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<div class="section" id="hashing">
<h1>Hashing<a class="headerlink" href="#hashing" title="Permalink to this headline">¶</a></h1>
<p>Internally, when Varnish stores content in the cache it stores the object
together with a hash key to find the object again. In the default setup
this key is calculated based on the content of the <em>Host</em> header or the
IP address of the server and the URL.</p>
<p>Behold the <cite>default vcl</cite>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">sub</span> <span class="n">vcl_hash</span> <span class="p">{</span>
<span class="n">hash_data</span><span class="p">(</span><span class="n">req</span><span class="o">.</span><span class="n">url</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="n">req</span><span class="o">.</span><span class="n">http</span><span class="o">.</span><span class="n">host</span><span class="p">)</span> <span class="p">{</span>
<span class="n">hash_data</span><span class="p">(</span><span class="n">req</span><span class="o">.</span><span class="n">http</span><span class="o">.</span><span class="n">host</span><span class="p">);</span>
<span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
<span class="n">hash_data</span><span class="p">(</span><span class="n">server</span><span class="o">.</span><span class="n">ip</span><span class="p">);</span>
<span class="p">}</span>
<span class="k">return</span> <span class="p">(</span><span class="n">lookup</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>As you can see it first checks in <cite>req.url</cite> then <cite>req.http.host</cite> if
it exists. It is worth pointing out that Varnish doesn’t lowercase the
hostname or the URL before hashing it so in theory having “Varnish.org/”
and “varnish.org/” would result in different cache entries. Browsers
however, tend to lowercase hostnames.</p>
<p>You can change what goes into the hash. This way you can make Varnish
serve up different content to different clients based on arbitrary
criteria.</p>
<p>Let’s say you want to serve pages in different languages to your users
based on where their IP address is located. You would need some Vmod to
get a country code and then put it into the hash. It might look like this.</p>
<p>In <cite>vcl_recv</cite>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">set</span> <span class="n">req</span><span class="o">.</span><span class="n">http</span><span class="o">.</span><span class="n">X</span><span class="o">-</span><span class="n">Country</span><span class="o">-</span><span class="n">Code</span> <span class="o">=</span> <span class="n">geoip</span><span class="o">.</span><span class="n">lookup</span><span class="p">(</span><span class="n">client</span><span class="o">.</span><span class="n">ip</span><span class="p">);</span>
</pre></div>
</div>
<p>And then add a <cite>vcl_hash</cite>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">sub</span> <span class="n">vcl_hash</span> <span class="p">{</span>
<span class="n">hash_data</span><span class="p">(</span><span class="n">req</span><span class="o">.</span><span class="n">http</span><span class="o">.</span><span class="n">X</span><span class="o">-</span><span class="n">Country</span><span class="o">-</span><span class="n">Code</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>As the default VCL will take care of adding the host and URL to the hash
we don’t have to do anything else. Be careful calling <code class="docutils literal notranslate"><span class="pre">return</span> <span class="pre">(lookup)</span></code>
as this will abort the execution of the default VCL and Varnish can end
up returning data based on more or less random inputs.</p>
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