How are the multiple Actors implementation in Scala different?
ScalaActorAkkaScala Problem Overview
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?
Scala Solutions
Solution 1 - Scala
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.
Design Philosophy
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Scalaz Actors
Minimal complexity. Maximal generality, modularity and extensibility.
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Lift Actors
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.
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Scala Actors
Provide the full Erlang actor model in Scala, lightweight/small memory footprint.
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Akka Actors
Simple and transparently distributable, high performance, lightweight and highly adaptable.
Versioning
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
Actor Model Support
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
Level of state isolation
If user defines public methods on their Actors, are they callable from the outside?
- Scalaz Actors: n/a. Actor is a sealed trait.
- Lift Actors: Yes
- Scala Actors: Yes
- Akka Actors: No, actor instance is shielded behind an ActorRef.
Actor type
- Scalaz Actors:
Actor[A] extends A => () - Lift Actors:
LiftActor,SpecializeLiftActor[T] - Scala Actors:
Reactor[T],Actor extends Reactor[Any] - Akka Actors:
Actor[Any]
Actor lifecycle management
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
Message send modes
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 Promise.
Message reply modes
TBD
Scalaz Actors Lift Actors Scala Actors Akka Actors reply-to-sender-in-message reply-to-message
Message processing
Supports nested receives?
- Scalaz Actors: --
- Lift Actors: Yes (with a little hand coding).
- Scala Actors: Yes, both thread-based receive and event-based react.
- Akka Actors: No, nesting receives can lead to memory leaks and degraded performance over time.
Message Execution Mechanism
TBD
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
Distribution/Remote Actors
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
Howtos
TBD
Scalaz Actors Lift Actors Scala Actors Akka Actors Define an actor Create an actor instance Start an actor instance Stop an actor instance
Solution 2 - Scala
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scala.actors was the first serious attempt to implement Erlang-style concurrency in Scala that has inspired other library designers for making a better (in some cases) and more performant implementations. The biggest problem (at least for me), is that unlike Erlang processes, complemented with OTP (that allows for building fault-tolerant systems), scala.actors only offer a good foundation, a set of stable primitives that must be used for building a more high-level frameworks - at the end of the day, you’ll have to write your own supervisors, catalogs of actors, finite state machines, etc. on top of actors.
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And here Akka comes to the rescue, offering a full-featured stack for actor-based development: more idiomatic actors, set of high-level abstractions for coordination (load balancers, actor pools, etc.) and building fault-tolerant systems (supervisors, ported from OTP, etc.), easily configurable schedulers (dispatchers), and so on. Sorry, if I sound rude, but I think, there will be no merge in 2.9.0+ - I’d rather expect Akka actors to gradually replace stdlib implementation.
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Scalaz. Normally I have this library in the list of dependencies of all my projects, and when, for some reason, I can’t use Akka, non-blocking Scalaz Promises (with all the goodness, like
sequence) combined with the standard actors are saving the day. I never used Scalaz actors as a replacement for scala.actors or Akka, however.