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  <section id="ignoring-the-vary-header-for-bots">
<h1>Ignoring the Vary header for bots<a class="headerlink" href="#ignoring-the-vary-header-for-bots" title="Link to this heading">¶</a></h1>
<p>Varnish supports HTTP variants out of the box, but the <em>Vary</em> header is
somewhat limited since it operates on complete header values. If you want for
example to conduct an A/B testing campaign or perform blue/green deployment
you can make clients “remember” their path with a first-party cookie.</p>
<p>When a search engine bot asks for contents however, there’s a high chance that
they don’t process cookies and in all likelihood you would prefer to serve a
response quickly. In that case you would probably prefer not to even try to
attribute a category to the client, but in that case you create a new variant
in your cache that is none of A, B, blue, green, or whatever your backend
serves.</p>
<p>If the way content is served makes no difference to the bot, because you
changed the color of a button or something else orthogonal to the content
itself, then you risk a cache miss with the detrimental effects of adding a
needless variant to the cache and serving it with extra latency.</p>
<p>If latency is paramount, you can use <code class="docutils literal notranslate"><span class="pre">req.hash_ignore_vary</span></code> to opt out of
the Vary match during the lookup and get the freshest variant.</p>
<p>Ignoring how the cookie is set, and assuming the backend always provides an
accurate <em>Cache-Control</em> even when cookies are present, below is an example of
an A/B testing setup where bots are served the freshest variant:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">cookie</span><span class="p">;</span>

<span class="n">include</span> <span class="s2">&quot;devicedetect.vcl&quot;</span><span class="p">;</span>

<span class="n">sub</span> <span class="n">vcl_recv</span> <span class="p">{</span>
    <span class="n">call</span> <span class="n">devicedetect</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">X</span><span class="o">-</span><span class="n">UA</span><span class="o">-</span><span class="n">Device</span> <span class="o">~</span> <span class="s2">&quot;bot&quot;</span><span class="p">)</span> <span class="p">{</span>
        <span class="nb">set</span> <span class="n">req</span><span class="o">.</span><span class="n">hash_ignore_vary</span> <span class="o">=</span> <span class="n">true</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">}</span>

<span class="n">sub</span> <span class="n">vcl_req_cookie</span> <span class="p">{</span>
    <span class="n">cookie</span><span class="o">.</span><span class="n">parse</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">Cookie</span><span class="p">);</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">AB</span><span class="o">-</span><span class="n">Test</span> <span class="o">=</span> <span class="n">cookie</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s2">&quot;ab-test&quot;</span><span class="p">);</span>
    <span class="k">return</span><span class="p">;</span>
<span class="p">}</span>

<span class="n">sub</span> <span class="n">vcl_deliver</span> <span class="p">{</span>
    <span class="n">unset</span> <span class="n">resp</span><span class="o">.</span><span class="n">http</span><span class="o">.</span><span class="n">Vary</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>It is also assumed that the backend replies with a <code class="docutils literal notranslate"><span class="pre">Vary:</span> <span class="pre">X-AB-Test</span></code> header
and varies on no other header.</p>
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