The MySQL Native Driver Plugin API is a feature of MySQL Native Driver, or mysqlnd. Mysqlnd plugins operate in the layer between PHP applications and the MySQL server. This is comparable to MySQL Proxy. MySQL Proxy operates on a layer between any MySQL client application, for example, a PHP application and, the MySQL server. Mysqlnd plugins can undertake typical MySQL Proxy tasks such as load balancing, monitoring and performance optimizations. Due to the different architecture and location, mysqlnd plugins do not have some of MySQL Proxy's disadvantages. For example, with plugins, there is no single point of failure, no dedicated proxy server to deploy, and no new programming language to learn (Lua).
A mysqlnd plugin can be thought of as an extension to mysqlnd. Plugins can intercept the majority of mysqlnd functions. The mysqlnd functions are called by the PHP MySQL extensions such as ext/mysql, ext/mysqli, and PDO_MYSQL. As a result, it is possible for a mysqlnd plugin to intercept all calls made to these extensions from the client application.
Internal mysqlnd function calls can also be intercepted, or replaced. There are no restrictions on manipulating mysqlnd internal function tables. It is possible to set things up so that when certain mysqlnd functions are called by the extensions that use mysqlnd, the call is directed to the appropriate function in the mysqlnd plugin. The ability to manipulate mysqlnd internal function tables in this way allows maximum flexibility for plugins.
Mysqlnd plugins are in fact PHP Extensions, written in C, that use the mysqlnd plugin API (which is built into MySQL Native Driver, mysqlnd). Plugins can be made 100% transparent to PHP applications. No application changes are needed because plugins operate on a different layer. The mysqlnd plugin can be thought of as operating in a layer below mysqlnd.
The following list represents some possible applications of mysqlnd plugins.
Read/Write Splitting. An example of this is the PECL/mysqlnd_ms (Master Slave) extension. This extension splits read/write queries for a replication setup.
Round-Robin, least loaded
Query Auditing. An example of this is the PECL/mysqlnd_sip (SQL Injection Protection) extension. This extension inspects queries and executes only those that are allowed according to a ruleset.
Caching. An example of this is the PECL/mysqlnd_qc (Query Cache) extension.
Sharding. An example of this is the PECL/mysqlnd_mc (Multi Connect) extension. This extension will attempt to split a SELECT statement into n-parts, using SELECT ... LIMIT part_1, SELECT LIMIT part_n. It sends the queries to distinct MySQL servers and merges the result at the client.
MySQL Native Driver Plugins Available
There are a number of mysqlnd plugins already available. These include:
PECL/mysqlnd_mc - Multi Connect plugin.
PECL/mysqlnd_ms - Master Slave plugin.
PECL/mysqlnd_qc - Query Cache plugin.
PECL/mysqlnd_pscache - Prepared Statement Handle Cache plugin.
PECL/mysqlnd_sip - SQL Injection Protection plugin.
PECL/mysqlnd_uh - User Handler plugin.