219 lines
7.4 KiB
Plaintext
219 lines
7.4 KiB
Plaintext
<!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: WaitGroup</title>
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<div class="example" id="waitgroup">
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<h2><a href="./">Go by Example</a>: WaitGroup</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/8cD2V9CgI0J"><img title="Run code" src="play.png" class="run" /></a>
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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="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></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>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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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">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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<span class="nx">fmt</span><span class="p">.</span><span class="nx">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></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>Sleep to simulate an expensive task.</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">time</span><span class="p">.</span><span class="nx">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="nx">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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</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>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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<div class="highlight"><pre> <span class="nx">wg</span><span class="p">.</span><span class="nx">Done</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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</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>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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<div class="highlight"><pre> <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></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>Launch several goroutines and increment the WorkGroup
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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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<div class="highlight"><pre> <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="nx">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="nx">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></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>Block until the WorkGroup 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">
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<div class="highlight"><pre> <span class="nx">wg</span><span class="p">.</span><span class="nx">Wait</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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</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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<div class="highlight"><pre><span class="gp">$</span> go run waitgroup.go
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<span class="go">Worker 5 starting</span>
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<span class="go">Worker 3 starting</span>
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<span class="go">Worker 4 starting</span>
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<span class="go">Worker 1 starting</span>
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<span class="go">Worker 2 starting</span>
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<span class="go">Worker 4 done</span>
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<span class="go">Worker 1 done</span>
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<span class="go">Worker 2 done</span>
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<span class="go">Worker 5 done</span>
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<span class="go">Worker 3 done</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>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> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/waitgroup">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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</body>
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