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waitgroups
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example: WaitGroups</title>
<link rel=stylesheet href="site.css">
</head>
<script>
onkeydown = (e) => {
if (e.key == "ArrowLeft") {
window.location.href = 'worker-pools';
}
if (e.key == "ArrowRight") {
window.location.href = 'rate-limiting';
}
}
</script>
<body>
<div class="example" id="waitgroups">
<h2><a href="./">Go by Example</a>: WaitGroups</h2>
<table>
<tr>
<td class="docs">
<p>To wait for multiple goroutines to finish, we can
use a <em>wait group</em>.</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/S98GjeaGBX0"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kn">import</span> <span class="p">(</span>
<span class="s">"fmt"</span>
<span class="s">"sync"</span>
<span class="s">"time"</span>
<span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>This is the function we’ll run in every goroutine.</p>
</td>
<td class="code leading">
<pre class="chroma">
<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="p">{</span>
<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>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Sleep to simulate an expensive task.</p>
</td>
<td class="code leading">
<pre class="chroma">
<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>
<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>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>This WaitGroup is used to wait for all the
goroutines launched here to finish. Note: if a WaitGroup is
explicitly passed into functions, it should be done <em>by pointer</em>.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">var</span> <span class="nx">wg</span> <span class="nx">sync</span><span class="p">.</span><span class="nx">WaitGroup</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Launch several goroutines and increment the WaitGroup
counter for each.</p>
</td>
<td class="code leading">
<pre class="chroma">
<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>
<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>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Avoid re-use of the same <code>i</code> value in each goroutine closure.
See <a href="https://golang.org/doc/faq#closures_and_goroutines">the FAQ</a>
for more details.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">i</span> <span class="o">:=</span> <span class="nx">i</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Wrap the worker call in a closure that makes sure to tell
the WaitGroup that this worker is done. This way the worker
itself does not have to be aware of the concurrency primitives
involved in its execution.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<span class="k">defer</span> <span class="nx">wg</span><span class="p">.</span><span class="nf">Done</span><span class="p">()</span>
<span class="nf">worker</span><span class="p">(</span><span class="nx">i</span><span class="p">)</span>
<span class="p">}()</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Block until the WaitGroup counter goes back to 0;
all the workers notified they’re done.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">wg</span><span class="p">.</span><span class="nf">Wait</span><span class="p">()</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Note that this approach has no straightforward way
to propagate errors from workers. For more
advanced use cases, consider using the
<a href="https://pkg.go.dev/golang.org/x/sync/errgroup">errgroup package</a>.</p>
</td>
<td class="code">
<pre class="chroma">
<span class="p">}</span>
</pre>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="gp">$</span> go run waitgroups.go
<span class="go">Worker 5 starting
</span><span class="go">Worker 3 starting
</span><span class="go">Worker 4 starting
</span><span class="go">Worker 1 starting
</span><span class="go">Worker 2 starting
</span><span class="go">Worker 4 done
</span><span class="go">Worker 1 done
</span><span class="go">Worker 2 done
</span><span class="go">Worker 5 done
</span><span class="go">Worker 3 done</span></pre>
</td>
</tr>
<tr>
<td class="docs">
<p>The order of workers starting up and finishing
is likely to be different for each invocation.</p>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="rate-limiting">Rate Limiting</a>.
</p>
<p class="footer">
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>
</p>
</div>
<script>
var codeLines = [];
codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"fmt\"\u000A \"sync\"\u000A \"time\"\u000A)\u000A');codeLines.push('func worker(id int) {\u000A 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');codeLines.push(' i :\u003D i\u000A');codeLines.push(' go func() {\u000A defer wg.Done()\u000A worker(i)\u000A }()\u000A }\u000A');codeLines.push(' wg.Wait()\u000A');codeLines.push('}\u000A');codeLines.push('');codeLines.push('');
</script>
<script src="site.js" async></script>
</body>
</html>