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171 lines
9.4 KiB
171 lines
9.4 KiB
11 months ago
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<a href="https://blog.juliobiason.me"><h1>Julio Biason .Me 4.3</h1></a>
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<p class="lead">Old school dev living in a 2.0 dev world</p>
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<div class="post">
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<h1 class="post-title">Thinking About Rust Actors</h1>
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<span class="post-date">
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2023-08-11
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<a href="https://blog.juliobiason.me/tags/rust/">#rust</a>
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<a href="https://blog.juliobiason.me/tags/actor-model/">#actor model</a>
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</span>
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<p>I recently wrote an application for work (so, sorry, can't show you the code)
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that, 'cause it was heavily I/O based, I decided to write it using
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<a href="https://tokio.rs/">Tokio</a> and the idea of <a href="https://ryhl.io/blog/actors-with-tokio/">Actor Model with
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it</a>.</p>
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<p>... which gave me some things to think about.</p>
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<span id="continue-reading"></span>
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<p>Before anything, actors in Rust are very different from the actors in languages
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with the actual Actor Model. In summary, you have your actors, which running
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independently, each actor have an Inbox for things to be processed and an
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"outbox" -- in quotes, 'cause that's not really it. An actor can receive a
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message, process it and then it can just be done with it or it can produce
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something that it is send to another actor -- that's its outbox, which usually
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differs from the Inbox 'cause the Inbox need to have a queue of sorts, but the
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Outbox doesn't (and that's why I've been using "outbox" with quotes before).</p>
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<p>All the messages are delivered by a "post office" of sorts, that connects all
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Actors:</p>
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<p><img src="https://blog.juliobiason.me/code/thinking-about-rust-actors/actors.png" alt="" title="A silly representation of the actor model" /></p>
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<p>On my implementation, the actor is actually a module with a <code>run()</code> function;
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this function exposes the <code>Sender</code> part of a MPSC
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(Multiple-Producer-Single-Consumer) channel which acts as the Inbox of it and
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the task PID, so the can <code>.await</code> the actor processing loop to avoid the main
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application from finishing with the actor still running. </p>
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<div style="border:1px solid grey; margin:7px; padding: 7px">
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<p>For now, I'm ignoring Tokio and async for next examples.</p>
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</div>
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<p>And because there is no "Post Office" kind of solver in Rust, I short-circuited
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the actors by giving the <code>Sender</code> channel of an actor as parameter to a second,
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so it knows where to send its messages. Something like:</p>
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<pre data-lang="rust" style="background-color:#2b303b;color:#c0c5ce;" class="language-rust "><code class="language-rust" data-lang="rust"><span style="color:#b48ead;">let</span><span> channel3 = actor3::run(...);
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</span><span style="color:#b48ead;">let</span><span> channel2 = actor2::run(channel3);
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</span><span>actor1::run(channel2);
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</span></code></pre>
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<p>In this short sample, whatever "actor1" produces, it sends directly to "actor2"
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though the channel the latter created; "actor2", on its part, produces
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something that is received by "actor3". And, with more actors, things just keep
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chaining.</p>
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<div style="border:1px solid grey; margin:7px; padding: 7px">
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<p>I am intentionally ignoring the internals of each actor and their <code>run()</code>
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function, but they are some variations of:</p>
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<pre data-lang="rust" style="background-color:#2b303b;color:#c0c5ce;" class="language-rust "><code class="language-rust" data-lang="rust"><span style="color:#b48ead;">fn </span><span style="color:#8fa1b3;">run</span><span>(..) -> (task::JoinHandle<()>, mpsc::Sender<TheKindOfMessageTheActorAccepts>) {
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</span><span> </span><span style="color:#b48ead;">let </span><span>(tx, </span><span style="color:#b48ead;">mut</span><span> rx) = mpsc::channel::<TheKindOfMessageTheActorAccepts>(</span><span style="color:#d08770;">SOME_SIZE</span><span>);
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</span><span> </span><span style="color:#b48ead;">let</span><span> task = tokio::spawn(async </span><span style="color:#b48ead;">move </span><span>{
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</span><span> </span><span style="color:#b48ead;">while let </span><span>Some(incoming) = rx.</span><span style="color:#96b5b4;">recv</span><span>().await {
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</span><span> </span><span style="color:#b48ead;">let</span><span> conversion = </span><span style="color:#96b5b4;">actor_process</span><span>(incoming);
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</span><span> </span><span style="color:#65737e;">// maybe send the conversion to the next actor?
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</span><span> }
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</span><span> });
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</span><span> (task, tx)
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</span><span>}
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</span></code></pre>
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</div>
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<p>But... 'cause the actors have (very similar) interfaces, that looks like a
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trait!</p>
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<p>So, what should be the Actor trait?</p>
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<p>First thing, its <code>run()</code> or similar function should expose its PID and its
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receiving channel. Something like:</p>
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<pre data-lang="rust" style="background-color:#2b303b;color:#c0c5ce;" class="language-rust "><code class="language-rust" data-lang="rust"><span style="color:#b48ead;">pub trait </span><span>Actor {
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</span><span> </span><span style="color:#b48ead;">fn </span><span style="color:#8fa1b3;">run</span><span>() -> (task::JoinHandle<()>, Sender<TheKindOfMessageTheActorAccepts>);
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</span><span>}
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</span></code></pre>
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<p>Why <code>TheKindOfMessageTheActorAccepts</code>? That's because each actor may have a
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different input message. If we take our short sample above, "actor2" may be
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receiving <code>usize</code>s and sending them as <code>String</code>s to "actor3".</p>
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<p>Because that type may change from actor to actor, it should be an associated
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type:</p>
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<pre data-lang="rust" style="background-color:#2b303b;color:#c0c5ce;" class="language-rust "><code class="language-rust" data-lang="rust"><span style="color:#b48ead;">pub trait </span><span>Actor {
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</span><span> </span><span style="color:#b48ead;">type </span><span>Input;
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</span><span>
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</span><span> </span><span style="color:#b48ead;">fn </span><span style="color:#8fa1b3;">run</span><span>() -> (task::JoinHandle<()>, Sender<</span><span style="color:#b48ead;">Self::</span><span>Input>);
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</span><span>}
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</span></code></pre>
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<p>So the basic idea is that, once the trait is implemented in a struct, we could
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managed it like:</p>
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<pre data-lang="rust" style="background-color:#2b303b;color:#c0c5ce;" class="language-rust "><code class="language-rust" data-lang="rust"><span style="color:#b48ead;">let</span><span> actor3 = Actor3::new(..);
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</span><span style="color:#b48ead;">let </span><span>(actor3_pid, actor3_channel) = actor3::run();
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</span></code></pre>
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<p>Wait, what about the chaining? We could do something simple like:</p>
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<pre data-lang="rust" style="background-color:#2b303b;color:#c0c5ce;" class="language-rust "><code class="language-rust" data-lang="rust"><span style="color:#b48ead;">let</span><span> actor3 = Actor3::new(..);
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</span><span style="color:#b48ead;">let </span><span>(actor3_pid, actor3_channel) = actor3::run();
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</span><span style="color:#b48ead;">let</span><span> actor2 = Actor2::new(actor3_channel);
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</span><span style="color:#b48ead;">let </span><span>(actor2_pid, actor2_channel) = actor2::run();
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</span></code></pre>
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<p>... which is kinda verbose, but does work.</p>
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<p>I have some ideas to make this part more fluent, but I need to do some more
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exploration about the topic (specially since I think we can leverage the type
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system to not allow connecting actors whose input type is not the same as the
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output type of the previous actor). Once I get those hammered down, I'll get a
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follow up post.</p>
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