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.
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.