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4.Distributed Databases

distributed database is a network of databases managed by multiple database servers that appears to a user as a single logical database. The data of all databases in the distributed database can be simultaneously accessed and modified. The primary benefit of a distributed database is that the data of physically separate databases can be logically combined and potentially made accessible to all users on a network.

Each computer that manages a database in the distributed database is called a node. The database to which a user is directly connected is called the local database. Any additional databases accessed by this user are called remote databases. When a local database accesses a remote database for information, the local database is a client of the remote server. This is an example of client/server architecture.

While a distributed database enables increased access to a large amount of data across a network, it must also hide the location of the data and the complexity of accessing it across the network. The distributed database management system must also preserve the advantages of administrating each local database as though it were not distributed.

Location Transparency

Location transparency occurs when the physical location of data is transparent to the applications and users of a database system. Several Oracle features, such as views, procedures, and synonyms, can provide location transparency. For example, a view that joins table data from several databases provides location transparency because the user of the view does not need to know from where the data originates.

Site Autonomy

Site autonomy means that each database participating in a distributed database is administered separately and independently from the other databases, as though each database were a non-networked database. Although each database can work with others, they are distinct, separate systems that are cared for individually.

Distributed Data Manipulation

The Oracle distributed database architecture supports all DML operations, including queries, inserts, updates, and deletes of remote table data. To access remote data, you make reference to the remote object's global object name. No coding or complex syntax is required to access remote data.

For example, to query a table named EMP in the remote database named SALES,reference the table's global object name:

SELECT * FROM emp@sales;

Two-Phase Commit

Oracle provides the same assurance of data consistency in a distributed environment as in a nondistributed environment. Oracle provides this assurance using the transaction model and a two-phase commit mechanism.

As in nondistributed systems, transactions should be carefully planned to include a logical set of SQL statements that should all succeed or fail as a unit. Oracle's two-phase commit mechanism guarantees that no matter what type of system or network failure occurs, a distributed transaction either commits on all involved nodes or rolls back on all involved nodes to maintain data consistency across the global distributed database.