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achieve an optimal design. As illustrated in Fig.
10.5
, two types of activities
can be distinguished: one that corresponds to analysis needs (as described
in Sect.
10.3.1
) and another that represents the steps involved in creating
a multidimensional schema from operational databases (as described in
Sect.
10.3.3
). Each type of activity results in the identification of elements
for the initial multidimensional schema.
Document
requirements
specification
Analysis-driven
approach
Determine
analysis needs
Identify users
Document
requirements
specification
Source-driven
approach
Identify source
systems
Apply derivation
process
Fig. 10.5
Steps for requirements specification in the analysis/source-driven
approach
10.4 Conceptual Design
Independently of whether the analysis-driven or the source-driven approach
has been used, the requirements specification phase should eventually provide
the necessary elements for building the initial conceptual data warehouse
schema. The purpose of this schema is to represent a set of data requirements
in a clear and concise manner that can be understood by the users. In the
following, we detail the various steps of the conceptual-design phase and
show examples of their execution. We use the MultiDim model described in
Chap.
4
to define the conceptual schemas, although other conceptual models
that provide an abstract representation of a data warehouse schema can also
be used.
10.4.1 Analysis-Driven Conceptual Design
The design of a conceptual schema is an iterative process composed of three
steps, shown in Fig.
10.6
, namely, the development of the initial schema,
the verification that the data in this schema are available in the source
systems, and the mapping between the data in the schema and the data
in the sources. In the case of missing data items, modification of the schema
must be performed, which may lead to changes in the mappings. Finally, note
that data can be directly obtained from the sources or can be derived from
one or many sources. We next detail the three steps above.
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