a34c967eaf
The buffered channel prevents goroutine leaks in case the channel doesn't end up being read (as indeed happens to c1). Updates #207
202 lines
9.1 KiB
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202 lines
9.1 KiB
Plaintext
Generated
<!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: Timeouts</title>
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window.location.href = 'non-blocking-channel-operations';
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<body>
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<div class="example" id="timeouts">
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<h2><a href="./">Go by Example</a>: Timeouts</h2>
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<table>
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<tr>
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<td class="docs">
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<p><em>Timeouts</em> are important for programs that connect to
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external resources or that otherwise need to bound
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execution time. Implementing timeouts in Go is easy and
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elegant thanks to channels and <code>select</code>.</p>
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</td>
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<td class="code empty leading">
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</td>
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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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<a href="http://play.golang.org/p/gyY_qDsRVUe"><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="s">"time"</span>
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<span class="kn">import</span> <span class="s">"fmt"</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>For our example, suppose we’re executing an external
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call that returns its result on a channel <code>c1</code>
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after 2s. Note that the channel is buffered, so the
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send in the goroutine is nonblocking. This is a
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common pattern to prevent goroutine leaks in case the
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channel is never read.</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">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>
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<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
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<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>
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<span class="nx">c1</span> <span class="o"><-</span> <span class="s">"result 1"</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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<p>Here’s the <code>select</code> implementing a timeout.
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<code>res := <-c1</code> awaits the result and <code><-Time.After</code>
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awaits a value to be sent after the timeout of
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1s. Since <code>select</code> proceeds with the first
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receive that’s ready, we’ll take the timeout case
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if the operation takes more than the allowed 1s.</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="nx">res</span> <span class="o">:=</span> <span class="o"><-</span><span class="nx">c1</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="nx">res</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">1</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">Println</span><span class="p">(</span><span class="s">"timeout 1"</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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<tr>
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<td class="docs">
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<p>If we allow a longer timeout of 3s, then the receive
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from <code>c2</code> will succeed and we’ll print the result.</p>
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</td>
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<td class="code">
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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="mi">1</span><span class="p">)</span>
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<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
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<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>
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<span class="nx">c2</span> <span class="o"><-</span> <span class="s">"result 2"</span>
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<span class="p">}()</span>
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<span class="k">select</span> <span class="p">{</span>
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<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>
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<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>
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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">Println</span><span class="p">(</span><span class="s">"timeout 2"</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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</table>
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<table>
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<tr>
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<td class="docs">
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<p>Running this program shows the first operation timing
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out and the second succeeding.</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 timeouts.go
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<span class="go">timeout 1</span>
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<span class="go">result 2</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>Using this <code>select</code> timeout pattern requires
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communicating results over channels. This is a good
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idea in general because other important Go features are
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based on channels and <code>select</code>. We’ll look at two
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examples of this next: timers and tickers.</p>
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</td>
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<td class="code empty">
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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="non-blocking-channel-operations">Non-Blocking Channel Operations</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/timeouts">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('');codeLines.push('package main\u000A');codeLines.push('import \"time\"\u000Aimport \"fmt\"\u000A');codeLines.push('func main() {\u000A');codeLines.push(' c1 := make(chan string, 1)\u000A go func() {\u000A time.Sleep(2 * time.Second)\u000A c1 \x3C- \"result 1\"\u000A }()\u000A');codeLines.push(' select {\u000A case res := \x3C-c1:\u000A fmt.Println(res)\u000A case \x3C-time.After(1 * time.Second):\u000A fmt.Println(\"timeout 1\")\u000A }\u000A');codeLines.push(' c2 := make(chan string, 1)\u000A go func() {\u000A time.Sleep(2 * time.Second)\u000A c2 \x3C- \"result 2\"\u000A }()\u000A select {\u000A case res := \x3C-c2:\u000A fmt.Println(res)\u000A case \x3C-time.After(3 * time.Second):\u000A fmt.Println(\"timeout 2\")\u000A }\u000A}\u000A');codeLines.push('');codeLines.push('');
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