Merge Replication

Merge replication is the process of distributing data from Publisher to Subscribers, allowing the Publisher and Subscribers to make updates while connected or disconnected, and then merging the updates between sites when they are connected.

Merge replication allows various sites to work autonomously and at a later time merge updates into a single, uniform result. The initial snapshot is applied to Subscribers, and then Microsoft® SQL Server™ 2000 tracks changes to published data at the Publisher and at the Subscribers. The data is synchronized between servers continuously, at a scheduled time, or on demand. Because updates are made at more than one server, the same data may have been updated by the Publisher or by more than one Subscriber. Therefore, conflicts can occur when updates are merged.

Merge replication includes default and custom choices for conflict resolution that you can define as you configure a merge publication. When a conflict occurs, a resolver is invoked by the Merge Agent and determines which data will be accepted and propagated to other sites.

Merge Replication is helpful when:

• Multiple Subscribers need to update data at various times and propagate those changes to the Publisher and to other Subscribers.
  
  
• Subscribers need to receive data, make changes offline, and later synchronize changes with the Publisher and other Subscribers.
  
  
• You do not expect many conflicts when data is updated at multiple sites (because the data is filtered into partitions and then published to different Subscribers or because of the uses of your application). However, if conflicts do occur, violations of ACID properties are acceptable.

Insert Diagram (servers and data flow – laptops to indicate occasionally connected)

Both queued updating and merge replication allow updates at the Publisher and at Subscribers while offline; however, there are significant differences between the two methods. For more information, see Merge Replication or Updatable Subscriptions.