X-WACoDa - Data Warehousing Design and Advanced Engineering Applications - page 42

xML Web Warehouses to XML warehousing, but they can definitely be

used for representing XML star schemas.
Rusu et al. (2005) propose a methodology,
based on the XQuery technology, for building
XML data warehouses. This methodology covers
processes such as data cleaning, summarization,
intermediating XML documents, updating/link-
ing existing documents and creating fact tables.
Facts and dimensions are represented by XML
documents built with XQueries.
Park et al. (2005) introduce a framework for
the multidimensional analysis of XML documents,
named XML-OLAP. XML-OLAP is based on an
XML warehouse where every fact and dimension
is stored as an XML document. The proposed
model features a single repository of XML docu-
ments for facts and multiple repositories of XML
documents for dimensions (one repository per
dimension).
Eventually, Boussaïd et al. (2006) propose an
XML-based methodology, named XWarehousing,
for warehousing complex data. They use XML
Schema as a modelling language to represent user
analysis needs. These needs are then compared
to complex data stored in heterogeneous XML
sources. Information needed for building an
XML cube is extracted from these sources and
transformed into OLAP facts.
Note that all these studies, though all dif-
ferent, more or less converge toward a unified
XML warehouse model. They mostly differ in
the way dimensions are handled and the number
of XML documents that are used to store facts
and dimensions.

The objective of these approaches is to gather
XMLWeb sources and integrate them into a Web
warehouse. Vrdoljak et al. (2003) introduce the
design of aWeb warehouse that originates from
XML Schemas describing operational sources.
This method consists in preprocessing XML
Schemas, in creating and transforming the schema
graph, in selecting facts and in creating a logical
schema that validates a data warehouse.
Golfareli et al. (2001) also propose a semi-
automatic approach for building a datamart con-
ceptual schema from XML sources. The authors
show how data warehouse multidimensional
design may be carried out starting directly from
XML sources. They also propose an algorithm
that solves the problem of correctly inferring the
information needed for data warehousing.
Finally, the designers of the Xyleme system
propose a dynamic warehouse for XML data from
the Web that supports query evaluation, change
control and data integration (Xyleme, 2001).
No particular warehouse model is proposed,
though.

xML Data Warehouses

In his XML-star schema, Pokorný (2002) models
a star schema in XML by defining dimension hier-
archies as sets of logically connected collections
of XML data, and facts as XML data elements.
Hümmer et al. (2003) propose a family of
templates enabling the description of a multidi-
mensional structure (dimension and fact data) for
integrating several data warehouses into a virtual
or federated warehouse. These templates, col-
lectively named XCube, consist of three kinds of
XML documents with respect to specific schemas:
XCubeSchema stores metadata; XCubeDimension
describes dimensions and their hierarchical levels;
and XCubeFact stores facts, i.e., measures and
the corresponding dimension references. These
federated templates are not actually directly related

xML Document Warehouses

Nassis et al. (2005) propose a conceptual approach
for designing and building an XML repository,
named xFACT. They exploit object-oriented
concepts and propose to select dimensions based
on user requirements. To enhance the XML data
warehouse's expressiveness, these dimensions
are represented by XML virtual views. In this