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<ol class="chapter"><li class="affix"><a href="introduction.html">Introduction</a></li><li><a href="1_background_information.html" class="active"><strong aria-hidden="true">1.</strong> Some background information</a></li><li><a href="2_waker_context.html"><strong aria-hidden="true">2.</strong> Waker and Context</a></li><li><a href="3_generators_pin.html"><strong aria-hidden="true">3.</strong> Generators</a></li><li><a href="4_pin.html"><strong aria-hidden="true">4.</strong> Pin</a></li><li><a href="6_future_example.html"><strong aria-hidden="true">5.</strong> Futures - our main example</a></li><li><a href="8_finished_example.html"><strong aria-hidden="true">6.</strong> Finished example (editable)</a></li><li class="affix"><a href="conclusion.html">Conclusion and exercises</a></li></ol>
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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="#futures" id="futures">Futures</a></h2>
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<p>So what is a future?</p>
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<p>A future is a representation of some operation which will complete in the
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future.</p>
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<p>Async in Rust uses a <code>Poll</code> based approach, in which an asynchronous task will
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have three phases.</p>
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<ol>
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<li><strong>The Poll phase.</strong> A Future is polled which result in the task progressing until
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a point where it can no longer make progress. We often refer to the part of the
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runtime which polls a Future as an executor.</li>
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<li><strong>The Wait phase.</strong> An event source, most often referred to as a reactor,
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registers that a Future is waiting for an event to happen and makes sure that it
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will wake the Future when that event is ready.</li>
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<li><strong>The Wake phase.</strong> The event happens and the Future is woken up. It's now up
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to the executor which polled the Future in step 1 to schedule the future to be
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polled again and make further progress until it completes or reaches a new point
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where it can't make further progress and the cycle repeats.</li>
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</ol>
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<p>Now, when we talk about futures I find it useful to make a distinction between
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<strong>non-leaf</strong> futures and <strong>leaf</strong> futures early on because in practice they're
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pretty different from one another.</p>
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<h3><a class="header" href="#leaf-futures" id="leaf-futures">Leaf futures</a></h3>
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<p>Runtimes create <em>leaf futures</em> which represents a resource like a socket.</p>
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<pre><code class="language-rust ignore noplaypen">// stream is a **leaf-future**
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let mut stream = tokio::net::TcpStream::connect("127.0.0.1:3000");
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</code></pre>
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<p>Operations on these resources, like a <code>Read</code> on a socket, will be non-blocking
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and return a future which we call a leaf future since it's the future which
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we're actually waiting on.</p>
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<p>It's unlikely that you'll implement a leaf future yourself unless you're writing
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a runtime, but we'll go through how they're constructed in this book as well.</p>
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<p>It's also unlikely that you'll pass a leaf-future to a runtime and run it to
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completion alone as you'll understand by reading the next paragraph.</p>
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<h3><a class="header" href="#non-leaf-futures" id="non-leaf-futures">Non-leaf-futures</a></h3>
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<p>Non-leaf-futures is the kind of futures we as <em>users</em> of a runtime writes
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ourselves using the <code>async</code> keyword to create a <strong>task</strong> which can be run on the
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executor.</p>
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<p>The bulk of an async program will consist of non-leaf-futures, which are a kind
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of pause-able computation. This is an important distinction since these futures represents a <em>set of operations</em>. Often, such a task will <code>await</code> a leaf future
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as one of many operations to complete the task.</p>
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<pre><code class="language-rust ignore noplaypen">// Non-leaf-future
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let non_leaf = async {
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let mut stream = TcpStream::connect("127.0.0.1:3000").await.unwrap();// <- yield
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println!("connected!");
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let result = stream.write(b"hello world\n").await; // <- yield
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println!("message sent!");
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...
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};
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</code></pre>
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<p>The key to these tasks is that they're able to yield control to the runtime's
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scheduler and then resume execution again where it left off at a later point.</p>
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<p>In contrast to leaf futures, these kind of futures does not themselves represent
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an I/O resource. When we poll these futures we either run some code or we yield
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to the scheduler while waiting for some resource to signal us that it's ready so
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we can resume where we left off.</p>
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<h2><a class="header" href="#runtimes" id="runtimes">Runtimes</a></h2>
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<p>Languages like C#, JavaScript, Java, GO and many others comes with 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>Rust is different from these languages in the sense that Rust doesn't come with
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a runtime for handling concurrency, so you need to use a library which provide
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this for you.</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 requires quite a bit of effort as well, but
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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, in other languages
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you'll just use the one provided for you.</p>
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<p>An async runtime can be divided into two parts:</p>
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<ol>
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<li>The Executor</li>
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<li>The Reactor</li>
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</ol>
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<p>When Rusts Futures were designed there was a desire to separate the job of
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notifying a <code>Future</code> that it can do more work, and actually doing the work
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on the <code>Future</code>.</p>
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<p>You can think of the former as the reactor's job, and the latter as the
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executors job. These two parts of a runtime interacts using the <code>Waker</code> type.</p>
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<p>The two most popular runtimes for <code>Futures</code> as of writing this is:</p>
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<ul>
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<li><a href="https://github.com/async-rs/async-std">async-std</a></li>
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<li><a href="https://github.com/tokio-rs/tokio">Tokio</a></li>
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</ul>
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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>A common interface representing an operation which will be completed in the
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future through the <code>Future</code> trait.</li>
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<li>An ergonomic way of creating tasks which can be suspended and resumed through
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the <code>async</code> and <code>await</code> keywords.</li>
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<li>A defined interface wake up a suspended task through the <code>Waker</code> type.</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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<h2><a class="header" href="#bonus-section" id="bonus-section">Bonus section</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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