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Merge replication is a feature offered by Microsoft SQL Servers that allows changes to be sent from one primary server, called a publisher, to one or more secondary servers, called subscribers.
Merge replication is one of the modes available in the Microsoft SQL Server for distributing data to various servers from a primary server. Merge replication is one of three types of replication, along with snapshot replication and transactional replication. Which type is used depends on the database's needs, how frequently changes are made to it and the SQL Server version being employed.
Merge replication is the most complex type of replication because it allows both publisher and subscriber to independently make changes to the database. In this scenario, it is debatable whether the publisher is strictly the primary server, because other servers can also make changes to the data. At any rate, the changes are then synchronized by merge agents that sit on both servers, as well as by a predetermined conflict resolution mechanism in case of clashing data changes. Such clashes may arise because merge replication does not require a real-time network connection between the publisher and the subscriber, which raises the very real possibility of one server changing data, and another server later changing the very same data to a different value.
Merge replication is commonly used by laptop and other mobile users who cannot be constantly connected to the publisher, but still need to carry around a copy of the database that they can make changes to.