With the release of Scala 2.9.0, the Typesafe Stack was also announced, which combines the Scala language with the Akka framework. Now, though Scala has actors in its standard library, Akka uses its own implementation. And, if we look for other implementations, we'll also find that Lift and Scalaz have implementations too!
So, what is the difference between these implementations?
This answer isn't really mine. It was produced by Viktor Klang (of Akka fame) with the help of David Pollak (of Lift fame), Jason Zaugg (of Scalaz fame), Philipp Haller (of Scala Actors fame).
All I'm doing here is formatting it (which would be easier if Stack Overflow supported tables).
There are a few places I'll fill later when I have more time.
Minimal complexity. Maximal generality, modularity and extensibility.
Minimal complexity, Garbage Collection by JVM rather than worrying about an explicit lifecycle, error handling behavior consistent with other Scala & Java programs, lightweight/small memory footprint, mailbox, statically similar to Scala Actors and Erlang actors, high performance.
Provide the full Erlang actor model in Scala, lightweight/small memory footprint.
Simple and transparently distributable, high performance, lightweight and highly adaptable.
Scalaz Actors Lift Actors Scala Actors Akka Actors Current stable ver. 5 2.1 2.9.0 0.10 Minimum Scala ver. 2.8 2.7.7 2.8 Minimum Java ver. 1.5 1.5 1.6
Scalaz Actors Lift Actors Scala Actors Akka Actors Spawn new actors Yes Yes Yes Yes inside of actor Send messages to Yes Yes Yes Yes known actor Change behavior Actors are Yes Yes: nested Yes: for next message immutable react/receive become/unbecome Supervision Not provided No Actor: Yes, Yes (link/trapExit) Reactor: No
If user defines public methods on their Actors, are they callable from the outside?
Actor[A] extends A => ()
Actor extends Reactor[Any]
Scalaz Actors Lift Actors Scala Actors Akka Actors Manual start No No Yes Yes Manual stop No No No Yes Restart-on-failure n/a Yes Yes Configurable per actor instance Restart semantics n/a Rerun actor Restore actor to stable state by re-allocating it and behavior throw away the old instance Restart configurability n/a n/a X times, X times within Y time Lifecycle hooks provided No lifecycle act preStart, postStop, preRestart, postRestart
Scalaz Actors Lift Actors Scala Actors Akka Actors Fire-forget a ! message actor ! msg actor ! msg actorRef ! msg a(message) Send-receive-reply (see 1) actor !? msg actor !? msg actorRef !! msg actor !! msg Send-receive-future (see 2) actor !! msg actorRef !!! msg Send-result-of- promise(message). future.onComplete( f => to ! f.result ) future to(actor) Compose actor with actor comap f No No No function (see 3)
(1) Any function f becomes such an actor:
val a: Msg => Promise[Rep] = f.promise val reply: Rep = a(msg).get
(2) Any function f becomes such an actor:
val a = f.promise val replyFuture = a(message)
(3) Contravariant functor:
actor comap f. Also Kleisli composition in
Scalaz Actors Lift Actors Scala Actors Akka Actors reply-to-sender-in-message reply-to-message
Supports nested receives?
Scalaz Actors Lift Actors Scala Actors Akka Actors Name for Execution Mechanism Execution Mechanism is configurable Execution Mechanism can be specified on a per-actor basis Lifecycle of Execution Mechanism must be explicitly managed Thread-per-actor execution mechanism Event-driven execution mechanism Mailbox type Supports transient mailboxes Supports persistent mailboxes
Scalaz Actors Lift Actors Scala Actors Akka Actors Transparent remote n/a No Yes Yes actors Transport protocol n/a n/a Java Akka Remote Protocol serialization (Protobuf on top of TCP) on top of TCP Dynamic clustering n/a n/a n/a In commercial offering
Scalaz Actors Lift Actors Scala Actors Akka Actors Define an actor Create an actor instance Start an actor instance Stop an actor instance