A 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
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 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 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
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
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;
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.
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