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  <section id="vcl-probe">
<span id="vcl-probe-7"></span><h1>VCL-probe<a class="headerlink" href="#vcl-probe" title="Link to this heading">¶</a></h1>
<section id="configuring-backend-health-probes">
<h2>Configuring Backend Health Probes<a class="headerlink" href="#configuring-backend-health-probes" title="Link to this heading">¶</a></h2>
<dl class="field-list simple">
<dt class="field-odd">Manual section<span class="colon">:</span></dt>
<dd class="field-odd"><p>7</p>
</dd>
</dl>
<section id="backend-health-probes">
<span id="reference-vcl-probes"></span><h3>Backend health probes<a class="headerlink" href="#backend-health-probes" title="Link to this heading">¶</a></h3>
<p>Varnish can be configured to periodically send a request to test if a
backend is answering and thus “healthy”.</p>
<p>Probes can be configured per backend:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">backend</span> <span class="n">foo</span> <span class="p">{</span>
    <span class="p">[</span><span class="o">...</span><span class="p">]</span>
    <span class="o">.</span><span class="n">probe</span> <span class="o">=</span> <span class="p">{</span>
        <span class="p">[</span><span class="o">...</span><span class="p">]</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>They can be named and shared between backends:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">probe</span> <span class="n">light</span> <span class="p">{</span>
    <span class="p">[</span><span class="o">...</span><span class="p">]</span>
<span class="p">}</span>

<span class="n">backend</span> <span class="n">foo</span> <span class="p">{</span>
    <span class="o">.</span><span class="n">probe</span> <span class="o">=</span> <span class="n">light</span><span class="p">;</span>
<span class="p">}</span>

<span class="n">backend</span> <span class="n">bar</span> <span class="p">{</span>
    <span class="o">.</span><span class="n">probe</span> <span class="o">=</span> <span class="n">light</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Or a <code class="docutils literal notranslate"><span class="pre">default</span></code> probe can be defined, which applies to all backends
without a specific <code class="docutils literal notranslate"><span class="pre">.probe</span></code> configured:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">probe</span> <span class="n">default</span> <span class="p">{</span>
    <span class="p">[</span><span class="o">...</span><span class="p">]</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The basic syntax is the same as for backends:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">probe</span> <span class="n">name</span> <span class="p">{</span>
    <span class="o">.</span><span class="n">attribute1</span> <span class="o">=</span> <span class="n">value</span><span class="p">;</span>
    <span class="o">.</span><span class="n">attribute2</span> <span class="o">=</span> <span class="s2">&quot;value&quot;</span><span class="p">;</span>
    <span class="p">[</span><span class="o">...</span><span class="p">]</span>
<span class="p">}</span>
</pre></div>
</div>
<p>There are no mandatory attributes, they all have defaults.</p>
</section>
<section id="attribute-url">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.url</span></code><a class="headerlink" href="#attribute-url" title="Link to this heading">¶</a></h3>
<p>The URL to query.  Defaults to <code class="docutils literal notranslate"><span class="pre">/</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">url</span> <span class="o">=</span> <span class="s2">&quot;/health-probe&quot;</span><span class="p">;</span>
</pre></div>
</div>
</section>
<section id="attribute-request">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.request</span></code><a class="headerlink" href="#attribute-request" title="Link to this heading">¶</a></h3>
<p>Can be used to specify a full HTTP/1.1 request to be sent:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">request</span> <span class="o">=</span> <span class="s2">&quot;GET / HTTP/1.1&quot;</span>
    <span class="s2">&quot;Host: example.com&quot;</span>
    <span class="s2">&quot;X-Magic: We&#39;re fine with this.&quot;</span>
    <span class="s2">&quot;Connection: close&quot;</span><span class="p">;</span>
</pre></div>
</div>
<p>Each of the strings will have <code class="docutils literal notranslate"><span class="pre">CRNL</span></code> appended and a final HTTP
header block terminating <code class="docutils literal notranslate"><span class="pre">CRNL</span></code> will be appended as well.</p>
<p>Because connection shutdown is part of the health check,
<code class="docutils literal notranslate"><span class="pre">Connection:</span> <span class="pre">close</span></code> is mandatory.</p>
</section>
<section id="attribute-expected-response">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.expected_response</span></code><a class="headerlink" href="#attribute-expected-response" title="Link to this heading">¶</a></h3>
<p>The expected HTTP status, defaults to <code class="docutils literal notranslate"><span class="pre">200</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">expected_response</span> <span class="o">=</span> <span class="mi">418</span><span class="p">;</span>
</pre></div>
</div>
</section>
<section id="attribute-expect-close">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.expect_close</span></code><a class="headerlink" href="#attribute-expect-close" title="Link to this heading">¶</a></h3>
<p>Whether or not to expect the backend to close the underlying connection.</p>
<p>Accepts <code class="docutils literal notranslate"><span class="pre">true</span></code> or <code class="docutils literal notranslate"><span class="pre">false</span></code>, defaults to <code class="docutils literal notranslate"><span class="pre">true</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">expect_close</span> <span class="o">=</span> <span class="n">false</span><span class="p">;</span>
</pre></div>
</div>
<p>Warning: when the backend does not close the connection,
setting <code class="docutils literal notranslate"><span class="pre">expect_close</span></code> to <code class="docutils literal notranslate"><span class="pre">false</span></code> makes probe tasks wait until
they time out before inspecting the response.</p>
</section>
<section id="attribute-timeout">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.timeout</span></code><a class="headerlink" href="#attribute-timeout" title="Link to this heading">¶</a></h3>
<p>How fast the probe must succeed, default is two seconds:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">timeout</span> <span class="o">=</span> <span class="mi">10</span><span class="n">s</span><span class="p">;</span>
</pre></div>
</div>
</section>
<section id="attribute-interval">
<h3>Attribute <code class="docutils literal notranslate"><span class="pre">.interval</span></code><a class="headerlink" href="#attribute-interval" title="Link to this heading">¶</a></h3>
<p>Time between probes, default is five seconds:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">interval</span> <span class="o">=</span> <span class="mi">1</span><span class="n">m</span><span class="p">;</span>
</pre></div>
</div>
</section>
<section id="the-backend-health-shift-register">
<h3>The backend health shift register<a class="headerlink" href="#the-backend-health-shift-register" title="Link to this heading">¶</a></h3>
<p>Backend health probes uses a 64 stage shift register to remember the
most recent health probes and to evaluate the total health of the backend.</p>
<p>In the CLI, a good backend health status looks like this:</p>
<div class="highlight-text notranslate"><div class="highlight"><pre><span></span>varnish&gt; backend.list -p boot.backend
Backend name    Admin    Probe    Health     Last change
boot.backend    probe    5/5      healthy    Wed, 13 Jan 2021 10:31:50 GMT
 Current states  good:  5 threshold:  4 window:  5
  Average response time of good probes: 0.000793
  Oldest ================================================== Newest
  4444444444444444444444444444444444444444444444444444444444444444 Good IPv4
  XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX Good Xmit
  RRRRRRRRRRRRRRRRRRRRRRR----RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR Good Recv
  HHHHHHHHHHHHHHHHHHHHHHH--------HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH Happy
</pre></div>
</div>
<p>Starting from the bottom, the last line shows that this backend has been
declared “Happy” for most the 64 health probes, but there were some
trouble some while ago.</p>
<p>However, in this case the <code class="docutils literal notranslate"><span class="pre">.window</span></code> is configured to five, and the
<code class="docutils literal notranslate"><span class="pre">.threshold</span></code> is set to four, so at this point in time, the backend
is considered fully healthy.</p>
<p>An additional <code class="docutils literal notranslate"><span class="pre">.initial</span></code> fills that many “happy” entries in the
shift register when the VCL is loaded, so that backends can quickly
become healthy, even if their health is normally considered over
many samples:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">.</span><span class="n">interval</span> <span class="o">=</span> <span class="mi">1</span><span class="n">s</span><span class="p">;</span>
<span class="o">.</span><span class="n">window</span> <span class="o">=</span> <span class="mi">60</span><span class="p">;</span>
<span class="o">.</span><span class="n">threshold</span> <span class="o">=</span> <span class="mi">45</span><span class="p">;</span>
<span class="o">.</span><span class="n">initial</span> <span class="o">=</span> <span class="mi">43</span><span class="p">;</span>
</pre></div>
</div>
<p>This backend will be considered healthy if three out of four health
probes in the last minute were good, but it becomes healthy as soon
as two good probes have happened after the VCL was loaded.</p>
<p>The default values are:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">.window</span></code> = 8</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">.threshold</span></code> = 3</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">.initial</span></code> = one less than <code class="docutils literal notranslate"><span class="pre">.threshold</span></code></p></li>
</ul>
<p>Note that the default <code class="docutils literal notranslate"><span class="pre">.initial</span></code> means that the backend will be marked
unhealthy until the first probe response come back successful.
This means that for backends created on demand (by vmods) cannot use the
default value for <code class="docutils literal notranslate"><span class="pre">.initial</span></code>, as the freshly created backend would very
likely still be unhealthy when the backend request happens.</p>
<section id="see-also">
<h4>SEE ALSO<a class="headerlink" href="#see-also" title="Link to this heading">¶</a></h4>
<ul class="simple">
<li><p><a class="reference internal" href="varnishd.html#varnishd-1"><span class="std std-ref">varnishd</span></a></p></li>
<li><p><a class="reference internal" href="vcl.html#vcl-7"><span class="std std-ref">VCL</span></a></p></li>
<li><p><a class="reference internal" href="vcl-backend.html#vcl-backend-7"><span class="std std-ref">VCL-backends</span></a></p></li>
<li><p><a class="reference internal" href="vmod_directors.html#vmod-directors-3"><span class="std std-ref">VMOD directors - Varnish Directors Module</span></a></p></li>
<li><p><a class="reference internal" href="vmod_std.html#vmod-std-3"><span class="std std-ref">VMOD std - Varnish Standard Module</span></a></p></li>
</ul>
</section>
<section id="history">
<h4>HISTORY<a class="headerlink" href="#history" title="Link to this heading">¶</a></h4>
<p>VCL was developed by Poul-Henning Kamp in cooperation with Verdens
Gang AS, Redpill Linpro and Varnish Software.  This manual page is
written by Per Buer, Poul-Henning Kamp, Martin Blix Grydeland,
Kristian Lyngstøl, Lasse Karstensen and others.</p>
</section>
<section id="copyright">
<h4>COPYRIGHT<a class="headerlink" href="#copyright" title="Link to this heading">¶</a></h4>
<p>This document is licensed under the same license as Varnish
itself. See LICENSE for details.</p>
<ul class="simple">
<li><p>Copyright (c) 2006 Verdens Gang AS</p></li>
<li><p>Copyright (c) 2006-2021 Varnish Software AS</p></li>
</ul>
</section>
</section>
</section>
</section>

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    <h3><a href="../index.html">Table of Contents</a></h3>
    <ul>
<li><a class="reference internal" href="#">VCL-probe</a><ul>
<li><a class="reference internal" href="#configuring-backend-health-probes">Configuring Backend Health Probes</a><ul>
<li><a class="reference internal" href="#backend-health-probes">Backend health probes</a></li>
<li><a class="reference internal" href="#attribute-url">Attribute <code class="docutils literal notranslate"><span class="pre">.url</span></code></a></li>
<li><a class="reference internal" href="#attribute-request">Attribute <code class="docutils literal notranslate"><span class="pre">.request</span></code></a></li>
<li><a class="reference internal" href="#attribute-expected-response">Attribute <code class="docutils literal notranslate"><span class="pre">.expected_response</span></code></a></li>
<li><a class="reference internal" href="#attribute-expect-close">Attribute <code class="docutils literal notranslate"><span class="pre">.expect_close</span></code></a></li>
<li><a class="reference internal" href="#attribute-timeout">Attribute <code class="docutils literal notranslate"><span class="pre">.timeout</span></code></a></li>
<li><a class="reference internal" href="#attribute-interval">Attribute <code class="docutils literal notranslate"><span class="pre">.interval</span></code></a></li>
<li><a class="reference internal" href="#the-backend-health-shift-register">The backend health shift register</a><ul>
<li><a class="reference internal" href="#see-also">SEE ALSO</a></li>
<li><a class="reference internal" href="#history">HISTORY</a></li>
<li><a class="reference internal" href="#copyright">COPYRIGHT</a></li>
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