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 [2]. To promote reuse, the UMDL
ontology is divided into nested modules (each of which is itself
called an ontology) [8]. 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.
Jose M. Vidal
jmvidal@umich.edu
Tue Sep 30 14:35:40 EDT 1997