<!DOCTYPE html>
<html>
<head>
<meta http-eqiv="content-type" content="text/html;charset=utf-8">
<title>Go by Example: Timeouts</title>
<link rel=stylesheet href="site.css">
</head>
<script type="text/javascript">
if (window.location.host == "gobyexample.com") {
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-34996217-1']);
_gaq.push(['_trackPageview']);
(function() {
var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
})();
}
</script>
<body>
<div class="example" id="timeouts">
<h2><a href="./">Go by Example</a>: Timeouts</h2>
<table>
<tr>
<td class="docs">
<p><em>Timeouts</em> are important for programs that connect to
external resources or that otherwise need to bound
execution time. Implementing timeouts in Go is easy and
elegant thanks to channels and <code>select</code>.</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<a href="http://play.golang.org/p/CoBnJiRyx3"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kn">import</span> <span class="s">"time"</span>
<span class="kn">import</span> <span class="s">"fmt"</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>For our example, suppose we’re executing an external
call that returns its result on a channel <code>c1</code>
after 2s.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">c1</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Second</span> <span class="o">*</span> <span class="mi">2</span><span class="p">)</span>
<span class="nx">c1</span> <span class="o"><-</span> <span class="s">"result 1"</span>
<span class="p">}()</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Here’s the <code>select</code> implementing a timeout.
<code>res := <-c1</code> awaits the result and <code><-Time.After</code>
awaits a value to be sent after the timeout of
1s. Since <code>select</code> proceeds with the first
receive that’s ready, we’ll take the timeout case
if the operation takes more than the allowed 1s.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="k">select</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">res</span> <span class="o">:=</span> <span class="o"><-</span><span class="nx">c1</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">res</span><span class="p">)</span>
<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="nx">time</span><span class="p">.</span><span class="nx">Second</span> <span class="o">*</span> <span class="mi">1</span><span class="p">):</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"timeout 1"</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>If we allow a longer timeout of 3s, then the receive
from <code>c2</code> will succeed and we’ll print the result.</p>
</td>
<td class="code">
<div class="highlight"><pre> <span class="nx">c2</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Second</span> <span class="o">*</span> <span class="mi">2</span><span class="p">)</span>
<span class="nx">c2</span> <span class="o"><-</span> <span class="s">"result 2"</span>
<span class="p">}()</span>
<span class="k">select</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">res</span> <span class="o">:=</span> <span class="o"><-</span><span class="nx">c2</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">res</span><span class="p">)</span>
<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="nx">time</span><span class="p">.</span><span class="nx">Second</span> <span class="o">*</span> <span class="mi">3</span><span class="p">):</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"timeout 2"</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
<p>Running this program shows the first operation timing
out and the second succeeding.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> go run timeouts.go
<span class="go">timeout 1</span>
<span class="go">result 2</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Using this <code>select</code> timeout pattern requires
communicating results over channels. This is a good
idea in general because other important Go features are
based on channels and <code>select</code>. We’ll look at two
examples of this next: timers and tickers.</p>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="non-blocking-channel-operations">Non-Blocking Channel Operations</a>.
</p>
<p class="footer">
by <a href="https://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/timeouts">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
</body>
</html>