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Omar Boucelma, Mehdi Essid, and Yassine Lassoued
capabilities are extracted using WFS queries (GetCapabilities and DescribeFeatureType). The global schema is specified as an XSD [37]. To understand how the system
works, let us describe the path followed by a query as illustrated in Fig. 7.6.
When submitted to the system, a (global) user query is rewritten into subqueries
expressed in terms of source schemas. First of all, the decomposition module decomposes the user query into a set of elementary subqueries (ESQs) expressed in terms
of the global schema. Each such (elementary) subquery aims to extract information
about a single feature. The decomposition module computes a global execution plan
(GEP) expressed as a join over ESQs.
Final Execution plan
Sources Capabilities
Global Schema
Mapping Rules
S1
S2
SubQueries
GetFeature
GQuery
XML/KVP request
Global execution plan
GetFeatureType
GetCapabilities
DescribeFeatureType
GML reply
VirGIS Integration System
Query flow
Data flow
GML
GML
WFS
Decomposition
Source Manager
Execution
WFS Query Engine
Join Engine
GQuery Engine
Recomposition
Mapping/Reformulation
Rewriting
WFS
XML/KVP to GQuery Wrapper
Fig. 7.6. VirGIS architecture