European Journal of Scientific Research - EuroJournals

Generic Architecture Based Agent for Context and Schema Mediation 375
mediation model and the queries processing. Finally, sections 3.8, 3.9 and 3.10 describe the
communication language between agents, a cooperation scenario and the prototype implementation.
2. Interoperability approaches of the information systems
We compare the various approaches according to the following criteria:
1. The formulation of the user query (in ontology/schema terms)
2. Capacity of the system to solve the majority of the semantic conflicts (solved conflicts type).
3. The degree of automation of the resolution process.
4. The resolution of the requests (treated statically/dynamically)
5. Transparency, evolutionarily (add/remove/modification of an information sources) and the
scalability (capacity to manage an exponential increase in information sources) of architecture.
Many solutions were proposed to solve the interoperability problem of information systems
(virtual integration) [3] [4] [2], The most recent systems are the broking information systems whose
objective is oriented directed on the semantic conflicts resolution (or semantic mediation). Three
approaches [6] [8] [2] are distinguished:
The schema mediation is a direct extension of the federate approach. Data conflicts solved
statically. It consists with associates to the mediator a knowledge set (mapping rules) allowing locating
the data sources. An execution plan is established by rules, which define where are the relevant data to
a query processing are. The Global schema is generally specified by interfaces object, logic or XML. In
this approach the transparency is assured, the degree of automation resolution is weak and the
scalability and the evolutionarily are respected compared to the context mediation. The majority of the
solutions not solve the semantic conflicts of values [02].
A first tendency can be illustrated using work of Kashyap and Seth carried in 1994 for the
context mediation approach. Sensitive to the semantic heterogeneity problem which can make difficult
query processing, they introduced the concept of context. The role of the mediator in this approach, it
identifies and locates, transforms and integrates the relevant data according to semantics associated
with a query [02]. The data conflicts resolution is dynamic and does not require the definition of a
mediation schema. The user's queries are generally formulated in terms of ontologies. The data are
integrated dynamically according to the semantic information contained in the description of the
contexts. This approach allows the best taken into account of the evolution of the local sources and
thus the automation degree is better compared to the schema mediation.
Two categories of context mediation are defined: the approach single domain SIMS [9], COIN
[10] works on a single domain where all the contexts are defined by using a universal of consensual
speech. The scalability and evolutionarily are respected but remains limited by the unicity of the
domain; and multi domains approaches Infosleuth [11], Observer [12]. In this approach the conflicts
resolution is a dynamic way, during the execution of the queries allows the best taken into account of
the evolution of the local sources and thus the automation degree is better compared to the schema
mediation. Concerning the semantic conflicts, the majority of the projects solve only the taxonomic
conflicts Coin [10]. The resolution of the values conflicts either is guided by the user Infosleuth [11],
or unsolved in the majority of the cases Observer [12] [13].
The hybrid approach combines between the schema mediation and the context mediation,
named DILEMMA [02][24] and based on the queries static resolution. The mediation is ensured by a
couple mediator/wrapper and a knowledge base associated with each system taking part in the
cooperation. The mediator comprises a queries processor and a facilitator. This last is composed of two
services: a fusion service (distant context, local context) and a generation service of mediation rules.
Shared information is described in a data model directed objects deductive and attached to an
applicability or local semantic context expressed using terminological logic. This approach offers a
better transparency like the schema mediation and makes it possible to solve the values semantic
conflicts, but in a manner a priori. The automation degree is better compared to the schema mediation