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Code Editor : vcl-grace.html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Misbehaving servers — Varnish version 5.2.1 documentation</title> <link rel="stylesheet" href="../_static/classic.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '5.2.1', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="top" title="Varnish version 5.2.1 documentation" href="../index.html" /> <link rel="up" title="VCL - Varnish Configuration Language" href="vcl.html" /> <link rel="next" title="Separate VCL files" href="vcl-separate.html" /> <link rel="prev" title="Hashing" href="vcl-hashing.html" /> </head> <body role="document"> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="vcl-separate.html" title="Separate VCL files" accesskey="N">next</a> |</li> <li class="right" > <a href="vcl-hashing.html" title="Hashing" accesskey="P">previous</a> |</li> <li class="nav-item nav-item-0"><a href="../index.html">Varnish version 5.2.1 documentation</a> »</li> <li class="nav-item nav-item-1"><a href="index.html" >The Varnish Users Guide</a> »</li> <li class="nav-item nav-item-2"><a href="vcl.html" accesskey="U">VCL - Varnish Configuration Language</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="misbehaving-servers"> <span id="users-guide-handling-misbehaving-servers"></span><h1>Misbehaving servers<a class="headerlink" href="#misbehaving-servers" title="Permalink to this headline">¶</a></h1> <p>A key feature of Varnish is its ability to shield you from misbehaving web- and application servers.</p> <div class="section" id="grace-mode"> <h2>Grace mode<a class="headerlink" href="#grace-mode" title="Permalink to this headline">¶</a></h2> <p>When several clients are requesting the same page Varnish will send one request to the backend and place the others on hold while fetching one copy from the backend. In some products this is called request coalescing and Varnish does this automatically.</p> <p>If you are serving thousands of hits per second the queue of waiting requests can get huge. There are two potential problems - one is a thundering herd problem - suddenly releasing a thousand threads to serve content might send the load sky high. Secondly - nobody likes to wait. To deal with this we can instruct Varnish to keep the objects in cache beyond their TTL and to serve the waiting requests somewhat stale content.</p> <p>So, in order to serve stale content we must first have some content to serve. So to make Varnish keep all objects for 2 minutes beyond their TTL use the following VCL:</p> <div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">sub</span> <span class="n">vcl_backend_response</span> <span class="p">{</span> <span class="nb">set</span> <span class="n">beresp</span><span class="o">.</span><span class="n">grace</span> <span class="o">=</span> <span class="mi">2</span><span class="n">m</span><span class="p">;</span> <span class="p">}</span> </pre></div> </div> <p>Now Varnish will be allowed to serve objects that are up to two minutes out of date. When it does it will also schedule a refresh of the object. This will happen asynchronously and the moment the new object is in it will replace the one we've already got.</p> <p>You can influence how this logic works by adding code in vcl_hit. The default looks like this:</p> <div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">sub</span> <span class="n">vcl_hit</span> <span class="p">{</span> <span class="k">if</span> <span class="p">(</span><span class="n">obj</span><span class="o">.</span><span class="n">ttl</span> <span class="o">>=</span> <span class="mi">0</span><span class="n">s</span><span class="p">)</span> <span class="p">{</span> <span class="o">//</span> <span class="n">A</span> <span class="n">pure</span> <span class="n">unadulterated</span> <span class="n">hit</span><span class="p">,</span> <span class="n">deliver</span> <span class="n">it</span> <span class="k">return</span> <span class="p">(</span><span class="n">deliver</span><span class="p">);</span> <span class="p">}</span> <span class="k">if</span> <span class="p">(</span><span class="n">obj</span><span class="o">.</span><span class="n">ttl</span> <span class="o">+</span> <span class="n">obj</span><span class="o">.</span><span class="n">grace</span> <span class="o">></span> <span class="mi">0</span><span class="n">s</span><span class="p">)</span> <span class="p">{</span> <span class="o">//</span> <span class="n">Object</span> <span class="ow">is</span> <span class="ow">in</span> <span class="n">grace</span><span class="p">,</span> <span class="n">deliver</span> <span class="n">it</span> <span class="o">//</span> <span class="n">Automatically</span> <span class="n">triggers</span> <span class="n">a</span> <span class="n">background</span> <span class="n">fetch</span> <span class="k">return</span> <span class="p">(</span><span class="n">deliver</span><span class="p">);</span> <span class="p">}</span> <span class="o">//</span> <span class="n">fetch</span> <span class="o">&</span> <span class="n">deliver</span> <span class="n">once</span> <span class="n">we</span> <span class="n">get</span> <span class="n">the</span> <span class="n">result</span> <span class="k">return</span> <span class="p">(</span><span class="n">fetch</span><span class="p">);</span> <span class="p">}</span> </pre></div> </div> <p>The grace logic is pretty obvious here. If you have enabled <a class="reference internal" href="vcl-backends.html#users-guide-advanced-backend-servers-health"><span class="std std-ref">Health checks</span></a> you can check if the backend is sick and only serve graced object then. Replace the second if-clause with something like this:</p> <div class="highlight-default"><div class="highlight"><pre><span></span>if (!std.healthy(req.backend_hint) && (obj.ttl + obj.grace > 0s)) { return (deliver); } else { return (fetch); } </pre></div> </div> <dl class="docutils"> <dt>So, to sum up, grace mode solves two problems:</dt> <dd><ul class="first last simple"> <li>it serves stale content to avoid request pile-up.</li> <li>it serves stale content if you allow it.</li> </ul> </dd> </dl> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../index.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">Misbehaving servers</a><ul> <li><a class="reference internal" href="#grace-mode">Grace mode</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="vcl-hashing.html" title="previous chapter">Hashing</a></p> <h4>Next topic</h4> <p class="topless"><a href="vcl-separate.html" title="next chapter">Separate VCL files</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/users-guide/vcl-grace.txt" rel="nofollow">Show Source</a></li> </ul> </div> <div id="searchbox" style="display: none" role="search"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <div><input type="text" name="q" /></div> <div><input type="submit" value="Go" /></div> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="vcl-separate.html" title="Separate VCL files" >next</a> |</li> <li class="right" > <a href="vcl-hashing.html" title="Hashing" >previous</a> |</li> <li class="nav-item nav-item-0"><a href="../index.html">Varnish version 5.2.1 documentation</a> »</li> <li class="nav-item nav-item-1"><a href="index.html" >The Varnish Users Guide</a> »</li> <li class="nav-item nav-item-2"><a href="vcl.html" >VCL - Varnish Configuration Language</a> »</li> </ul> </div> <div class="footer" role="contentinfo"> © Copyright 2010-2014, Varnish Software AS. 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