The presentation layer, shown in Figure 8-2, implements the program’s user interface for interactions with its consumers. In the pattern’s original form, this layer denotes a graphical interface, such as a web interface or a desktop application.

In modern systems, however, the presentation layer has a broader scope: that is, all means for triggering the program’s behavior, both synchronous and asynchronous. For example:

• Graphical user interface (GUI)

• Command-line interface (CLI)

• API for programmatic integration with other systems

• Subscription to events in a message broker

• Message topics for publishing outgoing events

All of these are the means for the system to receive requests from the external envi‐ ronment and communicate the output. Strictly speaking, the presentation layer is the program’s public interface.

Figure 8-2. Presentation layer