A 3-tier architecture is an application program that is organized into three major parts, comprising of:

• The data access layer tier at the bottom,
• The application tier (business logic) in the middle and
• The client tier (presentation) at the top.

Each tier is distributed to a different place or places in a network. These tiers do not necessarily correspond to physical locations on various computers on a network, but rather to logical layers of the application.

1. Presentation Layer (UI):

• This layer presents data to the user and optionally permits data manipulation and data entry, also this layer requests the data form Business layer.

• This layer accomplished through use of Dynamic HTML and client-side data sources and data cursors.

2. Business Logic Layer:

• The business logic acts as the server for client requests from workstations. It acts according Business rules fetch or insert data through the Data Layer.

• In turn, it determines what data is needed (and where it is located) and acts as a client in relation to a third tier of programming that might be located on a local or mainframe computer.

• Because these middle-tier components are not tied to a specific client, they can be used by all applications and can be moved to different locations, as response time and other rules require.

3. Data Access Layer:

• The third tier of the 3-tier system is made up of the DBMS that provides all the data for the above two layers.

• This is the actual DBMS access layer.

• Avoiding dependencies on the storage mechanisms allows for updates or changes without the application tier clients being affected by or even aware of the change.