To draw boundary lines in a software architecture, you first partition the system into components. Some of those components are core business rules; others are plugins that contain necessary functions that are not directly related to the core business. Then you arrange the code in those components such that the arrows between them point in one direction—toward the core business. You should recognize this as an application of the Dependency Inversion Principle and the Stable Abstractions Principle. Dependency arrows are arranged to point from lower-level details to higher-level abstractions. 1.
