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<h2 class="title">Architecture and Concepts</h2>
<p class="para">
The query cache plugin is implemented as a PHP extension.
It is written in C and operates under the hood of PHP. During the
startup of the PHP interpreter, it gets registered as a
<a href="book.mysqlnd.html" class="link">mysqlnd</a> plugin to replace selected
mysqlnd C methods. Hereby, it can change the behaviour of any
PHP MySQL extension (<a href="ref.mysqli.html" class="link">mysqli</a>,
<a href="ref.pdo-mysql.html" class="link">PDO_MYSQL</a>,
<a href="ref.mysql.html" class="link">mysql</a>) compiled to use the
mysqlnd library without changing the extensions API. This makes
the plugin compatible with each and every PHP MySQL application.
Because existing APIs are not changed, it is almost transparent
to use. Please, see the
<a href="mysqlnd.plugin.html" class="link">mysqlnd plugin API description</a>
for a discussion of the advantages of the plugin architecture and
a comparison with proxy based solutions.
</p>
<p class="para">
<em class="emphasis">Transparent to use</em>
</p>
<p class="para">
At PHP run time PECL/mysqlnd_qc can proxy queries send from PHP
(<a href="book.mysqlnd.html" class="link">mysqlnd</a>) to the MySQL server.
It then inspects the statement string to find whether it shall cache
its results. If so, result set is cached using a storage handler and
further executions of the statement are served from the cache for
a user-defined period. The Time to Live (TTL) of the cache entry
can either be set globally or on a per statement basis.
</p>
<p class="para">
A statement is either cached if the plugin is instructed to cache all
statements globally using a or, if the query string starts with the SQL hint
(<em>/*qc=on*/</em>). The plugin is capable of caching any
query issued by calling appropriate API calls of any of the existing
PHP MySQL extensions.
</p>
<p class="para">
<em class="emphasis">Flexible storage: various storage handler</em>
</p>
<p class="para">
Various storage handler are supported to offer different scopes for cache
entries. Different scopes allow for different degrees in sharing cache
entries among clients.
</p>
<p class="para">
<ul class="itemizedlist">
<li class="listitem">
<p class="para">
<em>default</em> (built-in): process memory, scope: process, one or more web requests depending on PHP deployment model used
</p>
</li>
<li class="listitem">
<p class="para">
<em>APC</em>: shared memory, scope: single server, multiple web requests
</p>
</li>
<li class="listitem">
<p class="para">
<em>SQLite</em>: memory or file, scope: single server, multiple web requests
</p>
</li>
<li class="listitem">
<p class="para">
<em>MEMCACHE</em>: main memory, scope: single or multiple server, multiple web requests
</p>
</li>
<li class="listitem">
<p class="para">
<em>user</em> (built-in): user-defined - any, scope: user-defined - any
</p>
</li>
</ul>
</p>
<p class="para">
Support for the <em>APC</em>, <em>SQLite</em> and <em>
MEMCACHE</em> storage handler has to be enabled at compile time. The <em>
default</em> and <em>user</em> handler are built-in. It is possible
to switch between compiled-in storage handlers on a per query basis at run time.
However, it is recommended to pick one storage handler and use it for all cache entries.
</p>
<p class="para">
<em class="emphasis">Built-in slam defense to avoid overloading</em>
</p>
<p class="para">
To avoid overload situations the cache plugin has a built-in slam defense mechanism.
If a popular cache entries expires many clients using the cache entries will try
to refresh the cache entry. For the duration of the refresh many clients may
access the database server concurrently. In the worst case, the database server
becomes overloaded and it takes more and more time to refresh the cache entry, which
in turn lets more and more clients try to refresh the cache entry. To prevent
this from happening the plugin has a slam defense mechanism. If slam defense is
enabled and the plugin detects an expired cache entry it extends the life time
of the cache entry before it refreshes the cache entry. This way other concurrent
accesses to the expired cache entry are still served from the cache for a certain
time. The other concurrent accesses to not trigger a concurrent refresh. Ideally,
the cache entry gets refreshed by the client which extended the cache entries lifespan
before other clients try to refresh the cache and potentially cause an overload
situation.
</p>
<p class="para">
<em class="emphasis">Unique approach to caching</em>
</p>
<p class="para">
PECL/mysqlnd_qc has a unique approach to caching result sets that is superior
to application based cache solutions. Application based solutions first fetch
a result set into PHP variables. Then, the PHP variables are serialized for
storage in a persistent cache, and then unserialized when fetching. The mysqlnd
query cache stores the raw wire protocol data sent from MySQL to PHP in its cache
and replays it, if still valid, on a cache hit. This way, it saves an extra
serialization step for a cache put that all application based solutions have to
do. It can store the raw wire protocol data in the cache without having to
serialize into a PHP variable first and deserializing the PHP variable for storing
in the cache again.
</p>
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