The Three-Layer Architecture

The Three-Layer Architecture is an instance of the Layers Pattern. This architecture is used by most desktop and distributed applications.

Here is the basic structure:

Notes

Carefully note that the dependencies are one-way. This implies that the persistence layer can be reused with different domain layers and that the domain layer can be reused with different presentation layers.

The term "tiers" is usually used when the layers are running on different computers. In this case we might think the presentation layer is a thin client, the domain layer is an application server, and the persistence layer is a database server. This, of course, is an elaboration of the client-server architecture.

We can generalize this pattern to the N-tier architecture by dividing the presentation and domain layers into finer layers.

The most popular example is what I call the 3.5 tier architecture in which a web server does some of the work of the presentation and domain layers.