Types of
Distributed Database
In Distributed database System (DDBS), data and
software are distributed over multiple sites connected by communication network.
Following two types of distributed databases are most
commonly used:
v
Types of Distributed Database
o
Homogeneous DDBMS
o Heterogeneous DDBMS
Homogeneous Distributed Database Management Systern(DDBMS)
Homogeneous DDBS is the simplest form of a distributed database where there are several sites, each running their own applications on the same DBS software. All sites have identical DBS software; all users (or clients) use identical software, are aware of one another and agree to cooperate in processing user's request.
The application can all see the same schema and run
the same transactions. That is, there is Location transparency in homogeneous DDBS. The
provision of location transparency forms the core of Distributed Database Management
System (DDBMS) development.
In Homogeneous DDBS. the use of a single DBMS avoids
any problems of mismatched database capabilities between nodes, since the data
is all managed within a single framework. In homogeneous DDBS, local sites
surrender a portion of their autonomy in terms of their rights to change schema
or DBMS software.
Homogeneous Distribute DBMS Environment
Design and implementation of a Homogeneous Distribute
DBMS Environment need involve only a single vendor. Any database request issued
at a client node does not require translation, because the database language
and data model are the same across all nodes in the network.
Heterogeneous Distributed Databment System(DDBMS)
In heterogeneous distributed database system, different sites run under the control of different DBSs essentially autonomously and are connected somehow to enable access to data from multiple sites. Different sites may use different schemas and different DBMS software.
The sites may not a aware of one another and they may provide
only limited facilities for cooperation in transaction processing. In other
word, if heterogeneous DDBS, each server (site) is an independent and autonomous centralised DBMS that has its own
local users, local transactions and
Database Administrator (DBA).
Heterogeneous Distributed DBMS Environment
In a truly heterogeneous distributed DBMS environment, the hardware, operating systems, communications systems, and DBMSs can all be different Different DBMSs may mean different data models alongwith different database languages for definition and manipulation. Any database request issued at a client node would have to be translated so that the server node responding to the request would understand how to execute the request.
Various degrees of heterogeneity can exist. For
example, within the distributed environment, different DBMSs can still be
compatible if they all support the relational data model and understand SQL, a
relational query language that is an American National Standards Institute and
International Standards Organisation standard. Presently, however, even among
SQL conforming systems, there is no general communications software that will
accept generic SQL statements from any other SQL conforming DBMS. This is an
area in which the pending Remote Data Access (RDA) standards are needed.
