This is a dataflow model of the Application Layer.

This language is close to the way a business person thinks about requirements: what data does the user see and what actions does the user take?

I'll be showing you a concrete example in a minute.

The main features:

1.     Think left-to-right data flow along wires from component to component. Roughly speaking, flow is from Domain-layer components on the left to Presentation-layer components on the right. Control flow, including response to user events, will take care of itself, as you will see. All flows along wires are left-to-right, from “source” component connectors to “sink” component connectors.

2.     Some components on the left will use Domain-Layer services.

3.     Some components on the right (I call them projectors) will use Presentation-Layer services. User-interface regions that can take user events, such as buttons and menu items, are projections of objects called "Do-Its". There is no special treatment of such user events; they follow the same implicit updating rules as other objects that can be projected onto the user interface, such as editable text and list boxes.