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docs/manual/mod/mod_cache.html.en
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@ -63,6 +63,9 @@
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignorequerystring">CacheIgnoreQueryString</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheignoreurlsessionidentifiers">CacheIgnoreURLSessionIdentifiers</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelastmodifiedfactor">CacheLastModifiedFactor</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelock">CacheLock</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelockmaxage">CacheLockMaxAge</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachelockpath">CacheLockPath</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachemaxexpire">CacheMaxExpire</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cacheminexpire">CacheMinExpire</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#cachestorenostore">CacheStoreNoStore</a></li>
@ -72,6 +75,7 @@
<ul id="topics">
<li><img alt="" src="../images/down.gif" /> <a href="#related">Related Modules and Directives</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#sampleconf">Sample Configuration</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#thunderingherd">Avoiding the Thundering Herd</a></li>
</ul><h3>See also</h3>
<ul class="seealso">
<li><a href="../caching.html">Caching Guide</a></li>
@ -106,6 +110,70 @@
</span>
&lt;/IfModule&gt;
</code></p></div>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="thunderingherd" id="thunderingherd">Avoiding the Thundering Herd</a></h2>
<p>When a cached entry becomes stale, <code class="module"><a href="../mod/mod_cache.html">mod_cache</a></code> will submit
a conditional request to the backend, which is expected to confirm whether the
cached entry is still fresh, and send an updated entity if not.</p>
<p>A small but finite amount of time exists between the time the cached entity
becomes stale, and the time the stale entity is fully refreshed. On a busy
server, a significant number of requests might arrive during this time, and
cause a <strong>thundering herd</strong> of requests to strike the backend
suddenly and unpredicably.</p>
<p>To keep the thundering herd at bay, the <code class="directive">CacheLock</code>
directive can be used to define a directory in which locks are created for
URLs <strong>in flight</strong>. The lock is used as a <strong>hint</strong>
by other requests to either suppress an attempt to cache (someone else has
gone to fetch the entity), or to indicate that a stale entry is being refreshed
(stale content will be returned in the mean time).
</p>
<h3>Initial caching of an entry</h3>
<p>When an entity is cached for the first time, a lock will be created for the
entity until the response has been fully cached. During the lifetime of the
lock, the cache will suppress the second and subsequent attempt to cache the
same entity. While this doesn't hold back the thundering herd, it does stop
the cache attempting to cache the same entity multiple times simultaneously.
</p>
<h3>Refreshment of a stale entry</h3>
<p>When an entity reaches its freshness lifetime and becomes stale, a lock
will be created for the entity until the response has either been confirmed as
still fresh, or replaced by the backend. During the lifetime of the lock, the
second and subsequent incoming request will cause stale data to be returned,
and the thundering herd is kept at bay.</p>
<h3>Locks and Cache-Control: no-cache</h3>
<p>Locks are used as a <strong>hint only</strong> to enable the cache to be
more gentle on backend servers, however the lock can be overridden if necessary.
If the client sends a request with a Cache-Control header forcing a reload, any
lock that may be present will be ignored, and the client's request will be
honoured immediately and the cached entry refreshed.</p>
<p>As a further safety mechanism, locks have a configurable maximum age.
Once this age has been reached, the lock is removed, and a new request is
given the opportunity to create a new lock. This maximum age can be set using
the <code class="directive">CacheLockMaxAge</code> directive, and defaults to 5
seconds.
</p>
<h3>Example configuration</h3>
<div class="example"><h3>Enabling the cache lock</h3><p><code>
#<br />
# Enable the cache lock<br />
#<br />
&lt;IfModule mod_cache.c&gt;<br />
<span class="indent">
CacheLock on<br />
CacheLockPath /tmp/mod_cache-lock<br />
CacheLockMaxAge 5<br />
</span>
&lt;/IfModule&gt;
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheDefaultExpire" id="CacheDefaultExpire">CacheDefaultExpire</a> <a name="cachedefaultexpire" id="cachedefaultexpire">Directive</a></h2>
@ -411,6 +479,67 @@ LastModified date.</td></tr>
CacheLastModifiedFactor 0.5
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheLock" id="CacheLock">CacheLock</a> <a name="cachelock" id="cachelock">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Enable the thundering herd lock.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheLock <var>on|off</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheLock off</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheLock</code> directive enables the thundering herd lock
for the given URL space.</p>
<p>In a minimal configuration the following directive is all that is needed to
enable the thundering herd lock in the default system temp directory.</p>
<div class="example"><p><code>
# Enable chache lock<br />
CacheLock on<br /><br />
</code></p></div>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheLockMaxAge" id="CacheLockMaxAge">CacheLockMaxAge</a> <a name="cachelockmaxage" id="cachelockmaxage">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Set the maximum possible age of a cache lock.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheLockMaxAge <var>integer</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheLockMaxAge 5</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheLockMaxAge</code> directive specifies the maximum
age of any cache lock.</p>
<p>A lock older than this value in seconds will be ignored, and the next
incoming request will be given the opportunity to re-establish the lock.
This mechanism prevents a slow client taking an excessively long time to refresh
an entity.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheLockPath" id="CacheLockPath">CacheLockPath</a> <a name="cachelockpath" id="cachelockpath">Directive</a></h2>
<table class="directive">
<tr><th><a href="directive-dict.html#Description">Description:</a></th><td>Set the lock path directory.</td></tr>
<tr><th><a href="directive-dict.html#Syntax">Syntax:</a></th><td><code>CacheLockPath <var>directory</var></code></td></tr>
<tr><th><a href="directive-dict.html#Default">Default:</a></th><td><code>CacheLockPath /tmp/mod_cache-lock</code></td></tr>
<tr><th><a href="directive-dict.html#Context">Context:</a></th><td>server config, virtual host</td></tr>
<tr><th><a href="directive-dict.html#Status">Status:</a></th><td>Extension</td></tr>
<tr><th><a href="directive-dict.html#Module">Module:</a></th><td>mod_cache</td></tr>
</table>
<p>The <code class="directive">CacheLockPath</code> directive allows you to specify the
directory in which the locks are created. By default, the system's temporary
folder is used. Locks consist of empty files that only exist for stale URLs
in flight, so is significantly less resource intensive than the traditional
disk cache.</p>
</div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="directive-section"><h2><a name="CacheMaxExpire" id="CacheMaxExpire">CacheMaxExpire</a> <a name="cachemaxexpire" id="cachemaxexpire">Directive</a></h2>