documentation/manual/working/scalaGuide/main/pekko/ScalaPekko.md
Pekko uses the Actor Model to raise the abstraction level and provide a better platform to build correct concurrent and scalable applications. For fault-tolerance it adopts the ‘Let it crash’ model, which has been used with great success in the telecoms industry to build applications that self-heal - systems that never stop. Actors also provide the abstraction for transparent distribution and the basis for truly scalable and fault-tolerant applications.
Pekko can work with several containers called actor systems. An actor system manages the resources it is configured to use in order to run the actors which it contains.
A Play application defines a special actor system to be used by the application. This actor system follows the application life-cycle and restarts automatically when the application restarts.
To start using Pekko, you need to write an actor. Below is a simple actor that simply says hello to whoever asks it to.
This actor follows a few Pekko conventions:
props method on its companion object that returns the props for creating itTo create and/or use an actor, you need an ActorSystem. This can be obtained by declaring a dependency on an ActorSystem, like so:
The actorOf method is used to create a new actor. Notice that we've declared this controller to be a singleton. This is necessary since we are creating the actor and storing a reference to it, if the controller was not scoped as singleton, this would mean a new actor would be created every time the controller was created, which would ultimate throw an exception because you can't have two actors in the same system with the same name.
The most basic thing that you can do with an actor is send it a message. When you send a message to an actor, there is no response, it's fire and forget. This is also known as the tell pattern.
In a web application however, the tell pattern is often not useful, since HTTP is a protocol that has requests and responses. In this case, it is much more likely that you will want to use the ask pattern. The ask pattern returns a Future, which you can then map to your own result type.
Below is an example of using our HelloActor with the ask pattern:
@ask
A few things to notice:
? operator on the actor.Future[Any], usually the first thing you will want to do after asking actor is map that to the type you are expecting, using the mapTo method.If you prefer, you can have Guice instantiate your actors and bind actor refs to them for your controllers and components to depend on.
For example, if you wanted to have an actor that depended on the Play configuration, you might do this:
Play provides some helpers to help providing actor bindings. These allow the actor itself to be dependency injected, and allows the actor ref for the actor to be injected into other components. To bind an actor using these helpers, create a module as described in the [[dependency injection documentation|ScalaDependencyInjection#Play-applications]], then mix in the PekkoGuiceSupport trait and use the bindActor method to bind the actor:
This actor will both be named configured-actor, and will also be qualified with the configured-actor name for injection. You can now depend on the actor in your controllers and other components:
The above is good for injecting root actors, but many of the actors you create will be child actors that are not bound to the lifecycle of the Play app, and may have additional state passed to them.
In order to assist in dependency injecting child actors, Play utilises Guice's AssistedInject support.
Let's say you have the following actor, which depends on configuration to be injected, plus a key:
Note that the key parameter is declared to be @Assisted, this tells that it's going to be manually provided.
We've also defined a Factory trait, this takes the key, and returns an Actor. We won't implement this, Guice will do that for us, providing an implementation that not only passes our key parameter, but also locates the Configuration dependency and injects that. Since the trait just returns an Actor, when testing this actor we can inject a factory that returns any actor, for example this allows us to inject a mocked child actor, instead of the actual one.
Now, the actor that depends on this can extend InjectedActorSupport, and it can depend on the factory we created:
It uses the injectedChild to create and get a reference to the child actor, passing in the key. The second parameter (key in this example) will be used as the child actor's name.
Finally, we need to bind our actors. In our module, we use the bindActorFactory method to bind the parent actor, and also bind the child factory to the child implementation:
This will get Guice to automatically bind an instance of ConfiguredChildActor.Factory, which will provide an instance of Configuration to ConfiguredChildActor when it's instantiated.
The default actor system configuration is read from the Play application configuration file. For example, to configure the default dispatcher of the application actor system, add these lines to the conf/application.conf file:
pekko.actor.default-dispatcher.fork-join-executor.parallelism-max = 64
pekko.actor.debug.receive = on
For Pekko logging configuration, see [[configuring logging|SettingsLogger]].
By default the name of the Play actor system is application. You can change this via an entry in the conf/application.conf:
play.pekko.actor-system = "custom-name"
Note: This feature is useful if you want to put your play application ActorSystem in an Pekko cluster.
While we recommend you use the built in actor system, as it sets up everything such as the correct classloader, lifecycle hooks, etc, there is nothing stopping you from using your own actor system. It is important however to ensure you do the following:
pekko config, which is used by Play's actor system already, as this will cause problems such as when the systems try to bind to the same remote portsPlay handles the shutdown of the Application and the Server using Pekko's Coordinated Shutdown. Find more information in the [[Coordinated Shutdown|Shutdown]] common section.
NOTE: Play only handles the shutdown of its internal ActorSystem. If you are using extra actor systems, make sure they are all terminated and feel free to migrate your termination code to Coordinated Shutdown.
You can make your Play application join an existing Pekko Cluster. In that case it is recommended that you leave the cluster gracefully. Play ships with Pekko's Coordinated Shutdown which will take care of that graceful leave.
If you already have custom Cluster Leave code it is recommended that you replace it with Pekko's handling. See Pekko docs for more details.
If you want to use a newer version of Pekko, one that is not used by Play yet, you can add the following to your build.sbt file:
Of course, other Pekko artifacts can be added transitively. Use sbt-dependency-graph to better inspect your build and check which ones you need to add explicitly.
If you haven't switched to the Netty server backend and therefore are using Play's default Pekko HTTP server backend, you also have to update Pekko HTTP. Therefore, you need to add its dependencies explicitly as well:
Note: When doing such updates, keep in mind that you need to follow Pekko's Binary Compatibility Rules. And if you are manually adding other Pekko artifacts, remember to keep the version of all the Pekko artifacts consistent since mixed versioning is not allowed.
Note: When resolving dependencies, sbt will get the newest one declared for this project or added transitively. It means that if Play depends on a newer Pekko (or Pekko HTTP) version than the one you are declaring, Play's version wins. See more details about how sbt does evictions here.