ActorsThe actor modelBasic conceptsA really short exampleA longer exampleActor is a traitTwo ways to create an ActorHow to start an ActorHow to tell an Actor to do one thingHow to tell an Actor to do several thingsSending and receiving messagesWaiting for a message that never comesGetting a result back from an ActorActors and shared stateCounting true/false values: OutlineThe main methodThe Counter classThe same program, all on one slideTypical resultsCounting 3sMain methodcount3sSequentiallycount3sConcurrentlyThe segment methodSlide 25Analysis of resultsMinimizing Thread creationUsing conventional shared stateDoing it rightThe EndJan 14, 2019ActorsConcurrency made easy(-er)2The actor modelMost of the problems with concurrency--from deadlocks to data corruption--result from having shared stateSolution: Don’t share state!An alternative to shared state is the actor model, in which independent processes send to and receive messages from one anotherThe actor model was developed in the Erlang language, and is being incorporated into many new languagesQuoting Alex Miller, http://www.javaworld.com/javaworld/jw-02-2009/jw-02-actor-concurrency1.html?page=2:The actor model consists of a few key principles:No shared stateLightweight processesAsynchronous message-passingMailboxes to buffer incoming messagesMailbox processing with pattern matching3Basic conceptsAn actor is an independent flow of controlYou can think of an actor as a Thread with extra featuresAn actor does not share its data with any other processThis means you can write it as a simple sequential process, and avoid a huge number of problems that result from shared stateHowever: It is possible to share state; it’s just a very bad ideaAny process can send a message to an actor with the syntax actor ! messageAn actor has a “mailbox” in which it receives messagesAn actor will process its messages one at a time, in the order that it receives them, and use pattern matching to decide what to do with each messageExcept: Messages which don’t match any pattern are ignored, but remain in the mailbox (this is bad)An actor doesn’t do anything unless/until it receives a message4A really short examplescala> import scala.actors.Actor._import scala.actors.Actor._scala> val me = selfme: scala.actors.Actor = scala.actors.ActorProxy@6013a567self is a method that returns the currently executing actorSince we didn’t call self from an actor, but just from a plain old Thread, it actually returns a proxy for the Threadscala> me ! 42Sending myself the message 42Doesn’t wait for an answer--just continues with the next codeNothing is printed because the value of this expression is Unitscala> receive { case x => println(x) }42The pattern x is a simple variable, so it will match anythingThe message is received and printed5A longer exampleimport scala.actors.Actorimport scala.actors.Actor._object TGIF { val worker = actor { loop { receive { case "Friday" => println("Thank God it's Friday!") case "Saturday" => exit case x => println("It's " + x + " and I'm working hard.") } } } def main(args: Array[String]) { val days = "Monday Tuesday Wednesday Thursday Friday Saturday Sunday" for (day <- days.split(" ")) worker ! day }}It's Monday and I'm working hard.It's Tuesday and I'm working hard.It's Wednesday and I'm working hard.It's Thursday and I'm working hard.Thank God it's Friday!Process .../bin/scala exited with code 06Actor is a traitA Scala trait is used like a Java interfaceYou can extend only one class, but you can with any number of traitsExample: class Employee extends Person with BenefitsExample: class Secretary extends Employee with ActorHowever, if you don’t explicitly extend a class, use extends for the first traitExample: class Person extends Life with Liberty with HappinessI don’t know the reasons for this rather strange exceptionA trait, like an interface, can require you to supply certain methodsIn an Actor, you must provide def act = ...7Two ways to create an Actor1. You can mix in the Actor traitExample: class Secretary extends Employee with ActorExample: class Worker extends ActorYour class extends a class, but withs a Trait1. Exception: If you don’t explicitly extend some class, you must use extends for the first trait1. I have no clue what the reason is for this ruleA Trait, like a Java interface, can require you to supply certain methodsThe Actor trait requires you to define an act method (with no parameters)nYou can use the actor factory methodExample: val myWorker = actor { ...code for the actor to execute... }The code is what you would otherwise put in the act method8How to start an ActorWhen you define a object that mixes in the Actor trait, you need to start it running explicitlyExample:class Worker extends Actor { ... }val worker1 = new Workerworker1 startWhen you use the actor factory method, the actor is started automaticallyAn actor doesn’t have to wait for messages before it starts doing work--you can write actors that already know what to do9How to tell an Actor to do one thingHere’s an actor that does one thing, once:class Worker extends Actor { def act = receive { case true => println("I am with you 1000%.") case false => println("Absolutely not!") case _ => println("Well, it's complicated....") }}val worker = new Worker().startworker ! 43Here’s another:val worker = actor { receive { case true => println("I am with you 1000%.") case false => println("Absolutely not!") case _ => println("Well, it's complicated....") }}worker ! 4310How to tell an Actor to do several thingsWhen an actor finishes its task, it quitsTo keep an actor going, put receive in a loopExample:class Counter(id: Int) extends Actor { var yes, no = 0 def act = loop { react { case true => yes += 1 case false => no += 1 case "printResults" => printf("Counter #%d got %d yes, %d no.\n", id, yes, no) case x => println("Counter " + id + " didn't understand " + x) } }}This is a special kind of loop defined in the Actor objectThere is also a loopWhile(condition) {...} methodOther kinds of loops will work with receive (but not react)11Sending and receiving messagesTo send a message, use actor ! messageThe thread sending the message keeps going--it doesn’t wait