A distributed database is a set of databases stored on multiple computers that typically appears to applications as a single database.
Consequently, an application can simultaneously access and modify the data in several databases in a network.
Each Oracle database in the system is controlled by its local Oracle server but cooperates to maintain the consistency of the global distributed database.
A database server is the Oracle 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 act as a client, a server, or both, depending on the situation.
The computer that manages the HQ database is acting as a database server when a statement is issued against its local data (for example, the second statement in each transaction issues a query against the local DEPT table),
and is acting as a client when it issues a statement against remote data (for example, the first statement in each transaction is issued against the remote table EMP in the SALES database).
The following images illustrate the process by which a connection is made between remote databases.