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Hana 9e216da9ef go.mod: add go.mod and move pygments to third_party 
After go1.16, go will use module mode by default,
even when the repository is checked out under GOPATH
or in a one-off directory. Add go.mod, go.sum to keep
this repo buildable without opting out of the module
mode.

> go mod init github.com/mmcgrana/gobyexample
> go mod tidy
> go mod vendor

In module mode, the 'vendor' directory is special
and its contents will be actively maintained by the
go command. pygments aren't the dependency the go will
know about, so it will delete the contents from vendor
directory. Move it to `third_party` directory now.

And, vendor the blackfriday package.

Note: the tutorial contents are not affected by the
change in go1.16 because all the examples in this
tutorial ask users to run the go command with the
explicit list of files to be compiled (e.g.
`go run hello-world.go` or `go build command-line-arguments.go`).
When the source list is provided, the go command does
not have to compute the build list and whether it's
running in GOPATH mode or module mode becomes irrelevant.
2021-02-15 16:45:26 -05:00

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<!DOCTYPE html>
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<title>Go by Example: Rate Limiting</title>
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<div class="example" id="rate-limiting">
<h2><a href="./">Go by Example</a>: Rate Limiting</h2>
<table>
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<td class="docs">
<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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<td class="docs">
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<td class="code leading">
<a href="http://play.golang.org/p/20c_m1AtOEI"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
<span class="s">&quot;fmt&quot;</span>
<span class="s">&quot;time&quot;</span>
<span class="p">)</span>
</pre></div>
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</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>First we&rsquo;ll look at basic rate limiting. Suppose
we want to limit our handling of incoming requests.
We&rsquo;ll serve these requests off a channel of the
same name.</p>
</td>
<td class="code leading">
<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">&lt;=</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">&lt;-</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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<td class="docs">
<p>This <code>limiter</code> channel will receive a value
every 200 milliseconds. This is the regulator in
our rate limiting scheme.</p>
</td>
<td class="code leading">
<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="mi">200</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Millisecond</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<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>
</td>
<td class="code leading">
<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">&lt;-</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">&quot;request&quot;</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>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<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>
</td>
<td class="code leading">
<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>
</pre></div>
</td>
</tr>
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<td class="docs">
<p>Fill up the channel to represent allowed bursting.</p>
</td>
<td class="code leading">
<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">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">&lt;-</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>
</td>
</tr>
<tr>
<td class="docs">
<p>Every 200 milliseconds we&rsquo;ll try to add a new
value to <code>burstyLimiter</code>, up to its limit of 3.</p>
</td>
<td class="code leading">
<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="mi">200</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Millisecond</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">burstyLimiter</span> <span class="o">&lt;-</span> <span class="nx">t</span>
<span class="p">}</span>
<span class="p">}()</span>
</pre></div>
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<td class="docs">
<p>Now simulate 5 more incoming requests. The first
3 of these will benefit from the burst capability
of <code>burstyLimiter</code>.</p>
</td>
<td class="code">
<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">&lt;=</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">&lt;-</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">&lt;-</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">&quot;request&quot;</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>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
<p>Running our program we see the first batch of requests
handled once every ~200 milliseconds as desired.</p>
</td>
<td class="code leading">
<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>
</td>
</tr>
<tr>
<td class="docs">
<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>
</td>
<td class="code">
<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>
</td>
</tr>
</table>
<p class="next">
Next example: <a href="atomic-counters">Atomic Counters</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/rate-limiting">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
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