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<div class="example" id="rate-limiting">
<h2><a href="./">Go by Example</a>: Rate Limiting</h2>
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<p><em><a href="http://en.wikipedia.org/wiki/Rate_limiting">Rate limiting</a></em>
is an important mechanism for controlling resource
utilization and maintaining quality of service. Go
elegantly supports rate limiting with goroutines,
channels, and <a href="tickers">tickers</a>.</p>
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<a href="http://play.golang.org/p/e7yzIk97-p"><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="s">"time"</span>
<span class="kn">import</span> <span class="s">"fmt"</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>
</pre></div>
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<p>First we’ll look at basic rate limiting. Suppose
we want to limit our handling of incoming requests.
We’ll serve these requests off a channel of the
same name.</p>
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<div class="highlight"><pre> <span class="nx">requests</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">int</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span>
<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>
<span class="nx">requests</span> <span class="o"><-</span> <span class="nx">i</span>
<span class="p">}</span>
<span class="nb">close</span><span class="p">(</span><span class="nx">requests</span><span class="p">)</span>
</pre></div>
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<p>This <code>limiter</code> channel will receive a value
every 200 milliseconds. This is the regulator in
our rate limiting scheme.</p>
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<div class="highlight"><pre> <span class="nx">limiter</span> <span class="o">:=</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Tick</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Millisecond</span> <span class="o">*</span> <span class="mi">200</span><span class="p">)</span>
</pre></div>
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<p>By blocking on a receive from the <code>limiter</code> channel
before serving each request, we limit ourselves to
1 request every 200 milliseconds.</p>
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<div class="highlight"><pre> <span class="k">for</span> <span class="nx">req</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">requests</span> <span class="p">{</span>
<span class="o"><-</span><span class="nx">limiter</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"request"</span><span class="p">,</span> <span class="nx">req</span><span class="p">,</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Now</span><span class="p">())</span>
<span class="p">}</span>
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<p>We may want to allow short bursts of requests in
our rate limiting scheme while preserving the
overall rate limit. We can accomplish this by
buffering our limiter channel. This <code>burstyLimiter</code>
channel will allow bursts of up to 3 events.</p>
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<div class="highlight"><pre> <span class="nx">burstyLimiter</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Time</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
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<p>Fill up the channel to represent allowed bursting.</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"><</span> <span class="mi">3</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
<span class="nx">burstyLimiter</span> <span class="o"><-</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Now</span><span class="p">()</span>
<span class="p">}</span>
</pre></div>
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<p>Every 200 milliseconds we’ll try to add a new
value to <code>burstyLimiter</code>, up to its limit of 3.</p>
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<div class="highlight"><pre> <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="nx">t</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Tick</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Millisecond</span> <span class="o">*</span> <span class="mi">200</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">burstyLimiter</span> <span class="o"><-</span> <span class="nx">t</span>
<span class="p">}</span>
<span class="p">}()</span>
</pre></div>
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<p>Now simulate 5 more incoming requests. The first
3 of these will benefit from the burst capability
of <code>burstyLimiter</code>.</p>
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<div class="highlight"><pre> <span class="nx">burstyRequests</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">int</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span>
<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>
<span class="nx">burstyRequests</span> <span class="o"><-</span> <span class="nx">i</span>
<span class="p">}</span>
<span class="nb">close</span><span class="p">(</span><span class="nx">burstyRequests</span><span class="p">)</span>
<span class="k">for</span> <span class="nx">req</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">burstyRequests</span> <span class="p">{</span>
<span class="o"><-</span><span class="nx">burstyLimiter</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"request"</span><span class="p">,</span> <span class="nx">req</span><span class="p">,</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Now</span><span class="p">())</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
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<p>Running our program we see the first batch of requests
handled once every ~200 milliseconds as desired.</p>
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<div class="highlight"><pre><span class="gp">$</span> go run rate-limiting.go
<span class="go">request 1 2012-10-19 00:38:18.687438 +0000 UTC</span>
<span class="go">request 2 2012-10-19 00:38:18.887471 +0000 UTC</span>
<span class="go">request 3 2012-10-19 00:38:19.087238 +0000 UTC</span>
<span class="go">request 4 2012-10-19 00:38:19.287338 +0000 UTC</span>
<span class="go">request 5 2012-10-19 00:38:19.487331 +0000 UTC</span>
</pre></div>
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<p>For the second batch of requests we serve the first
3 immediately because of the burstable rate limiting,
then serve the remaining 2 with ~200ms delays each.</p>
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<div class="highlight"><pre><span class="go">request 1 2012-10-19 00:38:20.487578 +0000 UTC</span>
<span class="go">request 2 2012-10-19 00:38:20.487645 +0000 UTC</span>
<span class="go">request 3 2012-10-19 00:38:20.487676 +0000 UTC</span>
<span class="go">request 4 2012-10-19 00:38:20.687483 +0000 UTC</span>
<span class="go">request 5 2012-10-19 00:38:20.887542 +0000 UTC</span>
</pre></div>
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<p class="next">
Next example: <a href="atomic-counters">Atomic Counters</a>.
</p>
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