Handling expected and unexpected actor termination

There are five ways that an actor can terminate. These are enumerated by the StopCause enum:

• Stopped: Successful termination
• Failed: Actor terminated itself due to some problem
• Killed: Actor was killed by another entity
• Dropped: The last ActorOwn referencing this actor was dropped
• Lost: This indicates that the connection to a remote actor has been lost. Remote actors are not implemented yet.

Both Failed and Killed have an associated boxed Error. When an actor is created, a notification handler Ret<StopCause> is normally provided. This receives the reason for the actor's termination when it terminates. So usually a parent actor will keep a hold of the ActorOwn for the child actor within its own state, and will receive the termination notification. However other patterns are also possible.

On receiving a termination notification, the parent actor might choose to restart the child, or terminate itself, or take some other action. The parent actor has a free choice on how to handle it. It is possible to downcast the Error to take a different action depending on the type of failure if necessary.