N-Tier Applications?

1) The Presentation Layer: Also called as the client layer comprises of components that are dedicated to presenting the data to the user. For example: Windows/Web Forms and buttons, edit boxes, Text boxes, labels, grids, etc.

2) The Business Rules Layer: This layer encapsulates the Business rules or the business logic of the encapsulations. To have a separate layer for business logic is of a great advantage. This is because any changes in Business Rules can be easily handled in this layer. As long as the interface between the layers remains the same, any changes to the functionality/processing logic in this layer can be made without impacting the others. A lot of client-server apps failed to implement successfully as changing the business logic was a painful process.

3) The Data Access Layer: This layer comprises of components that help in accessing the Database. If used in the right way, this layer provides a level of abstraction for the database structures. Simply put changes made to the database, tables, etc do not effect the rest of the application because of the Data Access layer. The different application layers send the data requests to this layer and receive the response from this layer.

The database is not accessed directly from any other layer/component. Hence the table names, field names are not hard coded anywhere else. This layer may also access any other services that may provide it with data, for instance Active Directory, Services etc. Having this layer also provides an additional layer of security for the database. As the other layers do not need to know the database credentials, connect strings and so on.

4) The Database Layer: This layer comprises of the Database Components such as DB Files, Tables, Views, etc. The Actual database could be created using SQL Server, Oracle, Flat files, etc.