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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<chapter xml:id="mysqlnd.persist" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
<title>Persistent Connections</title>
<para>
<emphasis role="bold">Using Persistent Connections</emphasis>
</para>
<para>
If <literal>mysqli</literal> is used with <literal>mysqlnd</literal>,
when a persistent connection is created it generates a
<literal>COM_CHANGE_USER</literal>
(<literal>mysql_change_user()</literal>) call on the server. This
ensures that re-authentication of the connection takes place.
</para>
<para>
As there is some overhead associated with the
<literal>COM_CHANGE_USER</literal> call, it is possible to switch this
off at compile time. Reusing a persistent connection will then
generate a <literal>COM_PING</literal> (<literal>mysql_ping</literal>)
call to simply test the connection is reusable.
</para>
<para>
Generation of <literal>COM_CHANGE_USER</literal> can be switched off
with the compile flag
<literal>MYSQLI_NO_CHANGE_USER_ON_PCONNECT</literal>. For example:
</para>
<programlisting>
<![CDATA[
shell# CFLAGS="-DMYSQLI_NO_CHANGE_USER_ON_PCONNECT" ./configure --with-mysql=/usr/local/mysql/ --with-mysqli=/usr/local/mysql/bin/mysql_config --with-pdo-mysql=/usr/local/mysql/bin/mysql_config --enable-debug && make clean && make -j6
]]>
</programlisting>
<para>
Or alternatively:
</para>
<programlisting>
<![CDATA[
shell# export CFLAGS="-DMYSQLI_NO_CHANGE_USER_ON_PCONNECT"
shell# configure --whatever-option
shell# make clean
shell# make
]]>
</programlisting>
<para>
Note that only <literal>mysqli</literal> on <literal>mysqlnd</literal>
uses <literal>COM_CHANGE_USER</literal>. Other extension-driver
combinations use <literal>COM_PING</literal> on initial use of a
persistent connection.
</para>
</chapter>
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