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<title>The mysqli Extension and Persistent Connections</title>
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<h1>The mysqli Extension and Persistent Connections</h1>
<p class="para">
Persistent connection support was introduced in PHP 5.3 for the
<em>mysqli</em> extension. Support was already present in
PDO MYSQL and ext/mysql. The idea behind persistent connections is
that a connection between a client process and a database can be
reused by a client process, rather than being created and destroyed
multiple times. This reduces the overhead of creating fresh
connections every time one is required, as unused connections are
cached and ready to be reused.
</p>
<p class="para">
Unlike the mysql extension, mysqli does not provide a separate function
for opening persistent connections. To open a persistent connection you
must prepend <em>p:</em> to the hostname when connecting.
</p>
<p class="para">
The problem with persistent connections is that they can be left in
unpredictable states by clients. For example, a table lock might be
activated before a client terminates unexpectedly. A new client
process reusing this persistent connection will get the connection
"<span class="quote">as is</span>". Any cleanup would need to be done by the new
client process before it could make good use of the persistent
connection, increasing the burden on the programmer.
</p>
<p class="para">
The persistent connection of the <em>mysqli</em> extension
however provides built-in cleanup handling code. The cleanup carried
out by <em>mysqli</em> includes:
</p>
<ul class="itemizedlist">
<li class="listitem">
<p class="para">
Rollback active transactions
</p>
</li>
<li class="listitem">
<p class="para">
Close and drop temporary tables
</p>
</li>
<li class="listitem">
<p class="para">
Unlock tables
</p>
</li>
<li class="listitem">
<p class="para">
Reset session variables
</p>
</li>
<li class="listitem">
<p class="para">
Close prepared statements (always happens with PHP)
</p>
</li>
<li class="listitem">
<p class="para">
Close handler
</p>
</li>
<li class="listitem">
<p class="para">
Release locks acquired with <span class="function"><strong>GET_LOCK()</strong></span>
</p>
</li>
</ul>
<p class="para">
This ensures that persistent connections are in a clean state on
return from the connection pool, before the client process uses them.
</p>
<p class="para">
The <em>mysqli</em> extension does this cleanup by
automatically calling the C-API function
<em>mysql_change_user()</em>.
</p>
<p class="para">
The automatic cleanup feature has advantages and disadvantages though.
The advantage is that the programmer no longer needs to worry about
adding cleanup code, as it is called automatically. However, the
disadvantage is that the code could <em class="emphasis">potentially</em>
be a little slower, as the code to perform the cleanup needs to run
each time a connection is returned from the connection pool.
</p>
<p class="para">
It is possible to switch off the automatic cleanup code, by compiling
PHP with
<strong><code>MYSQLI_NO_CHANGE_USER_ON_PCONNECT</code></strong>
defined.
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
The <em>mysqli</em> extension supports persistent
connections when using either MySQL Native Driver or MySQL Client
Library.
</p>
</p></blockquote>
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