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to generate ontology documents and an agent-based architecture to interpret the ontology documents.
The following section elucidates a real-world scenario to demonstrate the need for FIS to adjust to the
changes in data sources. We use this running example as a supporting disposition to provide a detailed
explanation of the agent-based framework for semantic extensibility proposed in this chapter. Finally,
we conclude with a summary section that identifies immediate benefits of the framework and suggests
future directions of research and development to address the limitations in this model.
Background
Conceptually, a federated information system can be treated as a three-tier architecture (adapted rep-
resentation from Busse et al., 1999) as shown in Figure 1. The data sources in the foundation layer can
be heterogeneous, autonomous, or distributed. The form of schema management in the federation layer
defines whether the FIS is tightly federated or loosely federated (Sheth & Larson, 1990). A tight federa-
tion creates a unified schema through the semantic integration of component schemas (Busse et al.). The
users in the presentation layer therefore need to have little knowledge about the individual component
schemas. However, tracking changes in the component schemas and correspondingly making changes
to the unified federated schema can be intractable and may be performed only by a domain expert. To
address this limitation, a loose federation adopts an approach where reliance on a central federated
schema is eliminated. This becomes a viable option only if a uniform query language is available to
Figure 1. Three-tier representation of a federated information system with Web service support
Presentation Layer
(Web applications, user interfaces)
Federation Layer
(uniform access language, schema and
meta data)
Relational /
Object
Database
Systems
XML Files /
Flat Files
Legacy
Systems
Application
APIS
Foundation Layer
(Data Sources from participating organizations)
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