A software architecture based on the separation of different functions into separate tiers. Typically there are three tiers, but there can be more or less as the situation demands. It is especially common in client/server environments.

  1. The bottom tier is normally the data access layer or DAL, and handles all the interaction with the database or other data source. It is the furthest removed from anything the user will ever see.
  2. The middle tier is the business logic layer or BLL, where any business rules are applied to change or reorganize the data.
  3. The top tier is the interface layer. This is where everything that the BLL got from the DAL is arranged into something that is pretty to look at or at least a GUI.

Microsoft uses the fictional company Fitch & Mather to demonstrate this idea to programmers.

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