Mysqlnd plugins and MySQL Proxy are different technologies using different approaches. Both are valid tools for solving a variety of common tasks such as load balancing, monitoring, and performance enhancements. An important difference is that MySQL Proxy works with all MySQL clients, whereas mysqlnd plugins are specific to PHP applications.
As a PHP Extension, a mysqlnd plugin gets installed on the PHP application server, along with the rest of PHP. MySQL Proxy can either be run on the PHP application server or can be installed on a dedicated machine to handle multiple PHP application servers.
Deploying MySQL Proxy on the application server has two advantages:
No single point of failure
Easy to scale out (horizontal scale out, scale by client)
MySQL Proxy (and mysqlnd plugins) can solve problems easily which otherwise would have required changes to existing applications.
However, MySQL Proxy does have some disadvantages:
MySQL Proxy is a new component and technology to master and deploy.
MySQL Proxy requires knowledge of the Lua scripting language.
MySQL Proxy can be customized with C and Lua programming. Lua is the preferred scripting language of MySQL Proxy. For most PHP experts Lua is a new language to learn. A mysqlnd plugin can be written in C. It is also possible to write plugins in PHP using » PECL/mysqlnd_uh.
MySQL Proxy runs as a daemon - a background process. MySQL Proxy can recall earlier decisions, as all state can be retained. However, a mysqlnd plugin is bound to the request-based lifecycle of PHP. MySQL Proxy can also share one-time computed results among multiple application servers. A mysqlnd plugin would need to store data in a persistent medium to be able to do this. Another daemon would need to be used for this purpose, such as Memcache. This gives MySQL Proxy an advantage in this case.
MySQL Proxy works on top of the wire protocol. With MySQL Proxy you have to parse and reverse engineer the MySQL Client Server Protocol. Actions are limited to those that can be achieved by manipulating the communication protocol. If the wire protocol changes (which happens very rarely) MySQL Proxy scripts would need to be changed as well.
Mysqlnd plugins work on top of the C API, which mirrors the libmysqlclient client and Connector/C APIs. This C API is basically a wrapper around the MySQL Client Server protocol, or wire protocol, as it is sometimes called. You can intercept all C API calls. PHP makes use of the C API, therefore you can hook all PHP calls, without the need to program at the level of the wire protocol.
Mysqlnd implements the wire protocol. Plugins can therefore parse, reverse engineer, manipulate and even replace the communication protocol. However, this is usually not required.
As plugins allow you to create implementations that use two levels (C API and wire protocol), they have greater flexibility than MySQL Proxy. If a mysqlnd plugin is implemented using the C API, any subsequent changes to the wire protocol do not require changes to the plugin itself.