DISTRIBUTED DATABASE SYSTEMS - Database Design and Development: An Essential Guide for IT Professionals - page 551

When managing global data access, a primary goal for DDBMS is transparency.
All aspects of data distribution must be transparent to the users. Users at any site
must not be concerned about the location of the data requested by them, nor should
they need to know how the data are obtained. Because of the significance of trans-
parency, a later section covers the topic in detail. Let us now examine the compo-
nents of DDBMS and see how they fit together.

Architecture and Components As you know, for a centralized database, the
DBMS supports three schema levels:

• External schema representing the set of user views of the database
• Conceptual schema representing the entire database
• Internal schema consisting of the physical representation of the entire database
as files, blocks, and records

These are clearly identifiable and applicable to all centralized database systems.
In the case of distributed database systems, however, variations on the types of
schemas are possible. We will present two such variations, one for a homogeneous
system and the other for a heterogeneous or federated system.

Homogeneous system: schema levels Figure 18-5 shows a configuration with the

following schemas:

• A collection of global external schemas
• A single global conceptual schema
• A fragmentation schema

Global
External
Schema

Local
Mapping
Schema
Local
Conceptual
Schema

Local
data

DB

Local
Internal
Schema

DBMS

London

DDBMS

Local
Mapping
Schema

Global
Conceptual
Schema

Global
External
Schema

Local
Conceptual
Schema

Global
External
Schema

Local
Mapping
Schema

Frag-
mentation
Schema

DBMS

Local
data

DB

Local
Conceptual
Schema
Local
Internal
Schema

Allo-
cation
Schema

Local
Internal
Schema

Tokyo

Local
data

DB

DBMS

New York

Figure 18-5 Homogeneous system: schema levels.