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<div class="example" id="timeouts">
<h2><a href="./">Go by Example</a>: Timeouts</h2>
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<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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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<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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<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>
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<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="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">&lt;-</span> <span class="s">&quot;result 1&quot;</span>
<span class="p">}()</span>
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<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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<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="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">&quot;timeout 1&quot;</span><span class="p">)</span>
<span class="p">}</span>
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<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>
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<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="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">&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="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">&quot;timeout 2&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
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<p>Running this program shows the first operation timing
out and the second succeeding.</p>
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<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>
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<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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<p class="next">
Next example: <a href="non-blocking-channel-operations">Non-Blocking Channel Operations</a>.
</p>
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