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  <section id="built-in-vcl">
<span id="vcl-built-in-code"></span><h1>Built-in VCL<a class="headerlink" href="#built-in-vcl" title="Link to this heading">¶</a></h1>
<p>Whenever a VCL program is loaded, the built-in VCL is appended to it. The
vcl built-in subs (<a class="reference internal" href="../reference/vcl-step.html#id7"><span class="std std-ref">VCL Steps</span></a>) have a special property, they can appear multiple
times and the result is concatenation of all built-in subroutines.</p>
<p>For example, let’s take the following snippet:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">sub</span> <span class="n">vcl_recv</span> <span class="p">{</span>
    <span class="c1"># loaded code for vcl_recv</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The effective VCL that is supplied to the compiler looks like:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">sub</span> <span class="n">vcl_recv</span> <span class="p">{</span>
    <span class="c1"># loaded code for vcl_recv</span>
    <span class="c1"># built-in code for vcl_recv</span>
<span class="p">}</span>
</pre></div>
</div>
<p>This is how it is guaranteed that all <a class="reference internal" href="../reference/states.html#reference-states"><span class="std std-ref">Varnish Processing States</span></a> have at least
one <code class="docutils literal notranslate"><span class="pre">return</span> <span class="pre">(&lt;action&gt;)</span></code>.</p>
<p>It is generally recommended not to invariably return from loaded code to
let Varnish execute the built-in code, because the built-in code provides
essentially a sensible default behavior for an HTTP cache.</p>
<section id="built-in-subroutines-split">
<h2>Built-in subroutines split<a class="headerlink" href="#built-in-subroutines-split" title="Link to this heading">¶</a></h2>
<p>It might however not always be practical that the built-in VCL rules take
effect at the very end of a state, so some subroutines like <code class="docutils literal notranslate"><span class="pre">vcl_recv</span></code>
are split into multiple calls to other subroutines.</p>
<p>By convention, those assistant subroutines are named after the variable
they operate on, like <code class="docutils literal notranslate"><span class="pre">req</span></code> or <code class="docutils literal notranslate"><span class="pre">beresp</span></code>. This allows for instance to
circumvent default behavior.</p>
<p>For example, <code class="docutils literal notranslate"><span class="pre">vcl_recv</span></code> in the built-in VCL prevents caching when clients
have a cookie. If you can trust your backend to always specify whether a
response is cacheable or not regardless of whether the request contained a
cookie you can do this:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">sub</span> <span class="n">vcl_req_cookie</span> <span class="p">{</span>
    <span class="k">return</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>With this, all other default behaviors from the built-in <code class="docutils literal notranslate"><span class="pre">vcl_recv</span></code> are
executed and only cookie handling is affected.</p>
<p>Another example is how the built-in <code class="docutils literal notranslate"><span class="pre">vcl_backend_response</span></code> treats a
negative TTL as a signal not to cache. It’s a historical mechanism to mark
a response as uncacheable, but only if the built-in <code class="docutils literal notranslate"><span class="pre">vcl_backend_response</span></code>
is not circumvented by a <code class="docutils literal notranslate"><span class="pre">return</span> <span class="pre">(&lt;action&gt;)</span></code>.</p>
<p>However, in a multi-tier architecture where a backend might be another
Varnish server, you might want to cache stale responses to allow the
delivery of graced objects and enable revalidation on the next fetch. This
can be done with the following snippet:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">sub</span> <span class="n">vcl_beresp_stale</span> <span class="p">{</span>
    <span class="k">if</span> <span class="p">(</span><span class="n">beresp</span><span class="o">.</span><span class="n">ttl</span> <span class="o">+</span> <span class="n">beresp</span><span class="o">.</span><span class="n">grace</span> <span class="o">&gt;</span> <span class="mi">0</span><span class="n">s</span><span class="p">)</span> <span class="p">{</span>
        <span class="k">return</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>This granularity, and the general goal of the built-in subroutines split
is to allow to circumvent a specific aspect of the default rules without
giving the entire logic up.</p>
</section>
<section id="built-in-vcl-reference">
<h2>Built-in VCL reference<a class="headerlink" href="#built-in-vcl-reference" title="Link to this heading">¶</a></h2>
<p>A copy of the <code class="docutils literal notranslate"><span class="pre">builtin.vcl</span></code> file might be provided with your Varnish
installation but <a class="reference internal" href="../reference/varnishd.html#varnishd-1"><span class="std std-ref">varnishd</span></a> is the reference to determine the code
that is appended to any loaded VCL.</p>
<p>The VCL compilation happens in two passes:</p>
<ul class="simple">
<li><p>the first one compiles the built-in VCL only,</p></li>
<li><p>and the second pass compiles the concatenation of the loaded and built-in
VCLs.</p></li>
</ul>
<p>Any VCL subroutine present in the built-in VCL can be extended, in which
case the loaded VCL code will be executed before the built-in code.</p>
</section>
</section>

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<li><a class="reference internal" href="#">Built-in VCL</a><ul>
<li><a class="reference internal" href="#built-in-subroutines-split">Built-in subroutines split</a></li>
<li><a class="reference internal" href="#built-in-vcl-reference">Built-in VCL reference</a></li>
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