6.6. The Architectural Perspective
The architectural perspective emphasizes the number and abstraction level of the components of a relationship, which together characterize the complexity of the relationship. We will briefly consider three architectural issues: degree (or arity), cardinality, and directionality.
These architectural concepts come from data modeling and they enable relationships to be described precisely and abstractly, which is essential for maintaining an organizing system that implements relationships among resources. Application and technology lifecycles have never been shorter, and vast amounts of new data are being created by increased tracking of online interactions and by all the active resources that are now part of the Internet of Things. Organizing systems built without clear architectural foundations cannot easily scale up in size and scope to handle these new requirements.
The degree or arity of a relationship is the number of entity types or categories of resources in the relationship. This is usually, though not always, the same as the number of arguments in the relationship expression.
Homer Simpson (husband) ⇔ is-married-to ⇔ Marge Simpson (wife)
is a relationship of degree 2, a binary relationship between two entity types, because the “is-married-to” relationship as we first defined it requires one of the arguments to be of entity type “husband” and one of them to be of type “wife.”
Now suppose we change the definition of marriage ...