This is a common approach for geographically distributed Oracle systems. A centralized processor does all the work, while remote data hubs access the data. The computers at the remote nodes act only as data servers, with all the processing being done by a large central processor. The main advantage to this approach is the proximity of the data to the user in a distributed network, and the main disadvantage is the lack of centralized control over backup and recovery of the data.
This is the standard configuration for Oracle Network Services.
Distributed Database System
A distributed database system consists of loosely coupled sites that share no physical component.
Database systems that run on each site are independent of each other
Transactions may access data at one or more sites
Homogeneous Distributed Databases
In a homogeneous distributed database
All sites have identical software
Are aware of each other and agree to cooperate in processing user requests.
Each site surrenders part of its autonomy in terms of right to change schemas or software
Appears to user as a single system
In a heterogeneous distributed database
Different sites may use different schemas and software
1) Difference in schema is a major problem for query processing, 2) Difference in softwrae is a major problem for transaction processing
Sites may not be aware of each other and may provide only limited facilities for cooperation in transaction processing
History of Distributed Databases
The ability to create a distributed database has existed since the late 1970s.
As you might expect, a variety of distributed database options exist. Figure 3-2 outlines the range of distributed database environments. These environments are briefly explained by the following:
I. Homogeneous The same DBMS is used at each node.
Autonomous Each DBMS works independently, passing messages back and forth to share data updates.
Nonautonomous A central, or master, DBMS coordinates database access and update across the nodes.
II. Heterogeneous Potentially different DBMSs are used at each node.
Systems Supports some or all of the functionality of one logical database.
Full DBMS functionality Supports all of the functionality of a distributed database.
Figure 3-2: Distributed database environments
Managing Distributed Databases
Due to the volatile nature of corporate computing that existed during the 1990's many corporations were faced with the issue of managing widely distributed database systems, spanning geographical areas, hardware platforms and database architectures.
Other shops embraced
downsizing and
rightsizing
and reated situations where many diverse islands of information were spread across many computer networks.
