"I thought of objects being like biological cells and/or individual computers on a network, only able to communicate with messages"
--Alan Kay, creator of Smalltalk, on the meaning of "object oriented programming"
Celluloid is a concurrent object framework for Ruby inspired by Erlang and the Actor model. Celluloid gives you thread-backed objects that run concurrently, providing the simplicity of Ruby objects for the most common use cases, but also the ability to call methods asynchronously, allowing the receiver to do things in the background while the caller carries on with its business. These concurrent objects are called "actors". Actors are somewhere in between the kind of object you're typically used to working with and a network service.
To use Celluloid, define a normal Ruby class that includes Celluloid::Actor
class Sheen
include Celluloid::Actor
def initialize(name)
@name = name
end
def set_status(status)
@status = status
end
def report
"#{@name} is #{@status}"
end
end
If you call Sheen.new, you'll wind up with a plain old Ruby object. To create a concurrent object instead of a regular one, use Sheen.spawn:
>> charlie = Sheen.spawn "Charlie Sheen"
=> #<Celluloid::Actor(Sheen:0x00000100a312d0) @name="Charlie Sheen">
>> charlie.set_status "winning!"
=> "winning!"
>> charlie.report
=> "Charlie Sheen is winning!"
>> charlie.set_status! "asynchronously winning!"
=> nil
>> charlie.report
=> "Charlie Sheen is asynchronously winning!"
Calling spawn creates a concurrent object running inside its own thread. You can call methods on this concurrent object just like you would any other Ruby object. The Sheen#set_status method works exactly like you'd expect, returning the last expression evaluated.
However, Celluloid's secret sauce kicks in when you call banged predicate methods (i.e. methods ending in !). Even though the Sheen class has no set_status! method, you can still call it. Why is this? Because bang methods have a special meaning in Celluloid. (Note: this also means you can't define bang methods on Celluloid classes and expect them to be callable from other objects)
Adding a bang to the end of a method instructs Celluloid that you would like for the given method to be called asynchronously. This means that rather than the caller waiting for a response, the caller sends a message to the concurrent object that you'd like the given method invoked, and then the caller proceeds without waiting for a response. The concurrent object receiving the message will then process the method call in the background.
Adding a bang to a method name is a convention in Ruby used to indicate that the method is in some way "dangerous", and in Celluloid this is no exception. You have no guarantees that just because you made an asynchronous call it was ever actually invoked. Asynchronous calls will never raise an exception, even if an exception occurs when the receiver is processing it. Worse, unhandled exceptions will crash the receiver, and making an asynchronous call to a crashed object will not raise an error.
However, you can still handle errors created by asynchronous calls using two features of Celluloid called supervisors and linking.
You may be familiar with tools like Monit or God which keep an eye on your applications and restart them when they crash. Celluloid supervisors work in a similar fashion, except instead of monitoring applications, they monitor individual actors and restart them when they crash. Crashes occur whenever an unhandled exception is raised anywhere within an actor.
To supervise an actor, start it with the supervise method. Using the Sheen class from the example above:
>> supervisor = Sheen.supervise "Charlie Sheen"
=> #<Celluloid::Supervisor(Sheen) "Charlie Sheen">
This created a new Celluloid::Supervisor actor, and also created a new Sheen actor, giving its initialize method the argument "Charlie Sheen". The supervise method has the same method signature as spawn. However, rather than returning the newly created actor, supervise returns the supervisor. To retrieve the actor that the supervisor is currently using, use the Celluloid::Supervisor#actor method:
>> supervisor = Sheen.supervise "Charlie Sheen"
=> #<Celluloid::Supervisor(Sheen) "Charlie Sheen">
>> charlie = supervisor.actor
=> #<Celluloid::Actor(Sheen:0x00000100a312d0)>
Supervisors can also automatically put actors into the actor registry using the supervise_as method:
>> Sheen.supervise_as :charlie, "Charlie Sheen"
=> #<Celluloid::Supervisor(Sheen) "Charlie Sheen">
>> charlie = Celluloid::Actor[:charlie]
=> #<Celluloid::Actor(Sheen:0x00000100a312d0)>
In this case, the supervisor will ensure that an actor of the Sheen class, created using the given arguments, is aways available by calling Celluloid::Actor[:charlie]. The first argument to supervise_as is the name you'd like the newly created actor to be registered under. The remaining arguments are passed to initialize just like you called new or spawn.
See the "Registry" section below for more information on the actor registry
Whenever any unhandled exceptions occur in any of the methods of an actor, that actor crashes and dies. Let's start with an example:
class JamesDean
include Celluloid::Actor
class CarInMyLaneError < StandardError; end
def drive_little_bastard
raise CarInMyLaneError, "that guy's gotta stop. he'll see us"
end
end
Now, let's have James drive Little Bastard and see what happens:
>> james = JamesDean.spawn
=> #<Celluloid::Actor(JamesDean:0x1068)>
>> james.drive_little_bastard!
=> nil
>> james
=> #<Celluloid::Actor(JamesDean:0x1068) dead>
When we told james asynchronously to drive Little Bastard, it killed him! If we were Elizabeth Taylor, co-star in James' latest film at the time of his death, we'd certainly want to know when he died. So how can we do that?
Actors can link to other actors they're interested in and want to receive crash notifications from. In order to receive these events, we need to use the trap_exit method to be notified of them. Let's look at how a hypothetical Elizabeth Taylor object could be notified that James Dean has crashed:
class ElizabethTaylor
include Celluloid::Actor
trap_exit :actor_died
def actor_died(actor, reason)
puts "Oh no! #{actor.inspect} has died because of a #{reason.class}"
end
end
We've now used the trap_exit method to configure a callback which is invoked whenever any linked actors crashed. Now we need to link Elizabeth to James so James' crash notifications get sent to her:
>> james = JamesDean.spawn
=> #<Celluloid::Actor(JamesDean:0x11b8)>
>> elizabeth = ElizabethTaylor.spawn
=> #<Celluloid::Actor(ElizabethTaylor:0x11f0)>
>> elizabeth.link james
=> #<Celluloid::Actor(JamesDean:0x11b8)>
>> james.drive_little_bastard!
=> nil
Oh no! #<Celluloid::Actor(JamesDean:0x11b8) dead> has died because of a JamesDean::CarInMyLaneError
Elizabeth called the link method to receive crash events from James. Because Elizabeth was linked to James, when James crashed, James' exit message was sent to her. Because Elizabeth was trapping the exit messages she received using the trap_exit method, the callback she specified was invoked, allowing her to take action (in this case, printing the error). But what would happen if she weren't trapping exits? Let's break James apart into two separate objects, one for James himself and one for Little Bastard, his car:
class PorscheSpider
include Celluloid::Actor
class CarInMyLaneError < StandardError; end
def drive_on_route_466
raise CarInMyLaneError, "head on collision :("
end
end
class JamesDean
include Celluloid::Actor
def initialize
@little_bastard = PorscheSpider.spawn_link
end
def drive_little_bastard
@little_bastard.drive_on_route_466
end
end
If you take a look in JamesDean#initialize, you'll notice that to create an instance of PorcheSpider, James is calling the spawn_link method.
This method works similarly to spawn, except it combines spawn and link into a single call.
Now what happens if we repeat the same scenario with Elizabeth Taylor watching for James Dean's crash?
>> james = JamesDean.spawn
=> #<Celluloid::Actor(JamesDean:0x1108) @little_bastard=#<Celluloid::Actor(PorscheSpider:0x10ec)>>
>> elizabeth = ElizabethTaylor.spawn
=> #<Celluloid::Actor(ElizabethTaylor:0x1144)>
>> elizabeth.link james
=> #<Celluloid::Actor(JamesDean:0x1108) @little_bastard=#<Celluloid::Actor(PorscheSpider:0x10ec)>>
>> james.drive_little_bastard!
=> nil
Oh no! #<Celluloid::Actor(JamesDean:0x1108) dead> has died because of a PorscheSpider::CarInMyLaneError
When Little Bastard crashed, it killed James as well. Little Bastard killed James, and because Elizabeth was trapping James' exit events, she received the notification of James' death.
Actors that are linked together propagate their error messages to all other actors that they're linked to. Unless those actors are trapping exit events, those actors too will die, like James did in this case. If you have many, many actors linked together in a large object graph, killing one will kill them all unless they are trapping exits.
This allows you to factor your problem into several actors. If an error occurs in any of them, it will kill off all actors used in a particular system. In general, you'll probably want to have a supervisor start a single actor which is in charge of a particular part of your system, and have that actor spawn_link to other actors which are part of the same system. If any error occurs in any of these actors, all of them will be killed off and the entire subsystem will be restarted by the supervisor in a clean state.
If, for any reason, you've linked to an actor and want to sever the link, there's a corresponding unlink method to remove links between actors.
Celluloid lets you register actors so you can refer to them symbolically. You can register Actors using Celluloid::Actor[]:
>> james = JamesDean.spawn
=> #<Celluloid::Actor(JamesDean:0x80c27ce0)>
>> Celluloid::Actor[:james] = james
=> #<Celluloid::Actor(JamesDean:0x80c27ce0)>
>> Celluloid::Actor[:james]
=> #<Celluloid::Actor(JamesDean:0x80c27ce0)>
The Celluloid::Actor constant acts as a hash, allowing you to register actors under the name of your choosing, and access actors by name rather than reference. This is important because actors may crash. If you're attempting to reference an actor explicitly by storing it in a variable, you may be holding onto a reference to a crashed copy of that actor, rather than talking to a working, freshly-restarted version.
The main use of the registry is for interfacing with actors that are automatically restarted by supervisors when they crash.
- Check out the latest master to make sure the feature hasn't been implemented or the bug hasn't been fixed yet
- Check out the issue tracker to make sure someone already hasn't requested it and/or contributed it
- Fork the project
- Start a feature/bugfix branch
- Commit and push until you are happy with your contribution
- Make sure to add tests for it. This is important so I don't break it in a future version unintentionally.
- Please try not to mess with the Rakefile, version, or history. If you want to have your own version, or is otherwise necessary, that is fine, but please isolate to its own commit so I can cherry-pick around it.
Copyright (c) 2011 Tony Arcieri. See LICENSE.txt for further details.