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
mysqlnd plugin to
replace selected mysqlnd C methods.
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
SELECT, the SQL hint /*ms=slave*/ or a slave had
been chosen for running the previous query, and the query started
with the SQL hint /*ms=last_used*/. In all other cases,
the query will be sent to the MySQL replication master server.
For better portability, applications should use the
MYSQLND_MS_LAST_USED_SWITCH predefined mysqlnd_ms
constants, instead of their literal values, such as
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.
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 BEGIN TRANSACTION, the plugin leaves it to the
user to handle.