CODEWITHSUNDEEP
scalaakkaevent-bus
romatthe
romatthe

Reputation: 1477

Starting Actors on-demand by identifier in Akka

I'm currently implementing a system that that receives inbound messages from an external monitoring system. I'm translating these messages into more concise 'events', and I'm using these to alter the state of 'Managed System' objects. Akka Actors seemed like a good use case for encapsulating mutable state in concurrent applications.

The managed systems are identified by a name (99% of the time this is a hostname). Whenever a proper event is received, the system routes the message to the correct actor based on the name property. At first I used to use actorSelection and the complete paths of said actors, but that was very ugly, and I saw several people advise against relying on the fully qualified name of an actor to deliver message. So I've set up a simple EventBus, which is great as I can now simply do:

eventBus.subscribe(subscriber1, "/managedSystem01")
eventBus.subscribe(subscriber2, "/managedSystem02")

eventBus.publish(MonitoringEvent("/managedSystem01", MonitoringMessage("managedSystem01", "N", "CPU_LOAD_HIGH", True)))
eventBus.publish(MonitoringEvent("/managedSystem02", MonitoringMessage("managedSystem02", "Y", "DISK_USAGE_HIGH", True)))

Of course, I now have the issue that, should I receive and event that concerns a managed system for which I've not spawned an actor yet (this is entirely possibly, it is impossible for me to get an absolute list of managed systems unfortunately), the message will be routed to the dead-letter mailbox.

Ideally I don't want this to happen. When it is unable to address a specific actor, I want to spawn a new one dynamically.

I suppose that, theoretically, I could subscribe to DeadLetter messages but:

  1. That sounds a little 'hacky', since those message are essentially reserved for the system
  2. Is it even possible to recover the original message (in my case, the MonitoringMessage) that is sent to the DeadLetter mailbox?

Alternatively is there a way to check if there are ZERO subscribers to a certain "topic"?

Upvotes: 1

Views: 288

Answers (1)

Konrad 'ktoso' Malawski
Konrad 'ktoso' Malawski

Reputation: 13130

What you describe ("send to Actor by some identifier, if it does not exist buffer until it gets created and then deliver to that newly on-demand created Actor") is implemented in Akka as Cluster Sharding.

While it is designed primarily for sharding load (work) across a cluster, you could use it locally as well, since your requirement is essentially a scaled down (to one node) version of problem that it solves. It takes care of starting new Actors if they don't exist for a given identifier etc, so you'd simply subscribe the shard-region to the events and it'll take care of creating the actors for you.

Upvotes: 1

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