Relationship

A relationship is the generic term for some kind of connection or dependency between elements of an architecture. It’s a broad umbrella. A call, a notification, an ownership, a containment, a creation, or an inheritance between two elements are all relationships.

The most common relationship in software architecture is the uses relationship. This is a dependency between two building blocks where, if A uses B, the correct execution of A depends on the presence of a correctly-working implementation of B. This is the relationship a dependency describes, and it underlies more specific relationship types such as association, aggregation, and composition in object-oriented design.

The strength and direction of a system’s relationships are a large part of what determines its coupling. Relationships that run in many directions, or that reach deep into another element’s internals rather than through its public interface, produce a more tightly coupled, harder-to-change system.