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<h2 class="title">Architecture</h2>
<p class="para">
The mysqlnd replication and load balancing 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, in the module init phase of the
PHP engine, it gets registered as a
<a href="book.mysqlnd.html" class="link">mysqlnd</a> plugin to replace selected
mysqlnd C methods.
</p>
<p class="para">
At PHP runtime, it inspects queries sent from
mysqlnd (PHP) to the MySQL server. If a query is recognized as read-only,
it will be sent to one of the configured slave servers. Statements are
considered read-only if they either start with <em>SELECT</em>,
the SQL hint <em>/*ms=slave*/</em> or a slave had been chosen for
running the previous query, and the query started with the SQL hint
<em>/*ms=last_used*/</em>. In all other cases, the query will
be sent to the MySQL replication master server.
</p>
<p class="para">
For better portability, applications should use the
<strong><code>MYSQLND_MS_MASTER_SWITCH</code></strong>,
<strong><code>MYSQLND_MS_SLAVE_SWITCH</code></strong>, and
<strong><code>MYSQLND_MS_LAST_USED_SWITCH</code></strong>
<a href="mysqlnd-ms.constants.html" class="link">predefined mysqlnd_ms constants</a>,
instead of their literal values, such as <em>/*ms=slave*/</em>.
</p>
<p class="para">
The plugin handles the opening and closing of database connections
to both master and slave servers. From an application
point of view, there continues to be only one connection handle. However,
internally, this one public connection handle represents a pool of
network connections that are managed by the plugin. The plugin proxies queries
to the master server, and to the slaves using multiple connections.
</p>
<p class="para">
Database connections have a state consisting of, for example, transaction
status, transaction settings, character set settings, and temporary tables.
The plugin will try to maintain the same state among all internal
connections, whenever this can be done in an automatic and transparent way.
In cases where it is not easily possible to maintain state among all
connections, such as when using <em>BEGIN TRANSACTION</em>, the
plugin leaves it to the user to handle.
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