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Showing content from http://ruby-concurrency.github.io/concurrent-ruby/master/Concurrent/Actor.html below:

Module: Concurrent::Actor — Concurrent Ruby

Edge Features are under active development and may change frequently.

• Deprecations are not added before incompatible changes.
• Edge version: major is always 0, minor bump means incompatible change, patch bump means compatible change.
• Edge features may also lack tests and documentation.
• Features developed in concurrent-ruby-edge are expected to move to concurrent-ruby when finalised.

• Light-weighted running on thread-pool.
• Inspired by Akka and Erlang.
• Modular.

This Actor model implementation makes actors very cheap to create and discard. Thousands of actors can be created, allowing you to break the program into smaller maintainable pieces, without violating the single responsibility principle.

Actor-based concurrency is all the rage in some circles. Originally described in 1973, the actor model is a paradigm for creating asynchronous, concurrent objects that is becoming increasingly popular. Much has changed since actors were first written about four decades ago, which has led to a serious fragmentation within the actor community. There is no universally accepted, strict definition of "actor" and actor implementations differ widely between languages and libraries.

Wiki definition is pretty good: The actor model in computer science is a mathematical model of concurrent computation that treats actors as the universal primitives of concurrent digital computation: in response to a message that it receives, an actor can make local decisions, create more actors, send more messages, and determine how to respond to the next message received.

Concurrency is hard to get right, actors are one of many ways how to simplify the problem.

An example:

class Counter < Concurrent::Actor::Context
    
    def initialize(initial_value)
    @count = initial_value
  end

    def on_message(message)
    if Integer === message
      @count += message
    end
  end
end 
counter = Counter.spawn(:first, 5)

counter.tell(1)
counter << 1

counter.ask(0).class
counter.ask(0).value

class Adder < Concurrent::Actor::RestartingContext
  def initialize(init)
    @count = init
  end

  def on_message(message)
    case message
    when :add
      @count += 1
    else
            pass
    end
  end
end 

adder = Adder.spawn(name: :adder, link: true, args: [1])
    adder.parent
    
adder.tell(:add).tell(:add)
    adder.ask!(:add)                                   adder.ask!(:bad) rescue $!
    adder.ask!(:add)                                   adder.ask!(:terminate!)                            

• Actor.spawn and Actor.spawn!
• AbstractContext.spawn and AbstractContext.spawn!

• Reference#tell Sends the message asynchronously to the actor and immediately returns self (the reference) allowing to chain message telling.
• Reference#ask
• Reference#ask! Sends the message synchronously and blocks until the message is processed. Raises on error.

Messages are processed in same order as they are sent by a sender. It may interleaved with messages from other senders though.

Messages sent between actors should be immutable. Gems like

• Algebrick - Typed struct on steroids based on algebraic types and pattern matching
• Hamster - Efficient, Immutable, Thread-Safe Collection classes for Ruby

are very helpful.

require 'algebrick'                                
Protocol = Algebrick.type do
  variants Add      = type { fields! a: Numeric, b: Numeric },
           Subtract = type { fields! a: Numeric, b: Numeric }
end                                                
class Calculator < Concurrent::Actor::RestartingContext
  include Algebrick::Matching

  def on_message(message)
            match message,
          (on Add.(~any, ~any) do |a, b|
            a + b
          end),
                    (on ~Subtract do |(a, b)|
            a - b
          end)
  end
end 

calculator = Calculator.spawn('calculator')
    addition = calculator.ask Add[1, 2]
    subtraction = calculator.ask Subtract[1, 0.5]
    results = (addition & subtraction)
    results.value!                                     
calculator.ask! :terminate!                        

New actor is defined by subclassing RestartingContext, Context and defining its abstract methods. AbstractContext can be subclassed directly to implement more specific behaviour see Root implementation.

• Context

  Basic Context of an Actor. It supports only linking and it simply terminates on error. Uses Behaviour.basic_behaviour_definition:

• RestartingContext.

  Context of an Actor for robust systems. It supports supervision, linking, pauses on error. Uses Behaviour.restarting_behaviour_definition

Example of ac actor definition:

Message = Struct.new :action, :value 

class AnActor < Concurrent::Actor::RestartingContext
  def initialize(init)
    @counter = init
  end

    def on_message(message)
    case message.action
    when :add
      @counter = @counter + message.value
    when :subtract
      @counter = @counter - message.value
    when :value
      @counter
    else
      pass
    end
  end

    def on_event(event)
    if event == :reset
      @counter = 0     end
  end
end 

an_actor = AnActor.spawn name: 'an_actor', args: 10 
an_actor << Message.new(:add, 1) << Message.new(:subtract, 2) 
an_actor.ask!(Message.new(:value, nil))            an_actor << :boo << Message.new(:add, 1) 
an_actor.ask!(Message.new(:value, nil))            an_actor << :terminate!
    

See methods of AbstractContext what else can be tweaked, e.g AbstractContext#default_reference_class

Reference is public interface of Actor instances. It is used for sending messages and can be freely passed around the application. It also provides some basic information about the actor, see PublicDelegations.

AdHoc.spawn('printer') { -> message { puts message } }

Spawned actor cannot be garbage-collected until it's terminated. There is a reference held in the parent actor.

• Core#name The name of actor instance, it should be uniq (not enforced). Allows easier orientation between actor instances.
• Core#path Path of this actor. It is used for easier orientation and logging. Path is constructed recursively with: parent.path + self.name up to a Actor.root, e.g. /an_actor/its_child.
• Core#parent A parent Actor. When actor is spawned the Actor.current becomes its parent. When actor is spawned from a thread outside of an actor (Actor.current is nil) Actor.root is assigned.

Actors have modular architecture, which is achieved by combining a light core with chain of behaviours. Each message or internal event propagates through the chain allowing the behaviours react based on their responsibility.

• Behaviour::Linking:

  Links the actor to other actors and sends actor's events to them, like: :terminated, :paused, :resumed, errors, etc. Linked actor needs to handle those messages.

  listener = AdHoc.spawn name: :listener do
    lambda do |message|
      case message
      when Reference
        if message.ask!(:linked?)
          message << :unlink
        else
          message << :link
        end
      else
        puts "got event #{message.inspect} from #{envelope.sender}"
      end
    end
  end
  
  an_actor = AdHoc.spawn name: :an_actor, supervise: true, behaviour_definition: Behaviour.restarting_behaviour_definition do
    lambda { |message| raise 'failed'}
  end
  
  listener.ask(an_actor).wait
  an_actor.ask(:fail).wait
  listener.ask(an_actor).wait
  an_actor.ask(:fail).wait
  an_actor << :terminate!
  

  produces only two events, other events happened after unlinking

  got event #<RuntimeError: failed> from #<Concurrent::Actor::Reference /an_actor (Concurrent::Actor::Utils::AdHoc)>
  got event :reset from #<Concurrent::Actor::Reference /an_actor (Concurrent::Actor::Utils::AdHoc)>
  

• Behaviour::Awaits:

  Accepts :await messages. Which allows to wait on Actor to process all previously send messages.

  actor << :a << :b
  actor.ask(:await).wait 

• Behaviour::Pausing:

  Allows to pause actors on errors. When paused all arriving messages are collected and processed after the actor is resumed or reset. Resume will simply continue with next message. Reset also reinitialized context.

• Behaviour::Supervising:

  Handles supervised actors. Handle configures what to do with failed child: :terminate!, :resume!, :reset!, or :restart!. Strategy sets :one_for_one (restarts just failed actor) or :one_for_all (restarts all child actors).

• Behaviour::Supervising:

  Handles supervised actors. Handle configures what to do with failed child: :terminate!, :resume!, :reset!, or :restart!. Strategy sets :one_for_one (restarts just failed actor) or :one_for_all (restarts all child actors).

• Behaviour::ExecutesContext:

  Delegates messages and events to AbstractContext instance.

• Behaviour::ErrorsOnUnknownMessage:

  Simply fails when message arrives here. It's usually the last behaviour.

• Behaviour::Termination:

  Handles actor termination. Waits until all its children are terminated, can be configured on behaviour initialization.

• Behaviour::RemovesChild:

  Removes terminated children.

If needed new behaviours can be added, or old one removed to get required behaviour.

• Context uses Array of behaviours and their construction parameters.

  [[Behaviour::SetResults, :terminate!],
   [Behaviour::RemovesChild],
   [Behaviour::Termination],
   [Behaviour::Linking],
   [Behaviour::Awaits],
   [Behaviour::ExecutesContext],
   [Behaviour::ErrorsOnUnknownMessage]]
  

• RestartingContext uses Array of behaviours and their construction parameters.

  [[Behaviour::SetResults, :pause!],
   [Behaviour::RemovesChild],
   [Behaviour::Termination],
   [Behaviour::Linking],
   [Behaviour::Pausing],
   [Behaviour::Supervising, :reset!, :one_for_one],
   [Behaviour::Awaits],
   [Behaviour::ExecutesContext],
   [Behaviour::ErrorsOnUnknownMessage]]
  

Actors are running on shared thread poll which allows user to create many actors cheaply. Downside is that these actors cannot be directly used to do IO or other blocking operations. Blocking operations could starve the global_fast_executor. However there are two options:

• Create an regular actor which will schedule blocking operations in global_io_executor (which is intended for blocking operations) sending results back to self in messages.
• Create an actor using global_io_executor instead of global_fast_executor, e.g. AnIOActor.spawn name: :blocking, executor: Concurrent.global_io_executor.

require 'concurrent'                               


class ActorDoingIO < Concurrent::Actor::RestartingContext
  def on_message(message)
      end

  def default_executor
    Concurrent.global_io_executor
  end
end 

actor_doing_io = ActorDoingIO.spawn :actor_doing_io
    actor_doing_io.executor == Concurrent.global_io_executor
    

class IOWorker < Concurrent::Actor::Context
  def on_message(io_job)
        sleep 0.1
    puts "#{path} second:#{(Time.now.to_f*100).floor} message:#{io_job}"
  end

  def default_executor
    Concurrent.global_io_executor
  end
end 

pool = Concurrent::Actor::Utils::Pool.spawn('pool', 2) do |index|
  IOWorker.spawn(name: "worker-#{index}")
end
    
pool << 1 << 2 << 3 << 4 << 5 << 6
    

sleep 1                                            

see AbstractContext#dead_letter_routing description:

Defines an actor responsible for dead letters. Any rejected message send with Reference#tell is sent there, a message with future is considered already monitored for failures. Default behaviour is to use AbstractContext#dead_letter_routing of the parent, so if no AbstractContext#dead_letter_routing method is overridden in parent-chain the message ends up in Actor.root.dead_letter_routing agent which will log warning.

Alleged supervisor will receive errors from its supervised actors. They'll have to be handled manually.

Use option behaviour_definition: Behaviour.restarting_behaviour_definition(:resume!) or behaviour_definition: Behaviour.restarting_behaviour_definition(:reset!, :one_for_all)

Any existing behavior can be subclassed

By subclassing Behaviour::Pausing and overriding Behaviour::Pausing#restart!. Implementing AbstractContext#on_event could be also considered.

We'll be happy to answer any other questions, just open an Issue or find us on https://gitter.im/ruby-concurrency/concurrent-ruby.

Simple benchmark Actor vs Celluloid, the numbers are looking good but you know how it is with benchmarks. Source code is in examples/actor/celluloid_benchmark.rb. It sends numbers between x actors and adding 1 until certain limit is reached.

Benchmark legend:

• mes. - number of messages send between the actors
• act. - number of actors exchanging the messages
• impl. - which gem is used

Rehearsal ---------------------------------------------------------
50000    2 concurrent  26.140000   0.610000  26.750000 (  7.761000)
50000    2 celluloid   41.440000   5.270000  46.710000 ( 17.535000)
50000  500 concurrent  11.340000   0.180000  11.520000 (  3.498000)
50000  500 celluloid   19.310000  10.680000  29.990000 ( 14.619000)
50000 1000 concurrent  10.640000   0.180000  10.820000 (  3.563000)
50000 1000 celluloid   17.840000  19.850000  37.690000 ( 18.892000)
50000 1500 concurrent  14.120000   0.290000  14.410000 (  4.618000)
50000 1500 celluloid   19.060000  28.920000  47.980000 ( 25.185000)
---------------------------------------------- total: 225.870000sec

 mes. act.      impl.       user     system      total        real
50000    2 concurrent   7.320000   0.530000   7.850000 (  3.637000)
50000    2 celluloid   13.780000   4.730000  18.510000 ( 10.756000)
50000  500 concurrent   9.270000   0.140000   9.410000 (  3.020000)
50000  500 celluloid   16.540000  10.920000  27.460000 ( 14.308000)
50000 1000 concurrent   9.970000   0.160000  10.130000 (  3.445000)
50000 1000 celluloid   15.930000  20.840000  36.770000 ( 18.272000)
50000 1500 concurrent  11.580000   0.240000  11.820000 (  3.723000)
50000 1500 celluloid   19.440000  29.060000  48.500000 ( 25.227000) (1)

Rehearsal ---------------------------------------------------------
50000    2 concurrent   4.180000   0.080000   4.260000 (  4.269435)
50000    2 celluloid    7.740000   3.100000  10.840000 ( 10.043875)
50000  500 concurrent   5.900000   1.310000   7.210000 (  6.565067)
50000  500 celluloid   12.820000   5.810000  18.630000 ( 17.320765)
50000 1000 concurrent   6.080000   1.640000   7.720000 (  6.931294)
50000 1000 celluloid   17.130000   8.320000  25.450000 ( 23.786146)
50000 1500 concurrent   6.940000   2.030000   8.970000 (  7.927330)
50000 1500 celluloid   20.980000  12.040000  33.020000 ( 30.849578)
---------------------------------------------- total: 116.100000sec

 mes. act.      impl.       user     system      total        real
50000    2 concurrent   3.730000   0.100000   3.830000 (  3.822688)
50000    2 celluloid    7.900000   2.910000  10.810000 (  9.924014)
50000  500 concurrent   5.420000   1.230000   6.650000 (  6.025579)
50000  500 celluloid   12.720000   5.540000  18.260000 ( 16.889517)
50000 1000 concurrent   5.420000   0.910000   6.330000 (  5.896689)
50000 1000 celluloid   16.090000   8.040000  24.130000 ( 22.347102)
50000 1500 concurrent   5.580000   0.760000   6.340000 (  6.038535)
50000 1500 celluloid   20.000000  11.680000  31.680000 ( 29.590774) (1)

Note (1): Celluloid is using thread per actor so this bench is creating about 1500 native threads. Actor is using constant number of threads.

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