Introduction to Distributed Database Systems

A distributed database appears to a user as a single database but is, in fact, a set of databases stored on multiple computers. The data on several computers can be simultaneously accessed and modified using a network. Each database server in the distributed database is controlled by its local DBMS, and each cooperates to maintain the consistency of the global database.

Clients, Servers, and Nodes :
A database server is the software managing a database, and a client is an application that requests information from a server. Each computer in a system is a node. A node in a distributed database system can be a client, a server, or both.
A client can connect directly or indirectly to a database server.


Site Autonomy :
Site autonomy means that each server participating in a distributed database is administered independently (for security and backup operations) from the other databases, as though each database was a non-distributed database. Although all the databases can work together, they are distinct, separate repositories of data and are administered individually. Some of the benefits of site autonomy are as follows:
- Nodes of the system can mirror the logical organization of companies or cooperating organizations that need to maintain an "arms length" relationship.
- Local data is controlled by the local database administrator. Therefore, each database administrator's domain of responsibility is smaller and more manageable.
- Independent failures are less likely to disrupt other nodes of the distributed database. The global database is partially available as long as one database and the network are available; no single database failure need halt all global operations or be a performance bottleneck.
- Failure recovery is usually performed on an individual node basis.
- A data dictionary exists for each local database.
- Nodes can upgrade software independently.

Homogenous Distributed Database Systems :
A homogenous distributed database system is a network of two or more Oracle databases that reside on one or more machines. An application can simultaneously access or modify the data in several databases in a single distributed environment. For example, a single query on local database MFG can retrieve joined data from the PRODUCTS table on the local database and the DEPT table on the remote HQ database.


Heterogeneous Distributed Database Systems :
In a heterogeneous distributed database system, at least one of the databases is a non-Oracle system. To the application, the heterogeneous distributed database system appears as a single, local, Oracle database; the local Oracle server hides the distribution and heterogeneity of the data.
The Oracle server accesses the non-Oracle system using Oracle8i Heterogeneous Services and a system-specific transparent gateway. For example, if you include a DB2 database in an Oracle distributed system, you need to obtain a DB2-specific transparent gateway so that the Oracle databases in the system can communicate with it.