Transaction handling is fundamentally changed. An SQL transaction is a unit of work that is run on one database server. The unit of work consists of one or more SQL statements.
By default the plugin is not aware of SQL transactions. The plugin may switch connections for load balancing at any point in time. Connection switches may happen in the middle of a transaction. This is against the nature of an SQL transaction. By default, the plugin is not transaction safe.
Any kind of MySQL load balancer must be hinted about the begin and end of a transaction. Hinting can either be done implicitly by monitoring API calls or using SQL hints. Both options are supported by the plugin, depending on your PHP version. API monitoring requires PHP 5.4.0 or newer. The plugin, like any other MySQL load balancer, cannot detect transaction boundaries based on the MySQL Client Server Protocol. Thus, entirely transparent transaction aware load balancing is not possible. The least intrusive option is API monitoring, which requires little to no application changes, depending on your application.
Please, find examples of using SQL hints or the API monitoring in the examples section. The details behind the API monitoring, which makes the plugin transaction aware, are described below.
Beginning with PHP 5.4.0, the mysqlnd library allows this plugin to subclass the library C API call set_autocommit(), to detect the status of autocommit mode.
The PHP MySQL extensions either issue a query (such as SET AUTOCOMMIT=0|1), or use the mysqlnd library call set_autocommit() to control the autocommit setting. If an extension makes use of set_autocommit(), the plugin can be made transaction aware. Transaction awareness cannot be achieved if using SQL to set the autocommit mode. The library function set_autocommit() is called by the mysqli_autocommit() and PDO::setAttribute(PDO::ATTR_AUTOCOMMIT) user API calls.
The plugin configuration option trx_stickiness=master can be used to make the plugin transactional aware. In this mode, the plugin stops load balancing if autocommit becomes disabled, and directs all statements to the master until autocommit gets enabled.
An application that does not want to set SQL hints for transactions but wants to use the transparent API monitoring to avoid application changes must make sure that the autocommit settings is changed exclusively through the listed API calls.
API based transaction boundary detection has been improved with PHP 5.5.0 and PECL/mysqlnd_ms 1.5.0 to cover not only calls to mysqli_autocommit() but also mysqli_begin(), mysqli_commit() and mysqli_rollback().