An application system consists of three logical layers (shown in yellow).
The presentation layer is what a system user sees or interacts with.
It can consist of visual objects such as screens, web pages or reports or
non-visual objects such as an interactive voice recognition interface that
you use over the telephone. When most people think of application
systems, they think mainly of the presentation layer. Unfortunately,
this layer represents a small portion of the effort involved in building
application systems.
The business logic layer, on the other hand, represents the business
rules that are enforced via programming logic (computer instructions)
regarding how those rules are applied. This business logic layer on
the surface, can appear to be very straight forward, however, it is rarely
so.
The data access layer consists of the definitions of database tables and
columns and the computer logic that is needed to navigate the database.
The data access layer enforces rules regarding the storage and access of
information. For example, dates must be valid dates and numeric fields
must never contain alphanumeric characters.
This diagrams is a "logical" representation of an application system.
When a system is physically implemented, application system components can
be physically deployed on different computer systems. For example, the
presentation of the web page you are looking at is being handled by your
personal computer. The logic required to consolidate and communicate
the visual objects that it needs is occurring on a web server located in
Austin, Texas.
