PSIs and Patterns

The topic map community has already recognised that when vocabularies are created and identifiers used for the elements of those vocabularies, then documentation is required. The Published Subject Indicators recommendation from OASIS places certain constraints on this documentation:

• The documentation must be retrievable from the URI used as the identifier for the published subject.

• The documentation must be in human-readable form.

• The documentation should provide human-readable and machine-processable metadata about itself.

However, the PSI recommendation is, perhaps rightly, silent about the content of such documentation and how it should be structured. The design pattern approach presented here goes beyond the PSI recommendation by recommending a pattern-based approach to subject identifier documentation for those identifiers which are intended as the components of a solution to one or more design problems.

The design pattern approach is intended to be a vehicle for communication between information professionals responsible for creating and maintaining topic map-based systems, rather than as a means for general communication of the meaning of PSIs. Although both forms are needed, there is no restriction placed on PSI documentation that prevents both pattern-based documentation and more user-oriented documentation from co-existing. It is also hoped that by the use of patterns to describe common solutions to topic map design problems that the quality and consistency of such designs can be improved and that tool-sets can be built to support the patterns, reducing the time and cost of implementation.

It is also worth noting at this point that having PSIs defined is not necessarily a pre-requisite for the documentation of a design pattern. The most general design patterns will be ones which are applied to the topic map meta model, rather than ones which apply to particular topic map applications, and so will not require the definition of any PSIs for their implementation. An example of such a pattern is the Hierarchical Naming Pattern given later in this paper.

Even when a pattern requires new subjects to be identified, there is no requirement that PSIs be used for establishing subject identity, although if the pattern is to be made public, then the subjects used must be published in some form and following the PSI recommendation makes sense.