Modules can have dependencies on other modules. These are expressed via the module’s metadata, as follows:
This states that the current module is dependent on mod_admin to be installed.
Sometimes, explicitly depending on a module name is not a good idea: there might be more modules that perform the same functions but are competing in implementation. In that case, such modules can export a mod_provides meta tag, so that dependent modules can depend on what one of these modules provide.
Example: mod_a and mod_b both provide some functionality, foo:
Now, another module, mod_bar, needs the “foo” functionality:
Now, the module manager will require either (or both!) of the mod_a and mod_b modules to be activated, before mod_bar can be activated.
A module automatically provides its own module name, as well as its name minus the mod_. So, mod_bar has implicitly the following provides constructs:
-module(mod_bar). -mod_provides([mod_bar, bar]).
These two provides are there even when a module adds its own provides clauses.
Note that when a site start, its modules are started up in order of module dependency, in such a way that a module’s dependencies are always started before the module itsef starts.