Actors in Tokio over Actix#

I have had the misfortune of working with the very powerful Actix Actor Model library in production over complex business logic and whilst it is probably the best known actor model framework for Rust it pushes you in various directions that you probably don't really want to go in - well not initially - and in all honesty I probably will not use it again.

It is probably part of my therapudic process that I feel compelled to write this article in the hope that others may avoid my mistakes.

I should add that whilst Actix is generally complex to use and may result in your business logic being unnecessarily convoluted it is still one of the fastest Actor Model libraries around and the suggestion I put forward in this blog will not be as fast as using Actix for an Actor Model system.

Speed is not everything however. These days I suggest following Alice Rhyll's wonderful actor model article on constructing actors using Tokio channels which is much more usable especially if you want to retain your sanity and have more control over the system you are building.

What sort of things do you need Actors for?#

Actors like lambda calculus are a whole computation paradigm so you certainly can make your entire system a set of interacting actors and that can work - however in a typical system it might make more sense to use actors to represent a logical boundary for services your system will use in order to manage concurrency.

A good example of this might be boundary services for example a network system, websocket, crypto rpc, database.

graph LR
    subgraph frameworks
        subgraph actorDB["DB Actor"]
            C[(Database)]
        end
        A[UI]
        subgraph actorNet["Net Actor"]
            D[Network]
        end
        subgraph adaptors
            CC["<< database >>"]
            DD["<< network >>"]
            subgraph domain
                B[Usecases]
            end
        end
    end
    A --> B
    B -.-> CC
    B -.-> DD
    CC -.-> C
    DD -.-> D
    D ---> B

style actorDB stroke-dasharray: 5 5, fill: #FFFACD, stroke: #FFD700, color: #1b4332, stroke-width: 3px
style actorNet stroke-dasharray: 5 5, fill: #FFFACD, stroke: #FFD700, color: #1b4332, stroke-width: 3px

Here I am thinking of building things like instant messenging applications - but you can apply this architecture to large distributed systems as well where actors might be individual microservices - or microservice systems.

The referentially transparent nature of actors makes them suitable to distributed systems where one actor can simply send a message to another actor and they need not have any indication of where that actor exists or how it is managed. This way you can get complex distributed applications that can be scaled in interesting ways or can even self assemble in peer to peer networks by giving each peer a role to accomplish.

A simple example#

Start function spawns loop and returns channel#

To have an actor you need a thing that asynchronously spins on messages and processes them one at a time. You also need a way to send the actor a message. A tokio channel is an easy well understood mechanism to achieve this.

Let's say we have a process function:

fn actor_process<M>(msg:M);

This will process all our messages a simple illustration here might be something like this:

fn actor_start() -> mpsc::Sender<M> {
    let (tx, mut rx) = mpsc::channel::<M>(100);
    tokio::spawn(async move {
        while let Some(msg) = rx.recv().await {
            actor_process(msg)
        }
    });

    tx
}

You could then do something like:

let sender = actor_start();
sender.send("say hello!");

And assuming that the processing function knows what to do with the &str "say hello!" you might find the actor_process() function doing something fancy like printing hello to the console perhaps.

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