Restore previous actor state after exception

Question

I have an actor which receive Int and accumulate result to the buffer. Some times actor can throw exception. What is the best way to restore previous state of buffer into actor after exception ?

BR.

Answer 1

In order to control what happens you need to set the supervisor strategy on the parent actor.

override val supervisorStrategy =
  OneForOneStrategy(maxNrOfRetries = 10, withinTimeRange = 1 minute) {
    case e: Exception                => {
      println(s"got exception ~ ${e.getMessage()}")
      Resume // Or Continue, Escalate, Restart, Stop
    }
  }

If you would like to handle the exception then resume the actor then your handler would return Resume

When resuming, the actor state remains unchanged.

Answer 2

In addition to manub's answer, you should also consider separating

• the part that accumulates the value, and
• the part that may fail

into two actors as follows:

The accumulating actor should create a child actor. This child actor does the job that can potentially fail, and then sends the result of its job as a message back to its parent. The parent actor receives the result of the job and does the accumulating, or a notification that the child actor has failed. This way, you can, for example, restart failed jobs using various supervision strategies.

For more information on such an approach, see for example this answer Handling Faults in Akka actors

Answer 3

In order to be able to recover the state of a stateful actor you should consider Akka Persistence. You will require a durable store (Cassandra or LevelDB for example) to persist state, and the framework will handle restoring the state for you (provided that you have a PersistentActor).

http://doc.akka.io/docs/akka/current/scala/persistence.html explains in detail how to achieve this.

Answer 4

You can set the Supervisor strategy to Resume. When your Actor throws an exception, the Supervisor will handle the exception and your Actor will throw away the current message and move on to the next one. (Newer versions of Akka let you set a maximum number of retires on the same message)

If instead, you would like your Actor to continue processing the message that caused the exception, you can use a try-catch-finally block.