[Lift] Re: New Lift Actor code
David, this is extremely interesting. Given the points you outlined this makes perfect sense to move from scala.actors - however, if come the 2.8 release EPFL fix the actors library so that it then becomes acceptable to use within lift again, would you want to move back to it? IMO, and as you said in your mail, you (or indeed we) have no interest in maintaing our own actors implementation and it seems like it would be most optiomal to use the EPFL implementation when it becomes appropriate to. Cheers, Tim On May 23, 6:19 am, David Pollak feeder.of.the.be...@gmail.com wrote: Folks, It is not lightly that I've made the decision to write an alternative Actor library and move the Lift code base from the Scala Actors to Lift Actors (working name). I want to spend a little time talking about the steps that led to the decision as well as the impact that it will have on Lift code. Since November, I've been chasing a series of memory leaks in the Actor library. Philipp Haller from EPFL has been responsive in addressing the individual memory leaks, but the issue seems to be one of whack-a-mole... each time one memory leak is fixed, another one appears. Further, the existing Actor architecture does not lend itself to the kind of Actor usage cycle that we find in Lift apps. Specifically: - Lift creates/destroys an Actor for each Comet request. This rapid creation/destruction of Actors caused memory back-ups, and the existing Actor code seems to be oriented to long running Actors rather than Actors with Object-length lifespans. - The FJ libraries used for Actor scheduling have problems on multi-core machines and are also a source of memory retention issues. - Replacing the FJ libraries with a scheduler based on java.util.concurrent exposes race/deadlock conditions related to the fact that some parts of the Actor processing (e.g., testing mailbox items against partial functions while the Actor itself is synchronized) - The Actors require external threads to function and it's not possible to create external threads in the Google App Engine (making Actor-based functionality including CometActors non-functioning in GAE apps) - Actors are fragile when exceptions are thrown - Actors have running and not running states (as compared with objects which can always respond to message sends). In practice, managing the running and not running states is as hard as managing memory in C. - There are hidden actors associated with each thread which display the above fragility and state management issues - And as a practical matter, I've got a couple of applications that are going into production over the next few weeks and cannot wait for the various fixes to make it into Scala 2.8 and the hacks and work-arounds that I've done to the 2.7.4 Actor libraries became too complex for my comfort. I have written a simple Actor class that is focused on message sending and processing of messages asynchronously. This means there's a single operation that you can perform on Actors, the message send operation. Actors can be specicialized (they only access messages of a certain type). In order to receive a response from an Actor, you can pass in a Future as part of the message and that Future may be satisfied asynchronously. This means that a sender of a message need not be an Actor and that the Actor recipient of a message cannot determine the sender of a message. Actors have two bits of internal state: a mailbox and a flag indicating that the Actor is currently processing messages in its mailbox. The amount of synchronization of Actors is minimal (on inserting messages into the mailbox, on removing messages from the mailbox, and on changing state to/from processing messages.) An Actor instance must provide a messageHandler method which returns a PartialFunction that is used to pattern match against the messages in the mailbox. The instance may also provide an optional exception handler that is called if an Exception is thrown during the handling of a message. The Actor is guaranteed to only be processing one message at a time and the Actor is guaranteed not to be in a monitor (synchronized) during the processing of messages. An Actor is guaranteed to maintain the order of the messages in its mailbox, however, messages that do not currently match the messageHandler will be retained in the order that they were received in the event that the messageHandler changes and they can be processed. The Lift Actors will, by default, use the java.util.concurrent library for thread pooling, although I have worked out a mechanism for thread-piggy-backing such that if the Actors are running in GAE, they need not use any additional thread (this will enable Lift's comet support in GAE.) There will also be a scheduler (much like the existing ActorPing) which will send a message to an Actor at some time in the
[Lift] Re: New Lift Actor code
On Sat, May 23, 2009 at 7:39 AM, Martin Ellis ellis@gmail.com wrote: On Sat, May 23, 2009 at 6:19 AM, David Pollak feeder.of.the.be...@gmail.com wrote: I am happy to share the Lift Actor code with EPFL and if it makes it into the Scala distribution as SimpleActors or something similar, I'm totally cool with that. I'm not interested in owning or maintaining an Actor library. I am however, dedicated to making sure that Lift apps can run in production for months (or even years) without retaining memory or having other problems that can impact the stability of applications. The cool thing about this is that it provides solid evidence that Scala - as a language - does satisfy the aim of being be a scalable language. Yes, this is absolutely right. It also points up what I missed in my original posting... the amazing value of the Scala Actors which include: - First, and most important to Lift, a conceptual framework for doing concurrency. Without the Actor model, Lift would not have such a rich model for building interactive applications. - A design that keeps true to the Erlang Actor model in that it supports linking, run states, and other things that make an OTP style library possible. (Hey Jonas, where's that OTP library?) - A design that has evolved from simply supporting send/wait-for-response (!?) to send and immediately receive Future and other cool features. - Blocking until Futures are satisfied without consuming a thread if the Future was within a react-based Actor. - An implementation that worked well in JDK 1.4. Many of the current memory and scheduling issues are a result of the fact that Scala's Actors worked on JDK 1.4, back when 1.4 was the target for the Scala distribution. Scala is a language that supports multiple Actor libraries, just as it supports multiple collections libraries. There are no built-in collections classes in Scala. All collections are implemented at the library level. And just as there were defects in some on the Scala collections classes that David MacIver fixed, there are existing defects in the Actor libraries. Just as there are specialized Map() collections that are appearing for Scala that maximize performance for particular data types and/or key distributions, we are creating a specialized Actor library that's optimized for the kind of use that we see in Lift and web apps in general. This is a testament to Scala's flexibility and to the foresight of including such a powerful concurrency library, Actors, as part of the distribution. But for those two things, Lift would not be nearly as cool as it is. So, please do not read this thread as a repudiation of the Scala Actor library, please read it as an expansion of what is possible within Scala. Thanks, David I'm referring to the fact that Scala actors are not part of the core language. They're just a library that can be replaced with a different library, which can also to provide the 'feel' of native language support for objects of that type. It's such a fundamental part of the language design that Programming in Scala talks about it in Chapter 1, Section 1. It's timely that you sent the email so soon after the link to the Guy Steele Growing a Language OOPSLA presentation (of which I am still in awe) went around on twitter. http://video.google.com/videoplay?docid=-8860158196198824415 I guess this demonstrates that Scala provides the features for growth that Steele says are needed for languages to be successful in the long term, and that he would have liked Java to have. Awesome. Nice, clear explanation, by the way. Should avoid any any NIH allegations on the diggs and reddits of the world ;o) Martin -- Lift, the simply functional web framework http://liftweb.net Beginning Scala http://www.apress.com/book/view/1430219890 Follow me: http://twitter.com/dpp Git some: http://github.com/dpp --~--~-~--~~~---~--~~ You received this message because you are subscribed to the Google Groups Lift group. To post to this group, send email to liftweb@googlegroups.com To unsubscribe from this group, send email to liftweb+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/liftweb?hl=en -~--~~~~--~~--~--~---
[Lift] Re: New Lift Actor code
First, and most important to Lift, a conceptual framework for doing concurrency. Without the Actor model, Lift would not have such a rich model for building interactive applications. A design that keeps true to the Erlang Actor model in that it supports linking, run states, and other things that make an OTP style library possible. (Hey Jonas, where's that OTP library?) Here it is the repo: http://github.com/jboner/scala-otp/tree/master Or do you mean that it has not happened much there for a while? I certainly plan to expand it quite a lot, even have some code I could make its way into it eventually. -- Jonas Bonér twitter: @jboner blog:http://jonasboner.com work: http://crisp.se work: http://scalablesolutions.se code: http://github.com/jboner --~--~-~--~~~---~--~~ You received this message because you are subscribed to the Google Groups Lift group. To post to this group, send email to liftweb@googlegroups.com To unsubscribe from this group, send email to liftweb+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/liftweb?hl=en -~--~~~~--~~--~--~---
