Monday, May 25, 2009

Refactoring Scala Actors: Progress Update

It's been a while since I've posted, so I thought I'd give everyone a status update.  This post covers several different semi-disjoint topics at a fairly high level.  I plan on diving into some of the issues later this week and beyond, but for now...

State-machine Based Actors

A while back Philipp Haller, the original author and current maintainer of the Scala actors library, contacted and basically said he found the changes I was making really interesting, but he really needed a smaller, more gradual set of patches.  It's a perfectly reasonable request, as I had pretty much completely ripped apart his library.  I had rethought my approach, anyway, so I went about moving my state-machine based actor implementation into its own package and rewiring some of the pieces so that they could share common base traits, common infrastructure, and interoperate with one another as if they were the same library.  So I shoved my code into scalax.actors, and started hacking insertion points for my code into the main library.

The first thing I thought I needed was a base trait that defines the basic structure and operations of an actor, so created a BaseActor in between AbstractActor and Actor (as well as my own StateActor):

trait BaseActor extends AbstractActor {
  def react(f: PartialFunction[Any, Unit]): Nothing
  def reactWithin(msec: Long)(f: PartialFunction[Any, Unit]): Nothing
  def receive[A](f: PartialFunction[Any, A]): A
  def receiveWithin[R](msec: Long)(f: PartialFunction[Any, R]): R
  /*def loop(body: => Unit): Nothing */
  /*def loopWhile(cond: => Boolean)(body: => Unit): Nothing */
  protected[actors] def mailbox: MessageQueue[Message[Any]]
  private[actors] final def mailboxForChannel: MessageQueue[Message[Any]] = mailbox
  def mailboxSize: Int
  def send(msg: Any, replyTo: OutputChannel[Any]): Unit
  def forward(msg: Any): Unit
  def reply(msg: Any): Unit
  /*protected[actors]*/ def sender: OutputChannel[Any]
  def ? : Any
  def start(): AbstractActor
  def freshReplyChannel: Channel[Any] = new Channel[Any](this)
  def scheduler: IScheduler //TODO: restrict access to scheduler??
}

I don't think BaseActor is going to be a permanent fixture because its contents probably belong in AbstractActor instead, but for now it serves its purpose.  One of the first things you should notice is that way to much stuff in there is public.  Most of it should be protected, or perhaps somewhere else entirely (like an InputChannel encapsulated by the actor).

Reworking MessageQueue

There's also the issue of the mailbox, which is a rather important and a den of mutable data that is passed all around with private[actors] qualifiers.  Basically it separates the Message from the elements within the MessageQueue, so that the MessageQueue can keep its internal structure private, and thus facilitating making it a trait so that an actor can provide its own specialized implementation.  I was about to submit a patch for the change, but a fix for a memory leak in FJTaskRunner came about that relied on clearing mutable fields in the message when a task is done processing.  I have an alternative fix by changing pieces of FJTaskScheduler2, but schedulers in general and FJTaskScheduler2 in specific are in flux right now due to bugs (here and here and probably elsewhere), and I want to tweak the design a bit, so I'm holding off.

Fixing Schedulers

Which brings me to schedulers...  Problems with plugging in custom schedulers (mostly fixed) are what originally caused me to dive into the guts of the actor library.  Closely related to schedulers is ActorGC, which is absolutely essential to actors (almost) transparently abstracting threads, but can also be problematic due to it's fundamentally non-deterministic nature (it relies on the garbage collector for some of its more advanced capabilities).  That being said, now in trunk ActorGC is optional, so environments that don't require an implicit shutdown of the actor worker threads can avoid the added complexity.  I intend to cover the details of ActorGC very soon.  There should also be a default scheduler with daemon semantics coming, which has a number of use cases.

Closing Matter

There's a lot more going on.  Some recent flare-ups on Scala Internals mailing, despite being a tad melodramatic, brought a welcome focus on actors for the next release of Scala.  The issue has also given rise to two minimalistic actor implementations, one in Lift and the other in Scalaz.  They both make interesting data points for design and potential interoperability (remember: one of my primary goals is an actor implementation that lets you plug in what you need).  There's issues around ActorProxy that I think will be a little hairy to sort out, but I'm confident they will be.  And finally, there's the omnipresent issue of ensuring actor's really make the guarantees that they claim (right now I think they do, but I wouldn't place money on it until I have tests to prove it).

That's it for now.  I'm going to try to take the time to blog about many of the above issues and more in depth in the coming weeks, and hopefully gain some insights from out in the cloud.

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