About Replication

Replication is a topology wide feature that always involves more than one participant. For this reason, when you troubleshoot problems with replication, you need to detect from exactly what point in the topology replication stopped working and which replication agreements are broken.

Replication works as follows:

1. A master receives a change. Once the change has been applied to the entry in the database, then because the server is a master, it stores the change in the change log database.

2. The master updates the Replica Update Vector (RUV) in memory.

3. The master notifies the replication threads that a new change has been recorded in the change log.

4. These replication threads contact replication partners to propagate the information.

For example, Master 1 receives a change, applies it to the entry and updates its change log. When the threads on master 1 contact the consumer, the consumer shows that master replica its RUV. The master looks at the RUV and compares it with the RUV it contains in memory to see if it contains more recent changes than the consumer. If, for example, it sees that the consumer contains a higher RUV, it does not send changes. If it contains a more recent change, it sends another request to the consumer asking for a lock on replica ID 1 so that it can make updates. If the lock is unavailable, the update will be made later. If the lock is available, then the master can proceed to make the change.