Introduction to Drupal modules

When developing Drupal it became clear that we wanted to have a system which is as modular as possible. A modular design will provide flexibility, adaptability, and continuity which in turn allows people to customize the site to their needs and likings.

A Drupal module is simply a file containing a set of routines written in PHP. When used, the module code executes entirely within the context of the site. Hence it can use all the functions and access all variables and structures of the main engine. In fact, a module is not any different from a regular PHP file: it is more of a notion that automatically leads to good design principles and a good development model. Modularity better suits the open-source development model, because otherwise you can't easily have people working in parallel without risk of interference.

The idea is to be able to run random code at given places in the engine. This random code should then be able to do whatever needed to enhance the functionality. The places where code can be executed are called "hooks" and are defined by a fixed interface.

In places where hooks are made available, the engine calls each module's exported functions. This is done by iterating through the modules directory where all modules must reside. Say your module is named foo (i.e. modules/foo.module) and if there was a hook called bar, the engine will call foo_bar() if this was exported by your module.

See also the overview of module hooks, which is generated from the Drupal source code.