...
- changes more frequently.
- is used only by apps needing the business the specific business functions.
Level 1 is used for modules that need to aggregate services from multiple lower level modules. These modules are intended to be closely related to particular web apps, and the code is not generally meant to be reusable across many apps. This functionality:
- changes quite frequently.
- is used typically only by one app.
The levels have these attributes:
- the higher the level, the less reusable the services provided by its modules.
- conversely, the lower the level, the more reusable its services.
- the higher the level, the more frequently the modules will change.
- conversely, the lower the level, the more stable the services provided by its modules.
Rules for Module Use
For this scheme to work, and to provide benefit, we need to impose the following rules, or guidelines, for using the modules, and for determining where new code should go:
- Downward calls only. A module can use services in another module only if the other module