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Continuous Operation

Each site is capable of processing independently from the other remote sites, and the addition of a new site will not affect the overall system. While each site maintains its own unique identity and control, it functions as a part of a unified federation such that other remote sites can access information from it in a seamless fashion.
Continuous operation also refers to the ability of each node to be available to the overall system 24 hours per day, seven days per week. To accomplish this goal, remote sites may have a "hot backup" tool such as the Oracle Enterprise Backup Utility (EBU) or the Oracle8 Recovery Manager (RMAN) to back up the database while it remains available for update.
Other Oracle continuous-operation tools include Oracle Parallel Server and Oracle snapshots.

One of the most common reasons for developing a distributed database system is to provide redundancy and fault tolerance. By the same token, a distributed system should not require scheduled outages to perform maintenance such as adding and removing a site or upgrading software. Of course, in the ideal world we would have zero downtime, scheduled or not, however unplanned outages are difficult to avoid entirely.
Continuous operation of a distributed system means that no maintenance tasks should require an outage of the entire system. Maintenance tasks may include upgrades to the operating system or RDBMS or the addition and deletion of participating sites.
If the Oracle distributed system is built on database links and simple replication (i.e., read-only snapshots), then there are no maintenance activities that would require an outage of the entire distributed environment. Sites can be added or removed at any time, and upgrades can be executed without impacting participating sites. However, if you are using the advanced replication facilities, Oracle imposes certain limitations. Most significantly, if you wish to add a new master or snapshot site to a replicated environment, you must coordinate the addition so that the data at the new site includes data changes that may have occurred while the new site is being instantiated.