Software Architect's Handbook

Open versus closed layers

Layered architectures can have layers that are designed to be open or closed. With a closed layer, requests that are flowing down the stack from the layer above must go through it and cannot bypass it. For example, in a three-layer architecture with presentation, business, and data layers, if the business layer is closed, the presentation layer must send all requests to the business layer and cannot bypass it to send a request directly to the data layer.

Closed layers provide layers of isolation, which makes code easier to change, write, and understand. This makes the layers independent of each other, such that changes made to one layer of the application will not affect components in the other layers. If the layers ...