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<title>Go by Example: Atomic Counters</title>
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<div class="example" id="atomic-counters">
<h2><a href="./">Go by Example</a>: Atomic Counters</h2>
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<p>The primary mechanism for managing state in Go is
communication over channels. We saw this for example
with <a href="worker-pools">worker pools</a>. There are a few other
options for managing state though. Here we&rsquo;ll
look at using the <code>sync/atomic</code> package for <em>atomic
counters</em> accessed by multiple goroutines.</p>
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<a href="http://play.golang.org/p/2h8nvrnaHP"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
<span class="kn">import</span> <span class="s">&quot;time&quot;</span>
<span class="kn">import</span> <span class="s">&quot;sync/atomic&quot;</span>
<span class="kn">import</span> <span class="s">&quot;runtime&quot;</span>
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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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<p>We&rsquo;ll use an unsigned integer to represent our
(always-positive) counter.</p>
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<div class="highlight"><pre> <span class="kd">var</span> <span class="nx">ops</span> <span class="kt">uint64</span> <span class="p">=</span> <span class="mi">0</span>
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<p>To simulate concurrent updates, we&rsquo;ll start 50
goroutines that each increment the counter about
once a millisecond.</p>
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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">0</span><span class="p">;</span> <span class="nx">i</span> <span class="p">&lt;</span> <span class="mi">50</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<span class="k">for</span> <span class="p">{</span>
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<p>To atomically increment the counter we
use <code>AddUint64</code>, giving it the memory
address of our <code>ops</code> counter with the
<code>&amp;</code> syntax.</p>
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<div class="highlight"><pre> <span class="nx">atomic</span><span class="p">.</span><span class="nx">AddUint64</span><span class="p">(</span><span class="o">&amp;</span><span class="nx">ops</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
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<p>Allow other goroutines to proceed.</p>
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<div class="highlight"><pre> <span class="nx">runtime</span><span class="p">.</span><span class="nx">Gosched</span><span class="p">()</span>
<span class="p">}</span>
<span class="p">}()</span>
<span class="p">}</span>
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<p>Wait a second to allow some ops to accumulate.</p>
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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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<p>In order to safely use the counter while it&rsquo;s still
being updated by other goroutines, we extract a
copy of the current value into <code>opsFinal</code> via
<code>LoadUint64</code>. As above we need to give this
function the memory address <code>&amp;ops</code> from which to
fetch the value.</p>
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<div class="highlight"><pre> <span class="nx">opsFinal</span> <span class="o">:=</span> <span class="nx">atomic</span><span class="p">.</span><span class="nx">LoadUint64</span><span class="p">(</span><span class="o">&amp;</span><span class="nx">ops</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;ops:&quot;</span><span class="p">,</span> <span class="nx">opsFinal</span><span class="p">)</span>
<span class="p">}</span>
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<p>Running the program shows that we executed about
40,000 operations.</p>
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<div class="highlight"><pre><span class="gp">$</span> go run atomic-counters.go
<span class="go">ops: 40200</span>
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<p>Next we&rsquo;ll look at mutexes, another tool for managing
state.</p>
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<p class="next">
Next example: <a href="mutexes">Mutexes</a>.
</p>
<p class="footer">
by <a href="https://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/atomic-counters">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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