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Actor Hierarchy

Actors in Riker form a hierarchy with each actor addressable by a path. An actor's place in the hierarchy is determined by the location of its parent. Let's take a look at what the actor hierarchy looks like immediately after the actor system has started:

riker
└─ user
└─ system
   └─ logger
   └─ sys_events
   └─ dead_letters
   └─ dl_logger
└─ temp

We can see that without starting any actors ourselves, there's already a number of actors running. At the base of the hierarchy is our application which by default is named riker unless a name was provided using SystemBuilder.

There's then three root actors, user, system and temp. These are guardian actors under which all other actors are created. Perhaps the most important of these is user, since most actors created as part of the application are created in this branch.

If we start an actor using system.actor_of(props, "my-actor") we can see it added under user:

my-app
└─ user
   └─ my-actor      <-- our new actor is added
└─ system
   └─ logger
   └─ sys_events
   └─ dead_letters
   └─ dl_logger
└─ temp

In this case the newly created my-actor has a path of /user/my-actor. Since it was started by using actor_of on ActorSystem it is considered a top-level actor.

Let's look at how the hierarchy changes when another actor is started, this time from within /user/my-actor's recv method using Context.actor_of.

impl Actor for MyActor {
    type Msg = String;

    fn recv(&mut self,
            ctx: &Context<String>,
            msg: String,
            sender: Sender) {

        ctx.actor_of(MyActor::props(), "my-child").unwrap();
    }
}

Here MyActor will start another actor, which is also an instance of MyActor.

my-app
└─ user
   └─ my-actor
      └─ my-child   <-- our new actor is added
└─ system
   └─ logger
   └─ sys_events
   └─ dead_letters
   └─ dl_logger
└─ temp

Since the new actor was started using my-actor's context it is added to the hierarchy as a child of my-actor. my-child's path becomes /user/my-actor/my-child.

Let's move on the next section where we'll look at how the actor hierarchy is used in supervision to build resilient, self healing applications.

Fault Tolerance