238 lines
11 KiB
Plaintext
Generated
238 lines
11 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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<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="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" />
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<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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</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"><code><span class="line"><span class="cl"><span class="kn">import</span> <span class="p">(</span>
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</span></span><span class="line"><span class="cl"> <span class="s">"fmt"</span>
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</span></span><span class="line"><span class="cl"> <span class="s">"sync"</span>
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</span></span><span class="line"><span class="cl"> <span class="s">"time"</span>
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</span></span><span class="line"><span class="cl"><span class="p">)</span></span></span></code></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.</p>
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</td>
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<td class="code leading">
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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>
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</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">"Worker %d starting\n"</span><span class="p">,</span> <span class="nx">id</span><span class="p">)</span></span></span></code></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"><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>
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</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">"Worker %d done\n"</span><span class="p">,</span> <span class="nx">id</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></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"><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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</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. Note: if a WaitGroup is
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explicitly passed into functions, it should be done <em>by pointer</em>.</p>
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</td>
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<td class="code leading">
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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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</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"><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"><=</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></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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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Avoid re-use of the same <code>i</code> value in each goroutine closure.
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See <a href="https://golang.org/doc/faq#closures_and_goroutines">the FAQ</a>
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for more details.</p>
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</td>
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<td class="code leading">
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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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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Wrap the worker call in a closure that makes sure to tell
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the WaitGroup that this worker is done. This way the worker
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itself does not have to be aware of the concurrency primitives
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involved in its execution.</p>
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</td>
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<td class="code leading">
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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>
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</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>
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</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>
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</span></span><span class="line"><span class="cl"> <span class="p">}()</span>
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</span></span><span class="line"><span class="cl"> <span class="p">}</span></span></span></code></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"><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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</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"><code><span class="line"><span class="cl"><span class="p">}</span></span></span></code></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"><code><span class="line"><span class="cl"><span class="gp">$</span> go run waitgroups.go
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</span></span><span class="line"><span class="cl"><span class="go">Worker 5 starting
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</span></span></span><span class="line"><span class="cl"><span class="go">Worker 3 starting
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</span></span></span><span class="line"><span class="cl"><span class="go">Worker 4 starting
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</span></span></span><span class="line"><span class="cl"><span class="go">Worker 1 starting
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</span></span></span><span class="line"><span class="cl"><span class="go">Worker 2 starting
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</span></span></span><span class="line"><span class="cl"><span class="go">Worker 4 done
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</span></span></span><span class="line"><span class="cl"><span class="go">Worker 1 done
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</span></span></span><span class="line"><span class="cl"><span class="go">Worker 2 done
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</span></span></span><span class="line"><span class="cl"><span class="go">Worker 5 done
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</span></span></span><span class="line"><span class="cl"><span class="go">Worker 3 done</span></span></span></code></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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