About Replication

Replication is the process of copying and maintaining database objects, such as tables, in multiple databases that make up a distributed database system. Changes applied at one site are captured and stored locally before being forwarded and applied at each of the remote locations. Oracle Replication is a fully integrated feature of the Oracle server; it is not a separate server.

Replication uses distributed database technology to share data between multiple sites, but a replicated database and a distributed database are not the same. In a distributed database, data is available at many locations, but a particular table resides at only one location. For example, the employees table resides at only the ny.world database in a distributed database system that also includes the hk.world and la.world databases. Replication means that the same data is available at multiple locations. For example, the employees table is available at ny.world, hk.world, and la.world.

Some of the common reasons for using replication are:

Reason	Description
Availability	Replication improves the availability of applications because it provides them with alternative data access options. If one site becomes unavailable, then users can continue to query or even update the remaining locations.
Performance	Replication provides fast, local access to shared data because it balances activity over multiple sites. Some users can access one server while other users access different servers, thereby reducing the load at all servers. Also, users can access data from the replication site that has the lowest access cost, which is typically the site that is geographically closest to them.
Network Load Reduction	Replication can be used to distribute data over multiple regional locations. Then, applications can access various regional servers instead of accessing one central server. This configuration can reduce network load dramatically.
Disconnected Computing	Users can continue to view and update the replica even when network access is unavailable. A materialized view is a complete or partial copy (replica) of a target table from a single point in time. Materialized views enable users to work on a subset of a database while disconnected from the central database server. Later, when a connection is established, users can synchronize (refresh) materialized views on demand. When users refresh materialized views, they update the central database with all of their changes, and they receive any changes that may have happened while they were disconnected.
Mass Deployment	Increasingly, organizations need to deploy many applications that require the ability to use and manipulate data. With Oracle Replication, deployment templates enable you to create multiple materialized view environments quickly. You can use variables to customize each materialized view environment for its individual needs. For example, you can use deployment templates for sales force automation. In this case, the template could contain variables for various sales regions and salespersons.

Note: Beginning with Oracle 9i Release 2, the advanced replication functionality is also provided by Oracle Streams. Although Oracle Corporation continues to support the advanced replication functionality, Oracle Streams provides greater flexibility and functionality.

To learn more about Oracle Streams, refer the Oracle Streams Replication Administrator's Guide.

About Replication

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