UIAs, QPAs and Auction agents use the same SCA, and thus subscribe to
the same ontologies: the UMDL SMS is not terminologically
heterogenous. Figure 3 illustrates the SCA's
interactions with the other agents. Agents are in ovals, ontologies
in rectangles, and the arrows connecting agents represent messages,
paraphrased in natural language. In the SMS these messages are
expressed in Loom. To communicate with the SCA, agents use
terminology from the nested ontologies. The thin line around the SCA
is jagged to show that the set of available terms is dynamic; both
QPAs and the AMA add concepts to the agent services ontology. Messages
use font styles that correspond to the ontology labels (plain,
italics, and upper-case), to highlight the source of their
terminology.
Figure 3: Agent interaction with the SCA in the SMS
Figure 3 illustrates the ability of the SCA to
construct new terminology from existing terms at runtime. Concepts in
the SCA's dynamic ontology hide knowledge, just as words chunk meaning
in natural language. Auctions sell some service, but they don't need
to know anything about that service. The AMA asks the SCA for a
service label for an auction, using the QPA's service label, but it
does not know anything about that service. The SCA classifies the
auction service using characteristics inferred from the QPA's service
label. Thus, the SCA can respond to the UIA's request for an auction,
which is phrased in terms from the static ontologies, rather than the
label which the AMA used to define the auction service. This
appropriate hiding of knowledge reduces overall system complexity, and
increases reusability and maintainability.
In future versions, auction managers will make extensive use
of the SCA's knowledge. First, ontologies are the natural place to
represent the type of auction that is appropriate for marketing a
particular service. Second, auction managers may need to define
markets for sets of services, rather than services with exactly the
same classification. Efficient resource allocation depends on
maintaining suitable numbers and ratios of seller and buyer
agents. These quantities can be controlled by adjusting the generality
or specificity of the service to be auctioned.
The most dramatic contribution of the SCA to the SMS derives from its
declarative description of services, and its ability to rank available
services given a target and search strategy. When a new QPA provides a
service that better meets a UIA's needs, the UIA switches to buying
services from the new QPA, without requiring any intervention to
modify the UIAs behavior. This occurs because the UIA periodically
repeats its search for the best available service. Furthermore, if the
new service subsequently becomes unavailable, the UIA reverts to its
previous supplier.
Jose M. Vidal
jmvidal@umich.edu
Tue Sep 30 14:35:40 EDT 1997