We use ontologies to encode declarative descriptions of complex agent services. Declarative descriptions are required to establish a space of services independent of the implementation of the current set of agents. Representing complexity in digital libraries, and any other ``real-world'' domain, demands expressiveness that is substantially more powerful than that of relational databases. Services have many descriptive dimensions (attributes), may be partially described at any level of granularity (with any combination of dimensions), and may be viewed from many perspectives (accessed by different sequences of attribute values). For example, an agent might seek a service to recommend a collection of articles on volcanos, for high-school audiences, to be contracted for via auction. Alternatively, the agent might seek an auction that sells a service to recommend collections. (In Figure 2, follow links either from ``recommend-dlcollection'' at the top, or ``auction'' at the bottom of the figure). In an ontology, both perspectives can be represented simultaneously, and retrieval supports queries on any level of granularity. Declarative formalisms with less expressiveness than ontologies, such as relational databases, force a commitment to a particular ordering of dimensions and a particular perspective.
Figure 2: Multi-dimensional, multi-perspective complexity
Because ontologies are costly to develop, many researchers are working on ways to define ontologies so that they may be reused in different problem solving contexts . To promote reuse, the UMDL ontology is divided into nested modules (each of which is itself called an ontology) . The most general includes library content and services that we consider to be part of a ``generic'' digital library. The second module adds concepts specific to the UMDL implementation, such as auctions. The third module describes agent services. We call this last ontology ``dynamic'' because agents define new service concepts at runtime. In contrast, ``static'' ontologies are either fixed, or are changed slowly over time by committees of persons.