# Maintenance extensions

Not all maintenance logic can go in the initial application's Maintenance. For instance, how can you retrieve the description of a foreign key whose table doesn't exist in that application?

Therefore, an "extension" methodology needs to exist whereby an existing page can have behavior added to it at predetermined plug-in points.

This is done by having a list of maintenance extensions that can be supplied for any given maintenance. At runtime, this list is kept and when a maintenance is initialized, new instances of its extensions are created. These extensions are called after any original maintenance behavior, and in the order of loaded applications. This means that the extensions should have no dependence on which other extensions have run, excepting the original maintenance having run.

- Developing Maintenance Extensions

Parent topic: Hooking into Maintenance Class User Exits