The ExpressionEngine CMS uses the idea of "hooks" [1], which are basically global events that are fired at key points in other modules and extensions. Extensions can register to be notified of these hooks, which allows them to modify data at those key points, or modify output.
Extensions can check when they are first instantiated if their schema changes have been made, and if not, apply them.
Comments
The ExpressionEngine CMS uses the idea of "hooks" [1], which are basically global events that are fired at key points in other modules and extensions. Extensions can register to be notified of these hooks, which allows them to modify data at those key points, or modify output.
Extensions can check when they are first instantiated if their schema changes have been made, and if not, apply them.
It works out pretty well.
[1] http://expressionengine.com/user_guide/development/extension...