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<title>Go by Example: Mutexes</title>
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<div class="example" id="mutexes">
<h2><a href="./">Go by Example</a>: Mutexes</h2>
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<p>In the previous example we saw how to manage simple
counter state using atomic operations. For more complex
state we can use a <em><a href="http://en.wikipedia.org/wiki/Mutual_exclusion">mutex</a></em>
to safely access data across multiple goroutines.</p>
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<a href="http://play.golang.org/p/-_LKNceBru"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
<span class="s">&quot;fmt&quot;</span>
<span class="s">&quot;math/rand&quot;</span>
<span class="s">&quot;runtime&quot;</span>
<span class="s">&quot;sync&quot;</span>
<span class="s">&quot;sync/atomic&quot;</span>
<span class="s">&quot;time&quot;</span>
<span class="p">)</span>
</pre></div>
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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>
</pre></div>
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<p>For our example the <code>state</code> will be a map.</p>
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<div class="highlight"><pre> <span class="kd">var</span> <span class="nx">state</span> <span class="p">=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">map</span><span class="p">[</span><span class="kt">int</span><span class="p">]</span><span class="kt">int</span><span class="p">)</span>
</pre></div>
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<p>This <code>mutex</code> will synchronize access to <code>state</code>.</p>
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<div class="highlight"><pre> <span class="kd">var</span> <span class="nx">mutex</span> <span class="p">=</span> <span class="o">&amp;</span><span class="nx">sync</span><span class="p">.</span><span class="nx">Mutex</span><span class="p">{}</span>
</pre></div>
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<p>To compare the mutex-based approach with another
we&rsquo;ll see later, <code>ops</code> will count how many
operations we perform against the state.</p>
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<div class="highlight"><pre> <span class="kd">var</span> <span class="nx">ops</span> <span class="kt">int64</span> <span class="p">=</span> <span class="mi">0</span>
</pre></div>
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<p>Here we start 100 goroutines to execute repeated
reads against the state.</p>
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<div class="highlight"><pre> <span class="k">for</span> <span class="nx">r</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">r</span> <span class="p">&lt;</span> <span class="mi">100</span><span class="p">;</span> <span class="nx">r</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="nx">total</span> <span class="o">:=</span> <span class="mi">0</span>
<span class="k">for</span> <span class="p">{</span>
</pre></div>
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<p>For each read we pick a key to access,
<code>Lock()</code> the <code>mutex</code> to ensure
exclusive access to the <code>state</code>, read
the value at the chosen key,
<code>Unlock()</code> the mutex, and increment
the <code>ops</code> count.</p>
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<div class="highlight"><pre> <span class="nx">key</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="nx">mutex</span><span class="p">.</span><span class="nx">Lock</span><span class="p">()</span>
<span class="nx">total</span> <span class="o">+=</span> <span class="nx">state</span><span class="p">[</span><span class="nx">key</span><span class="p">]</span>
<span class="nx">mutex</span><span class="p">.</span><span class="nx">Unlock</span><span class="p">()</span>
<span class="nx">atomic</span><span class="p">.</span><span class="nx">AddInt64</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>
</pre></div>
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<p>In order to ensure that this goroutine
doesn&rsquo;t starve the scheduler, we explicitly
yield after each operation with
<code>runtime.Gosched()</code>. This yielding is
handled automatically with e.g. every
channel operation and for blocking
calls like <code>time.Sleep</code>, but in this
case we need to do it manually.</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>
</pre></div>
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<p>We&rsquo;ll also start 10 goroutines to simulate writes,
using the same pattern we did for reads.</p>
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<div class="highlight"><pre> <span class="k">for</span> <span class="nx">w</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">w</span> <span class="p">&lt;</span> <span class="mi">10</span><span class="p">;</span> <span class="nx">w</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>
<span class="nx">key</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="nx">val</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">100</span><span class="p">)</span>
<span class="nx">mutex</span><span class="p">.</span><span class="nx">Lock</span><span class="p">()</span>
<span class="nx">state</span><span class="p">[</span><span class="nx">key</span><span class="p">]</span> <span class="p">=</span> <span class="nx">val</span>
<span class="nx">mutex</span><span class="p">.</span><span class="nx">Unlock</span><span class="p">()</span>
<span class="nx">atomic</span><span class="p">.</span><span class="nx">AddInt64</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>
<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>
</pre></div>
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<p>Let the 10 goroutines work on the <code>state</code> and
<code>mutex</code> for a second.</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>
</pre></div>
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<p>Take and report a final operations count.</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">LoadInt64</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>
</pre></div>
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<p>With a final lock of <code>state</code>, show how it ended up.</p>
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<div class="highlight"><pre> <span class="nx">mutex</span><span class="p">.</span><span class="nx">Lock</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;state:&quot;</span><span class="p">,</span> <span class="nx">state</span><span class="p">)</span>
<span class="nx">mutex</span><span class="p">.</span><span class="nx">Unlock</span><span class="p">()</span>
<span class="p">}</span>
</pre></div>
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<p>Running the program shows that we executed about
3,500,000 operations against our <code>mutex</code>-synchronized
<code>state</code>.</p>
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<div class="highlight"><pre><span class="gp">$</span> go run mutexes.go
<span class="go">ops: 3598302</span>
<span class="go">state: map[1:38 4:98 2:23 3:85 0:44]</span>
</pre></div>
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<p>Next we&rsquo;ll look at implementing this same state
management task using only goroutines and channels.</p>
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<p class="next">
Next example: <a href="stateful-goroutines">Stateful Goroutines</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/mutexes">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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