Axiom

Axiom brings a highly-scalable actor model to the Rust language based on the many lessons learned over years of Actor model implementations in Akka and Erlang. Axiom is, however, not a direct re-implementation of either of the two aforementioned actor models but rather a new implementation deriving inspiration from the good parts of those models.

Getting Started

An actor model is an architectural asynchronous programming paradigm characterized by the use of actors for all processing activities.

Actors have the following characteristics:

1. An actor can be interacted with only by means of messages.
2. An actor processes only one message at a time.
3. An actor will process a message only once.
4. An actor can send a message to any other actor without knowledge of that actor's internals.
5. Actors send only immutable data as messages, though they may have mutable internal state.
6. Actors are location agnostic; they can be sent a message from anywhere in the cluster.

Note that within the language of Rust, rule five can't be enforced by Rust but is a best practice which is important for developers creating actors based on Axiom. In Erlang and Elixir rule five cannot be violated because of the structure of the language but this also leads to performance limitations. It's better to allow internal mutable state and encourage the good practice of not sending mutable state as messages.

What is important to understand is that these rules combined together makes each actor operate like a micro-service in the memory space of the program using them. Since actor messages are immutable, actors can trade information safely and easily without copying large data structures.

Although programming in the actor model is quite an involved process you can get started with Axiom in only a few lines of code.

use axiom::actors::*;
use std::sync::Arc;

let system = ActorSystem::create(10, 1);

let aid = ActorSystem::spawn(&system,
    0 as usize,
    |_state: &mut usize, _aid: Arc<ActorId>, message: &Arc<Message>| Status::Processed,
);

ActorId::send(&aid, Arc::new(11));

This code creates an actor system, spawns an actor and finally sends the actor a message. That is really all there is to it but of course it doesn't end there. If you want to create an actor with a struct that is simple as well. Let's create one that handles a couple of different message types:

use axiom::actors::*;
use std::sync::Arc;

let system = ActorSystem::create(10, 1);

struct Data {
    value: i32,
}

impl Data {
    fn handle_bool(&mut self, _aid: Arc<ActorId>, message: &bool) -> Status {
        if *message {
            self.value += 1;
        } else {
            self.value -= 1;
        }
        Status::Processed // assertion will fail but we still have to return.
    }

    fn handle_i32(&mut self, _aid: Arc<ActorId>, message: &i32) -> Status {
        self.value += *message;
        Status::Processed // assertion will fail but we still have to return.
    }

    fn handle(&mut self, aid: Arc<ActorId>, message: &Arc<Message>) -> Status {
        if let Some(msg) = message.downcast_ref::<bool>() {
            self.handle_bool(aid, msg)
        } else if let Some(msg) = message.downcast_ref::<i32>() {
            self.handle_i32(aid, msg)
        } else {
            assert!(false, "Failed to dispatch properly");
            Status::Stop // assertion will fail but we still have to return.
        }
    }
}

let data = Data { value: 0 };

let aid = ActorSystem::spawn(&system, data, Data::handle);

ActorId::send(&aid, Arc::new(11));
ActorId::send(&aid, Arc::new(true));
ActorId::send(&aid, Arc::new(true));
ActorId::send(&aid, Arc::new(false));

This code creates an actor out of an arbitrary struct. Since the only requirement to make an actor is to have a function that is compliant with the [axiom::actors::Processor] trait, anything can be an actor. If this struct had been declared somewhere outside of your control you could use it in an actor as state by declaring your own handler function and making the calls to the 3rd party structure.

It's important to keep in mind that the starting state is moved into the actor and you will not have external access to it afterwards. This is by design and although you could conceivably use a std::sync::Arc to a structure as state, that would definitely be a bad idea as it would break the rules we laid out for actors.

There is a lot more to learn and explore and your best resource is the test code for Axiom. The developers have a belief that test code should be well architected and well commented to act as a set of examples for users of Axiom.

Design Principals of Axiom

Based on previous experience with other actor models I wanted to design Axiom around some core principles:

1. At its core an actor is just an function that processes messages. The simplest actor is a function that takes a message and simply ignores it. The benefit to the functional approach over the Akka model is that it allows the user to create actors easily and simply. I like to refer to this quantum programming; the notion of building a complex system from the smallest components. Software based on the actor model can get complicated; keeping it simple at the core is fundamental to solid architecture.
2. An actor can supervise other actors. In the Erlang model there is a strong separation between the actor, known as a process, and a supervisor which manages other Erlang processes. This separation makes implementing certain kinds of applications cumbersome. Axiom, by contrast, borrows from the Akka approach allowing any actor to supervise other actors and act as routers to those child actors.
3. Actors can be a Finite State Machine (FSM). Actors receive and process messages nominally in the order received. However, there are certain circumstances where an actor has to change to another state and process other messages, skipping certain messages to be processed later.
4. When skipping messages, the messages must not move. Akka allows the skipping of messages by stashing the message in another data structure and then restoring this stash later. This process has many inherent flaws. Instead Axiom allows an actor to skip messages in its channel but leave them where they are, increasing performance and avoiding many problems.
5. Actors use a bounded capacity channel. In Axiom the message capacity for the actor's channel is bounded, resulting in greater simplicity and an emphasis on good actor design.
6. Axiom should be kept as small as possible. Axiom is the core of the actor model and should not be expanded to include everything possible for actors. That should be the job of libraries that extend Axiom. Axiom itself should be an example of quantum programming.
7. The tests are the best place for examples. The tests of Axiom will be extensive and well maintained and should be a resource for those wanting to use Axiom. They should not be a dumping ground for copy-paste or throwaway code. The best tests will look like architected code.