248 lines
8.5 KiB
Plaintext
Generated
248 lines
8.5 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: WaitGroups</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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window.location.href = 'worker-pools';
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window.location.href = 'rate-limiting';
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<body>
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<div class="example" id="waitgroups">
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<h2><a href="./">Go by Example</a>: WaitGroups</h2>
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<table>
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<tr>
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<td class="docs">
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<p>To wait for multiple goroutines to finish, we can
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use a <em>wait group</em>.</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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<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/vXBl8zQpDYj"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
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</pre>
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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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<pre class="chroma"><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">"sync"</span>
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<span class="s">"time"</span>
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<span class="p">)</span>
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</pre>
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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>This is the function we’ll run in every goroutine.
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Note that a WaitGroup must be passed to functions by
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pointer.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="kd">func</span> <span class="nf">worker</span><span class="p">(</span><span class="nx">id</span> <span class="kt">int</span><span class="p">,</span> <span class="nx">wg</span> <span class="o">*</span><span class="nx">sync</span><span class="p">.</span><span class="nx">WaitGroup</span><span class="p">)</span> <span class="p">{</span>
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</pre>
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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>On return, notify the WaitGroup that we’re done.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="k">defer</span> <span class="nx">wg</span><span class="p">.</span><span class="nf">Done</span><span class="p">()</span>
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</pre>
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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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<pre class="chroma"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"Worker %d starting\n"</span><span class="p">,</span> <span class="nx">id</span><span class="p">)</span>
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</pre>
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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>Sleep to simulate an expensive task.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">time</span><span class="p">.</span><span class="nf">Sleep</span><span class="p">(</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="nf">Printf</span><span class="p">(</span><span class="s">"Worker %d done\n"</span><span class="p">,</span> <span class="nx">id</span><span class="p">)</span>
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<span class="p">}</span>
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</pre>
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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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<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
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</pre>
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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>This WaitGroup is used to wait for all the
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goroutines launched here to finish.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="kd">var</span> <span class="nx">wg</span> <span class="nx">sync</span><span class="p">.</span><span class="nx">WaitGroup</span>
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</pre>
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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>Launch several goroutines and increment the WaitGroup
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counter for each.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="k">for</span> <span class="nx">i</span> <span class="o">:=</span> <span class="mi">1</span><span class="p">;</span> <span class="nx">i</span> <span class="o"><=</span> <span class="mi">5</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
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<span class="nx">wg</span><span class="p">.</span><span class="nf">Add</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="nf">worker</span><span class="p">(</span><span class="nx">i</span><span class="p">,</span> <span class="o">&</span><span class="nx">wg</span><span class="p">)</span>
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<span class="p">}</span>
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</pre>
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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>Block until the WaitGroup counter goes back to 0;
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all the workers notified they’re done.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">wg</span><span class="p">.</span><span class="nf">Wait</span><span class="p">()</span>
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</pre>
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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>Note that this approach has no straightforward way
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to propagate errors from workers. For more
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advanced use cases, consider using the
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<a href="https://pkg.go.dev/golang.org/x/sync/errgroup">errgroup package</a>.</p>
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</td>
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<td class="code">
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<pre class="chroma">
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<span class="p">}</span>
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</pre>
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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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</td>
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<td class="code leading">
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<pre class="chroma"><span class="gp">$</span> go run waitgroups.go
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<span class="go">Worker 5 starting
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</span><span class="go">Worker 3 starting
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</span><span class="go">Worker 4 starting
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</span><span class="go">Worker 1 starting
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</span><span class="go">Worker 2 starting
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</span><span class="go">Worker 4 done
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</span><span class="go">Worker 1 done
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</span><span class="go">Worker 2 done
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</span><span class="go">Worker 5 done
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</span><span class="go">Worker 3 done</span></pre>
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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>The order of workers starting up and finishing
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is likely to be different for each invocation.</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="rate-limiting">Rate Limiting</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> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">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 (\u000A \"fmt\"\u000A \"sync\"\u000A \"time\"\u000A)\u000A');codeLines.push('func worker(id int, wg *sync.WaitGroup) {\u000A');codeLines.push(' defer wg.Done()\u000A');codeLines.push(' fmt.Printf(\"Worker %d starting\\n\", id)\u000A');codeLines.push(' time.Sleep(time.Second)\u000A fmt.Printf(\"Worker %d done\\n\", id)\u000A}\u000A');codeLines.push('func main() {\u000A');codeLines.push(' var wg sync.WaitGroup\u000A');codeLines.push(' for i :\u003D 1; i \u003C\u003D 5; i++ {\u000A wg.Add(1)\u000A go worker(i, \u0026wg)\u000A }\u000A');codeLines.push(' wg.Wait()\u000A');codeLines.push('}\u000A');codeLines.push('');codeLines.push('');
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