The concept of a relationship is pervasive in human societies in both informal and formal senses. Humans are inescapably related to generations of ancestors, and in most cases they also have social networks of friends, co-workers, and casual acquaintances to whom they are related in various ways. We often hear that our access to information, money, jobs, and political power is all about “who you know,” so we strive to “network” with other people to build relationships that might help us expand our access. In information systems, relationships between resources embody the organization that enables finding, selection, retrieval, and other interactions.
Most organizing systems are based on many relationships to enable the system to satisfy some intentional purposes with individual resources or the collection as a whole. In the domain of information resources, common resources include web pages, journal articles, books, datasets, metadata records, and XML documents, among many others. Important relationships in the information domain that facilitate purposes like finding, identifying, and selecting resources include “is the author of,” “is published by,” “has publication date,” “is derived from,” “has subject keyword,” “is related to,” and many others.
When we talk about relationships we specify both the resources that are associated along with a name or statement about the reason for the association. Just identifying the resources involved is not enough because several different relationships can exist among the same resources; the same person can be your brother, your employer, and your landlord. Furthermore, for many relationships the directionality or ordering of the participants in a relationship statement matters; the person who is your employer gives a paycheck to you, not vice versa. Kent points out that when we describe a relationship we sometimes use whole phrases, such as “is-employed-by,” if our language does not contain a single word that expresses the meaning of the relationship.