Interface subsystem. This subsystem is responsible for managing all interaction with the enduser.Hence; it is frequently referred to as the user interface.
Rules subsystem. This subsystem manages the application logic in terms of a defined model of business rules.
Transaction subsystem. This subsystem acts as the link between the data subsystem and the rules and interface subsystems. Querying, insertion and update activity is triggered at the interface, validated by the rules subsystem and packaged as units (transactions) that will initiate actions(responses or changes) in the data subsystem.
Data subsystem. This subsystem is responsible for managing the underlying data needed by an application.