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Eli Bendersky
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public/context-in-http-servers
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public/context-in-http-servers
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<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>Go by Example: Context in HTTP Servers</title>
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<link rel=stylesheet href="site.css">
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</head>
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<script>
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onkeydown = (e) => {
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if (e.key == "ArrowLeft") {
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window.location.href = 'http-servers';
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}
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if (e.key == "ArrowRight") {
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window.location.href = 'spawning-processes';
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}
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}
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</script>
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<body>
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<div class="example" id="context-in-http-servers">
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<h2><a href="./">Go by Example</a>: Context in HTTP Servers</h2>
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<table>
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<tr>
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<td class="docs">
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<p>In the previous example we looked at setting up a simple
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<a href="http-servers">HTTP server</a>. HTTP servers are useful for
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demonstrating the usage of <code>context.Context</code> for
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controlling cancellation.</p>
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</td>
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<td class="code leading">
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<a href="http://play.golang.org/p/h1gVSe70R00"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
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<span class="s">"fmt"</span>
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<span class="s">"net/http"</span>
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<span class="s">"time"</span>
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<span class="p">)</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">hello</span><span class="p">(</span><span class="nx">w</span> <span class="nx">http</span><span class="p">.</span><span class="nx">ResponseWriter</span><span class="p">,</span> <span class="nx">req</span> <span class="o">*</span><span class="nx">http</span><span class="p">.</span><span class="nx">Request</span><span class="p">)</span> <span class="p">{</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>A <code>context.Context</code> is created for each request by
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the <code>net/http</code> machinery, and is available with
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the <code>Context()</code> method.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">ctx</span> <span class="o">:=</span> <span class="nx">req</span><span class="p">.</span><span class="nx">Context</span><span class="p">()</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"server: hello handler started"</span><span class="p">)</span>
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<span class="k">defer</span> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"server: hello handler ended"</span><span class="p">)</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Wait for 3 seconds before sending a reply to the
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client. This could simulate some work the server is
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doing. While working, keep an eye on the context’s
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<code>Done()</code> channel for a signal that we should cancel
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the work and return as soon as possible.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="k">select</span> <span class="p">{</span>
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<span class="k">case</span> <span class="o"><-</span><span class="nx">time</span><span class="p">.</span><span class="nx">After</span><span class="p">(</span><span class="mi">3</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">):</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Fprintf</span><span class="p">(</span><span class="nx">w</span><span class="p">,</span> <span class="s">"hello\n"</span><span class="p">)</span>
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<span class="k">case</span> <span class="o"><-</span><span class="nx">ctx</span><span class="p">.</span><span class="nx">Done</span><span class="p">():</span>
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<span class="nx">err</span> <span class="o">:=</span> <span class="nx">ctx</span><span class="p">.</span><span class="nx">Err</span><span class="p">()</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"server:"</span><span class="p">,</span> <span class="nx">err</span><span class="p">)</span>
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<span class="nx">internalError</span> <span class="o">:=</span> <span class="nx">http</span><span class="p">.</span><span class="nx">StatusInternalServerError</span>
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<span class="nx">http</span><span class="p">.</span><span class="nx">Error</span><span class="p">(</span><span class="nx">w</span><span class="p">,</span> <span class="nx">err</span><span class="p">.</span><span class="nx">Error</span><span class="p">(),</span> <span class="nx">internalError</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>As before, we register our handler on the “/hello”
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route, and start serving.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre> <span class="nx">http</span><span class="p">.</span><span class="nx">HandleFunc</span><span class="p">(</span><span class="s">"/hello"</span><span class="p">,</span> <span class="nx">hello</span><span class="p">)</span>
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<span class="nx">http</span><span class="p">.</span><span class="nx">ListenAndServe</span><span class="p">(</span><span class="s">":8090"</span><span class="p">,</span> <span class="kc">nil</span><span class="p">)</span>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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</table>
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<table>
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<tr>
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<td class="docs">
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<p>Run the server in the background.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> go run context-in-http-servers.go <span class="p">&</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Simulate a client request to <code>/hello</code>, hitting
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Ctrl+C shortly after starting to signal
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cancellation.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre><span class="gp">$</span> curl localhost:8090/hello
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<span class="go">server: hello handler started</span>
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<span class="go">^C</span>
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<span class="go">server: context canceled</span>
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<span class="go">server: hello handler ended</span>
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</pre></div>
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</td>
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</tr>
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</table>
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<p class="next">
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Next example: <a href="spawning-processes">Spawning Processes</a>.
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</p>
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<p class="footer">
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by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/context-in-http-servers">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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</p>
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</div>
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<script>
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var codeLines = [];
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codeLines.push('package main\u000A');codeLines.push('import (\u000A \"fmt\"\u000A \"net/http\"\u000A \"time\"\u000A)\u000A');codeLines.push('func hello(w http.ResponseWriter, req *http.Request) {\u000A');codeLines.push(' ctx := req.Context()\u000A fmt.Println(\"server: hello handler started\")\u000A defer fmt.Println(\"server: hello handler ended\")\u000A');codeLines.push(' select {\u000A case \x3C-time.After(3 * time.Second):\u000A fmt.Fprintf(w, \"hello\\n\")\u000A case \x3C-ctx.Done():\u000A err := ctx.Err()\u000A fmt.Println(\"server:\", err)\u000A internalError := http.StatusInternalServerError\u000A http.Error(w, err.Error(), internalError)\u000A }\u000A}\u000A');codeLines.push('func main() {\u000A');codeLines.push(' http.HandleFunc(\"/hello\", hello)\u000A http.ListenAndServe(\":8090\", nil)\u000A}\u000A');codeLines.push('');codeLines.push('');
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</script>
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<script src="site.js" async></script>
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</body>
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</html>
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