Chapter 2. Identifier Patterns


The single most important part of the Linked Data approach is the adoption of web-scale identifiers (URIs) to identify things of interest: people, events, places, statistical observations, colours. Anything that we want to publish data about on the web needs to have a URI, allowing it to be referenced, browsed and linked using existing web tools. The existing tools of the web of documents are already designed to work well with things that have URIs. We can "like" them, discuss them, and refer to them in documents.

In RDF we capture data as statements about resources. RDF allows resources to have global identifiers or to be un-named "blank nodes". While blank nodes may offer flexibility for some use cases, in a Linked Data context blank nodes limit our ability to collaboratively annotate data. A blank node cannot be the target of a link and we can't annotate it with new information from new sources. As one of the biggest benefits of the Linked Data approach is that "anyone can say anything anywhere", use of blank nodes undermines some of the advantages we can gain from wide adoption of the RDF model. Even within the closed world of a single application dataset, use of blank nodes can quickly become limiting when integrating new data.

Successful publishing of Linked Data requires the careful selection of good, clean, stable URIs for the resources in a dataset. This means that the most important first step in any Linked Data project is deciding on an appropriate identifier scheme: the conventions for how URIs will be assigned to resources. This is not to say that conventions won't evolve or be extended over time, but some upfront thought about identifiers is always beneficial.

This chapter introduces a collection of design patterns that promote some principled ways to assign identifiers within a dataset. All of these patterns are in wide use today and so are tried and tested in the field.

Many of these patterns are also prevalent in modern web frameworks and are more generally applicable to URL design for web applications.

Table of Contents

Hierarchical URIs
Literal Keys
Natural Keys
Patterned URIs
Proxy URIs
Rebased URI
Shared Keys
URL Slug