Introduction to Merge Replication

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Merge replication performs multidirectional replication between the publisher and one or more subscribers. This allows multiple systems to have updatable copies of the publication and to modify their own copies. A modification on the publisher will be replicated to the subscribers. A modification on a subscriber will be replicated to the publisher and then replicated to the other subscribers.

Unlike transactional replication, merge replication works by installing triggers on the publisher and on the subscribers. Whenever a change is made to the publication or a copy of it, the appropriate trigger is fired, which causes a replication command to be queued up to be sent to the distribution database. This command is eventually sent to the distribution database and then sent to participating systems. Because merge replication operates this way, it requires much more overhead, especially on the publisher, than does transactional replication.

As you will learn in this chapter, the key components involved in the merge replication system are the Merge Agent and the distribution database. The Merge Agent reconciles (merges) incremental changes that have occurred since the last reconciliation. When you use merge replication, no Distribution Agent is used—the Merge Agent communicates with both the publisher and the distributor. The Snapshot Agent is used only to create the initial database. The Merge Agent performs the following tasks.

1. The Merge Agent uploads all changes from the subscriber.
2. All of the rows without a conflict (rows not modified on both the publisher and the subscriber) are uploaded immediately; those with a conflict (rows modified on both systems) are sent to the conflict resolver. The resolver is a module that is used to resolve conflicts in merge replication. You can configure this module to resolve conflicts based on your needs.
3. All changes are applied to the publisher.
4. The Merge Agent uploads all changes from the publisher.
5. All of the rows without a conflict are uploaded immediately; those with a conflict are sent to the conflict resolver.
6. All changes are applied to the subscriber.

This process will repeat as scheduled. With push subscriptions, the Merge Agent runs on the distributor. With pull subscriptions, the Merge Agent runs on the subscriber. Each merge publication has its own Merge Agent.