Application programs may be divided into three layers:
Application or Business Processing Logic
Database or Business Rule Logic
In the past, the traditional program architecture or application programming model was based on executing application programs on a single hardware platform. Under this model, the application program could combine the presentation logic, application logic and database logic into a single large program. Business applications were made up of hundreds of these large all-in-one programs.
For example, traditional IBM i application programs are large RPG-based programs which are coded to handle screens, processing logic, and access to the IBM i database. These types of programs are time consuming to produce and difficult to maintain. They have limited or no portability options. There is limited code reuse and application logic is often duplicated in many places.
A traditional architecture, which forces developers to combine all layers into a single program, has serious limitations once a network or distributed computing environment is introduced. The 1.2.5 Tiered Application Architecture provides greater flexibility.