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<h1 class="menu-title">Futures Explained in 200 Lines of Rust</h1>
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<main>
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<h1><a class="header" href="#some-background-information" id="some-background-information">Some background information</a></h1>
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<blockquote>
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<p><strong>Relevant for:</strong></p>
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<ul>
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<li>High level introduction to concurrency in Rust</li>
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<li>Knowing what Rust provides and not when working with async code</li>
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<li>Understanding why we need runtimes </li>
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<li>Getting pointers to further reading on concurrency in general</li>
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</ul>
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</blockquote>
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<p>Before we start implementing our <code>Futures</code> , we'll go through some background
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information that will help demystify some of the concepts we encounter.</p>
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<p>Actually, after going through these concepts, implementing futures will seem
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pretty simple. I promise.</p>
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<h2><a class="header" href="#async-in-rust" id="async-in-rust">Async in Rust</a></h2>
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<p>Let's get some of the common roadblocks out of the way first.</p>
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<p>Async in Rust is different from most other languages in the sense that Rust
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has a very lightweight runtime.</p>
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<p>Languages like C#, JavaScript, Java and GO, already includes a runtime
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for handling concurrency. So if you come from one of those languages this will
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seem a bit strange to you.</p>
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<p>In Rust you will have to make an active choice about which runtime to use.</p>
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<h3><a class="header" href="#what-rusts-standard-library-takes-care-of" id="what-rusts-standard-library-takes-care-of">What Rust's standard library takes care of</a></h3>
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<ol>
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<li>The definition of an interruptible task</li>
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<li>An efficient technique to start, suspend, resume and store tasks which are
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executed concurrently.</li>
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<li>A defined way to wake up a suspended task</li>
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</ol>
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<p>That's really what Rusts standard library does. As you see there is no definition
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of non-blocking I/O, how these tasks are created or how they're run.</p>
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<h3><a class="header" href="#what-you-need-to-find-elsewhere" id="what-you-need-to-find-elsewhere">What you need to find elsewhere</a></h3>
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<p>A runtime. Well, in Rust we normally divide the runtime into two parts:</p>
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<ul>
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<li>The Reactor</li>
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<li>The Executor</li>
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</ul>
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<p>Reactors create leaf <code>Futures</code>, and provides things like non-blocking sockets,
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an event queue and so on.</p>
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<p>Executors, accepts one or more asynchronous tasks called <code>Futures</code> and takes
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care of actually running the code we write, suspend the tasks when they're
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waiting for I/O and resume them.</p>
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<p>In theory, we could choose one <code>Reactor</code> and one <code>Executor</code> that have nothing
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to do with each other besides that one creates leaf <code>Futures</code> and the other one
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runs them, but in reality today you'll most often get both in a <code>Runtime</code>.</p>
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<p>There are mainly two such runtimes today <a href="https://github.com/async-rs/async-std">async_std</a> and <a href="https://github.com/tokio-rs/tokio">tokio</a>.</p>
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<p>Quite a bit of complexity attributed to <code>Futures</code> are actually complexity rooted
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in runtimes. Creating an efficient runtime is hard. </p>
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<p>Learning how to use one correctly can require quite a bit of effort as well, but you'll see that there are several similarities between these kind of runtimes so
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learning one makes learning the next much easier.</p>
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<p>The difference between Rust and other languages is that you have to make an
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active choice when it comes to picking a runtime. Most often you'll just use
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the one provided for you.</p>
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<h2><a class="header" href="#futures-10-and-futures-30" id="futures-10-and-futures-30">Futures 1.0 and Futures 3.0</a></h2>
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<p>I'll not spend too much time on this, but it feels wrong to not mention that
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there have been several iterations on how async should work in Rust.</p>
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<p><code>Futures 3.0</code> works with the relatively new <code>async/await</code> syntax in Rust and
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it's what we'll learn.</p>
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<p>Now, since this is rather recent, you can encounter creates that use <code>Futures 1.0</code>
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still. This will get resolved in time, but unfortunately it's not always easy
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to know in advance.</p>
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<p>A good sign is that if you're required to use combinators like <code>and_then</code> then
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you're using <code>Futures 1.0</code>.</p>
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<p>While they're not directly compatible, there is a tool that let's you relatively
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easily convert a <code>Future 1.0</code> to a <code>Future 3.0</code> and vice a versa. You can find
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all you need in the <a href="https://github.com/rust-lang/futures-rs"><code>futures-rs</code></a> crate and all
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<a href="https://rust-lang.github.io/futures-rs/blog/2019/04/18/compatibility-layer.html">information you need here</a>.</p>
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<h2><a class="header" href="#first-things-first" id="first-things-first">First things first</a></h2>
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<p>If you find the concepts of concurrency and async programming confusing in
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general, I know where you're coming from and I have written some resources to
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try to give a high level overview that will make it easier to learn Rusts
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<code>Futures</code> afterwards:</p>
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<ul>
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<li><a href="https://cfsamson.github.io/book-exploring-async-basics/1_concurrent_vs_parallel.html">Async Basics - The difference between concurrency and parallelism</a></li>
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<li><a href="https://cfsamson.github.io/book-exploring-async-basics/2_async_history.html">Async Basics - Async history</a></li>
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<li><a href="https://cfsamson.github.io/book-exploring-async-basics/5_strategies_for_handling_io.html">Async Basics - Strategies for handling I/O</a></li>
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<li><a href="https://cfsamson.github.io/book-exploring-async-basics/6_epoll_kqueue_iocp.html">Async Basics - Epoll, Kqueue and IOCP</a></li>
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</ul>
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<p>Learning these concepts by studying futures is making it much harder than
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it needs to be, so go on and read these chapters if you feel a bit unsure. </p>
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<p>I'll be right here when you're back.</p>
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<p>However, if you feel that you have the basics covered, then let's get moving!</p>
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