mirror of https://github.com/rust-lang/async-book
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<ol class="chapter"><li class="chapter-item expanded "><a href="../01_getting_started/01_chapter.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../01_getting_started/02_why_async.html"><strong aria-hidden="true">1.1.</strong> Why Async?</a></li><li class="chapter-item expanded "><a href="../01_getting_started/03_state_of_async_rust.html"><strong aria-hidden="true">1.2.</strong> The State of Asynchronous Rust</a></li><li class="chapter-item expanded "><a href="../01_getting_started/04_async_await_primer.html"><strong aria-hidden="true">1.3.</strong> async/.await Primer</a></li></ol></li><li class="chapter-item expanded "><a href="../02_execution/01_chapter.html"><strong aria-hidden="true">2.</strong> Under the Hood: Executing Futures and Tasks</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../02_execution/02_future.html"><strong aria-hidden="true">2.1.</strong> The Future Trait</a></li><li class="chapter-item expanded "><a href="../02_execution/03_wakeups.html"><strong aria-hidden="true">2.2.</strong> Task Wakeups with Waker</a></li><li class="chapter-item expanded "><a href="../02_execution/04_executor.html"><strong aria-hidden="true">2.3.</strong> Applied: Build an Executor</a></li><li class="chapter-item expanded "><a href="../02_execution/05_io.html"><strong aria-hidden="true">2.4.</strong> Executors and System IO</a></li></ol></li><li class="chapter-item expanded "><a href="../03_async_await/01_chapter.html"><strong aria-hidden="true">3.</strong> async/await</a></li><li class="chapter-item expanded "><a href="../04_pinning/01_chapter.html"><strong aria-hidden="true">4.</strong> Pinning</a></li><li class="chapter-item expanded "><a href="../05_streams/01_chapter.html"><strong aria-hidden="true">5.</strong> Streams</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../05_streams/02_iteration_and_concurrency.html"><strong aria-hidden="true">5.1.</strong> Iteration and Concurrency</a></li></ol></li><li class="chapter-item expanded "><a href="../06_multiple_futures/01_chapter.html"><strong aria-hidden="true">6.</strong> Executing Multiple Futures at a Time</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../06_multiple_futures/02_join.html"><strong aria-hidden="true">6.1.</strong> join!</a></li><li class="chapter-item expanded "><a href="../06_multiple_futures/03_select.html"><strong aria-hidden="true">6.2.</strong> select!</a></li><li class="chapter-item expanded "><a href="../06_multiple_futures/04_spawning.html"><strong aria-hidden="true">6.3.</strong> Spawning</a></li><li class="chapter-item expanded "><div><strong aria-hidden="true">6.4.</strong> TODO: Cancellation and Timeouts</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">6.5.</strong> TODO: FuturesUnordered</div></li></ol></li><li class="chapter-item expanded "><a href="../07_workarounds/01_chapter.html"><strong aria-hidden="true">7.</strong> Workarounds to Know and Love</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../07_workarounds/02_err_in_async_blocks.html"><strong aria-hidden="true">7.1.</strong> ? in async Blocks</a></li><li class="chapter-item expanded "><a href="../07_workarounds/03_send_approximation.html"><strong aria-hidden="true">7.2.</strong> Send Approximation</a></li><li class="chapter-item expanded "><a href="../07_workarounds/04_recursion.html"><strong aria-hidden="true">7.3.</strong> Recursion</a></li><li class="chapter-item expanded "><a href="../07_workarounds/05_async_in_traits.html"><strong aria-hidden="true">7.4.</strong> async in Traits</a></li></ol></li><li class="chapter-item expanded "><a href="../08_ecosystem/00_chapter.html"><strong aria-hidden="true">8.</strong> The Async Ecosystem</a></li><li class="chapter-item expanded "><a href="../09_example/00_intro.html"><strong aria-hidden="true">9.</strong> Final Project: HTTP Server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../09_example/01_running_async_code.html"><strong aria-hidden="true">9.1.</strong> Running Asynchronous Code</a></li><li class="chapter-item expanded "><a href="../09_example/02_handling_connections_concurrently.html" class="active"><strong aria-hidden="true">9.2.</strong> Handling Connections Concurrently</a></li><li class="chapter-item expanded "><a href="../09_example/03_tests.html"><strong aria-hidden="true">9.3.</strong> Testing the Server</a></li></ol></li><li class="chapter-item expanded "><div><strong aria-hidden="true">10.</strong> TODO: I/O</div></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">10.1.</strong> TODO: AsyncRead and AsyncWrite</div></li></ol></li><li class="chapter-item expanded "><div><strong aria-hidden="true">11.</strong> TODO: Asynchronous Design Patterns: Solutions and Suggestions</div></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">11.1.</strong> TODO: Modeling Servers and the Request/Response Pattern</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">11.2.</strong> TODO: Managing Shared State</div></li></ol></li><li class="chapter-item expanded "><a href="../12_appendix/01_translations.html"><strong aria-hidden="true">12.</strong> Appendix: Translations of the Book</a></li></ol>
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<h1 class="menu-title">Asynchronous Programming in Rust</h1>
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<h1 id="handling-connections-concurrently"><a class="header" href="#handling-connections-concurrently">Handling Connections Concurrently</a></h1>
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<p>The problem with our code so far is that <code>listener.incoming()</code> is a blocking iterator.
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The executor can't run other futures while <code>listener</code> waits on incoming connections,
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and we can't handle a new connection until we're done with the previous one.</p>
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<p>In order to fix this, we'll transform <code>listener.incoming()</code> from a blocking Iterator
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to a non-blocking Stream. Streams are similar to Iterators, but can be consumed asynchronously.
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For more information, see the <a href="../05_streams/01_chapter.html">chapter on Streams</a>.</p>
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<p>Let's replace our blocking <code>std::net::TcpListener</code> with the non-blocking <code>async_std::net::TcpListener</code>,
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and update our connection handler to accept an <code>async_std::net::TcpStream</code>:</p>
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<pre><code class="language-rust ignore">use async_std::prelude::*;
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async fn handle_connection(mut stream: TcpStream) {
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let mut buffer = [0; 1024];
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stream.read(&mut buffer).await.unwrap();
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//<-- snip -->
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stream.write(response.as_bytes()).await.unwrap();
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stream.flush().await.unwrap();
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}</code></pre>
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<p>The asynchronous version of <code>TcpListener</code> implements the <code>Stream</code> trait for <code>listener.incoming()</code>,
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a change which provides two benefits.
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The first is that <code>listener.incoming()</code> no longer blocks the executor.
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The executor can now yield to other pending futures
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while there are no incoming TCP connections to be processed.</p>
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<p>The second benefit is that elements from the Stream can optionally be processed concurrently,
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using a Stream's <code>for_each_concurrent</code> method.
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Here, we'll take advantage of this method to handle each incoming request concurrently.
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We'll need to import the <code>Stream</code> trait from the <code>futures</code> crate, so our Cargo.toml now looks like this:</p>
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<pre><code class="language-diff">+[dependencies]
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+futures = "0.3"
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[dependencies.async-std]
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version = "1.6"
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features = ["attributes"]
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</code></pre>
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<p>Now, we can handle each connection concurrently by passing <code>handle_connection</code> in through a closure function.
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The closure function takes ownership of each <code>TcpStream</code>, and is run as soon as a new <code>TcpStream</code> becomes available.
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As long as <code>handle_connection</code> does not block, a slow request will no longer prevent other requests from completing.</p>
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<pre><code class="language-rust ignore">use async_std::net::TcpListener;
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use async_std::net::TcpStream;
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use futures::stream::StreamExt;
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#[async_std::main]
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async fn main() {
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let listener = TcpListener::bind("127.0.0.1:7878").await.unwrap();
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listener
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.incoming()
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.for_each_concurrent(/* limit */ None, |tcpstream| async move {
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let tcpstream = tcpstream.unwrap();
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handle_connection(tcpstream).await;
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})
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.await;
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}</code></pre>
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<h1 id="serving-requests-in-parallel"><a class="header" href="#serving-requests-in-parallel">Serving Requests in Parallel</a></h1>
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<p>Our example so far has largely presented concurrency (using async code)
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as an alternative to parallelism (using threads).
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However, async code and threads are not mutually exclusive.
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In our example, <code>for_each_concurrent</code> processes each connection concurrently, but on the same thread.
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The <code>async-std</code> crate allows us to spawn tasks onto separate threads as well.
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Because <code>handle_connection</code> is both <code>Send</code> and non-blocking, it's safe to use with <code>async_std::task::spawn</code>.
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Here's what that would look like:</p>
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<pre><pre class="playground"><code class="language-rust">use async_std::task::spawn;
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#[async_std::main]
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async fn main() {
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let listener = TcpListener::bind("127.0.0.1:7878").await.unwrap();
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listener
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.incoming()
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.for_each_concurrent(/* limit */ None, |stream| async move {
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let stream = stream.unwrap();
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spawn(handle_connection(stream));
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})
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.await;
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}</code></pre></pre>
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<p>Now we are using both concurrency and parallelism to handle multiple requests at the same time!
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See the <a href="../08_ecosystem/00_chapter.html#single-threading-vs-multithreading">section on multithreaded executors</a>
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for more information.</p>
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