Databases Reference
In-Depth Information
9.2.1
Five-Level Model for Heterogeneous Multi-DBMS
A heterogeneous multi-DBMS requires integration of the export schemas
to be performed so the federation can be accessed as a single resource. This
section examines how the three-level ANSI/SPARC architecture for central-
ized DBMSs [2] has been extended to a five-level architecture for heterogene-
ous DDBs. Several variations of the five-level model have been proposed
[1, 3, 4], and the description here broadly follows that of [1], except that
local DBs are termed component databases in [1] and global schemas are
termed federated schemas.
A heterogeneous multi-DBMS must integrate the export schemas of
the local DBs into one or more global schemas, which provide an integrated
view through which global queries and transactions can access the federation.
This view must be constructed while preserving the autonomy of the local
DBs, that is, leaving control of them in the hands of the local DBAs.
The five-level model illustrated in Figure 9.2 achieves that by requiring
the following five types of schema to be present in a heterogeneous DDB
system.
A local schema for each local DB. The local schema is the conceptual
schema of the local DB. Each local DB continues to operate as an
autonomous entity, and the content of its local schema is under the
control of its local DBAs. Each local DB will also have a physical
schema and possibly a number of external schemas that are views of
its local schema. However, those schemas are not considered to be
part of the heterogeneous multi-DBMS architecture.
·
A component schema corresponding to each local schema. The local DBs
may support different data models and different DDL/DMLs.
Thus, the local schemas have to be translated into some common
data model (CDM) before they can be integrated. This may be some
variant of the E/R model or an object-oriented data model. Each
component schema is the translation of its corresponding local
schema into this CDM.
·
One or more export schemas corresponding to each component schema.
Each export schema is a view over the component schema that the
local DBAs want to make available to the federation. The export
schemas define what part of the locally held information can be
accessed by global queries and transactions.
·
One or more global schemas. Each global schema is obtained by inte-
grating one or more of the export schemas into a single schema. A
·
