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Eli Bendersky baf22e84e7 Move <img> for play.golang.org to separate line in the template 
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Fixes #235
2019-06-04 19:03:37 -07:00

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<!DOCTYPE html>
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<title>Go by Example: Timeouts</title>
<link rel=stylesheet href="site.css">
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<div class="example" id="timeouts">
<h2><a href="./">Go by Example</a>: Timeouts</h2>
<table>
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<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>
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<td class="code leading">
<a href="http://play.golang.org/p/MgcfA-xpJO9">
<img title="Run code" class="run" src="data:image/png;base64,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" />
</a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<td class="docs">
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<td class="code leading">
<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;time&quot;</span>
<span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
</pre></div>
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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>
</pre></div>
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<td class="docs">
<p>For our example, suppose we&rsquo;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="mi">2</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">)</span>
<span class="nx">c1</span> <span class="o">&lt;-</span> <span class="s">&quot;result 1&quot;</span>
<span class="p">}()</span>
</pre></div>
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<td class="docs">
<p>Here&rsquo;s the <code>select</code> implementing a timeout.
<code>res := &lt;-c1</code> awaits the result and <code>&lt;-Time.After</code>
awaits a value to be sent after the timeout of
1s. Since <code>select</code> proceeds with the first
receive that&rsquo;s ready, we&rsquo;ll take the timeout case
if the operation takes more than the allowed 1s.</p>
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<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">&lt;-</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">&lt;-</span><span class="nx">time</span><span class="p">.</span><span class="nx">After</span><span class="p">(</span><span class="mi">1</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Second</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">&quot;timeout 1&quot;</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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<td class="docs">
<p>If we allow a longer timeout of 3s, then the receive
from <code>c2</code> will succeed and we&rsquo;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="mi">2</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">)</span>
<span class="nx">c2</span> <span class="o">&lt;-</span> <span class="s">&quot;result 2&quot;</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">&lt;-</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">&lt;-</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>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;timeout 2&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
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<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>
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<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&rsquo;ll look at two
examples of this next: timers and tickers.</p>
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<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://markmcgranaghan.com">Mark McGranaghan</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>
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