Oracle7 Server Distributed Systems Volume I: Distributed Data
Introduction to Distributed Systems
An Oracle7 distributed system can be a blend of distributed database and distributed processing systems.
Distributed Processing and Distributed Databases
Distributed processing and distributed databases are not the same thing, although they have similarities. In a distributed processing system, processing data (searching for information, storing results) is distributed. In a distributed database, the data is distributed in databases on more than one machine.
Distributed Database System Basics
A distributed database system appears to a user as a single server but is, in fact, a set of two or more servers. The data on each server can be simultaneously accessed and modified via a network. Each server in the distributed system is controlled by its local database administrator (DBA), and each server cooperates to maintain the consistency of the global database.
Note that in Figure 1 - 4, the workstations are the clients and connect
to the database servers over the communications network. The two servers, HQ and SALES also communicate over the network to maintain data consistency, as changes to the SALES database may
have impact on the HQ database as when data replication has
been implemented.
Figure 1 - 4. A Distributed Environment
Figure 1 - 5 illustrates how the HQ and SALES database servers
work together.
Figure 1 - 5. An Example of Cooperative Server Architecture
Note also that the INSERT statement includes the location of the database to be accessed (SALES) because the client from which the SQL statement is issued is connected directly to the HQ database
server only.