Interfaces with external systems should be consistently handled throughout the system. The architecture need not include a specific, detailed design for each system interface. It is often enough to simply identify the existence of the interface as a significant part of the architecture and create a Component to encapsulate the detail, so that it can be developed later.

The Entity-Control-Boundary Pattern provides the basis for a useful technique to support this. Specifically, if the system communicates with another system, define one or more components to describe the communication protocol.  The use of a component allows the interface to the external system to be defined and stabilized, while leaving the design details of the system interface hidden as the system evolves.