The advanced replication synchronization technology relies on a master/replica model. A single computer keeps a master or official copy of the data. All other nodes (replicas) in the system can also have a copy of some or all of the master data. When the replica changes data, that data is unofficial until it has been sent to, and accepted by, the master. Once the data has been accepted on the master, the replica that sent that data (and other replicas, as well) can request a copy of the new official data. In this way, replicas can temporarily become out of sync with the master, but such differences can be corrected quickly.
The advanced replication architecture supports a virtually unlimited number of replicas to have access to data. Each replica can read and write the data, as long as users know that local writes must be accepted at the master before they are official. Replicas do not need to be connected to the network 100% of the time. A replica can be stored on a mobile device or computer that is only connected to the network part of the time. This allows replicas to operate for long periods independently of the master, providing flexibility. It also means that the entire system is not disabled just because the master database server is shut down formaintenance or to correct a problem. Distributed systems based on the advanced replication technology are inherently robust with respect to isolated failures.
A replica database can hold not only copies of some or all master data, but also local data that is not shared with the master. A single computer can hold a mix of local and shared data.
The master database stores the data and the synchronization definitions. The synchronization definition data includes catalog, database schema, publication definitions, registrations, subscriptions, user access definitions of the replica databases, and so on.
The replica database stores local data and transactions that are to be propagated to the master. All replica data, or a suitable part of it, can be refreshed from the master database whenever needed by sending a REFRESH command to one or more publications. The local data includes data that is typically a subset of the master database, as well as system tables that contain information specific to the particular database.
In a multi-tier synchronization environment, synchronized databases can be configured to serve a dual role as both a master and a replica. These roles are established by creating a catalog and defining it to be both a replica and a master.