public/waitgroups

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    <div class="example" id="waitgroups">
      <h2><a href="./">Go by Example</a>: WaitGroups</h2>
      
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            <p>To wait for multiple goroutines to finish, we can
use a <em>wait group</em>.</p>

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            <a href="https://go.dev/play/p/S98GjeaGBX0"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">package</span> <span class="nx">main</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">import</span> <span class="p">(</span>
</span></span><span class="line"><span class="cl">    <span class="s">&#34;fmt&#34;</span>
</span></span><span class="line"><span class="cl">    <span class="s">&#34;sync&#34;</span>
</span></span><span class="line"><span class="cl">    <span class="s">&#34;time&#34;</span>
</span></span><span class="line"><span class="cl"><span class="p">)</span></span></span></code></pre>
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            <p>This is the function we&rsquo;ll run in every goroutine.</p>

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          <pre class="chroma"><code><span class="line"><span class="cl"><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></span><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;Worker %d starting\n&#34;</span><span class="p">,</span> <span class="nx">id</span><span class="p">)</span></span></span></code></pre>
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            <p>Sleep to simulate an expensive task.</p>

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          <pre class="chroma"><code><span class="line"><span class="cl">    <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></span><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;Worker %d done\n&#34;</span><span class="p">,</span> <span class="nx">id</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span></span></span></code></pre>
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            <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>

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          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="kd">var</span> <span class="nx">wg</span> <span class="nx">sync</span><span class="p">.</span><span class="nx">WaitGroup</span></span></span></code></pre>
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            <p>Launch several goroutines and increment the WaitGroup
counter for each.</p>

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          <pre class="chroma"><code><span class="line"><span class="cl">    <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">&lt;=</span> <span class="mi">5</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">        <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></span></span></code></pre>
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            <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>

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          <pre class="chroma"><code><span class="line"><span class="cl">        <span class="nx">i</span> <span class="o">:=</span> <span class="nx">i</span></span></span></code></pre>
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            <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>

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          <pre class="chroma"><code><span class="line"><span class="cl">        <span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">            <span class="k">defer</span> <span class="nx">wg</span><span class="p">.</span><span class="nf">Done</span><span class="p">()</span>
</span></span><span class="line"><span class="cl">            <span class="nf">worker</span><span class="p">(</span><span class="nx">i</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">        <span class="p">}()</span>
</span></span><span class="line"><span class="cl">    <span class="p">}</span></span></span></code></pre>
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            <p>Block until the WaitGroup counter goes back to 0;
all the workers notified they&rsquo;re done.</p>

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          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">wg</span><span class="p">.</span><span class="nf">Wait</span><span class="p">()</span></span></span></code></pre>
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            <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>

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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="gp">$</span> go run waitgroups.go
</span></span><span class="line"><span class="cl"><span class="go">Worker 5 starting
</span></span></span><span class="line"><span class="cl"><span class="go">Worker 3 starting
</span></span></span><span class="line"><span class="cl"><span class="go">Worker 4 starting
</span></span></span><span class="line"><span class="cl"><span class="go">Worker 1 starting
</span></span></span><span class="line"><span class="cl"><span class="go">Worker 2 starting
</span></span></span><span class="line"><span class="cl"><span class="go">Worker 4 done
</span></span></span><span class="line"><span class="cl"><span class="go">Worker 1 done
</span></span></span><span class="line"><span class="cl"><span class="go">Worker 2 done
</span></span></span><span class="line"><span class="cl"><span class="go">Worker 5 done
</span></span></span><span class="line"><span class="cl"><span class="go">Worker 3 done</span></span></span></code></pre>
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            <p>The order of workers starting up and finishing
is likely to be different for each invocation.</p>

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      <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>

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