InitState(state) }
4) Aggregate: during normal operation - forward events to listeners after
persisting event. Perhaps using AtLeastOnceDelivery to make sure we don't
loose any notifications
What do you think? Essentially I'm just looking to get some advice on how
to best solve this. Any suggestions are appreciated.
Cheers
Rune
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