Multi-master replication

Multi-master replication is a method of database replication which allows data to be stored by a group of computers, and updated by any member of the group. All members are responsive to client data queries. The multi-master replication system is responsible for propagating the data modifications made by each member to the rest of the group, and resolving any conflicts that might arise between concurrent changes made by different members.

Multi-master replication can be contrasted with master-slave replication, in which a single member of the group is designated as the "master" for a given piece of data and is the only node allowed to modify that data item. Other members wishing to modify the data item must first contact the master node. Allowing only a single master makes it easier to achieve consistency among the members of the group, but is less flexible than multi-master replication.

Multi-master replication can also be contrasted with failover clustering where passive slave servers are replicating the master data in order to prepare for takeover in the event that the master stops functioning. The master is the only server active for client interaction.

The primary purposes of multi-master replication are increased availability and faster server response time.

Many directory servers based on implement multi-master replication.

One of the more prevalent multi-master replication implementations in directory servers is Microsoft's Active Directory. Within Active Directory, objects that are updated on one Domain Controller are then replicated to other domain controllers through multi-master replication. It is not required for all domain controllers to replicate with each other domain controller as this would cause excessive network traffic in large Active Directory deployments. Instead, domain controllers have a complex update pattern that ensures that all servers are updated in a timely fashion without excessive replication traffic. Some Active Directory needs are however better served by Flexible single master operation.

CA Directory supports multi-master replication.

OpenDS (and its successor product OpenDJ) implemented multi-master since version 1.0. The OpenDS/OpenDJ multi-master replication is asynchronous, it uses a log with a publish-subscribe mechanism that allows scaling to a large number of nodes. OpenDS/OpenDJ replication does conflict resolution at the entry and attribute level. OpenDS/OpenDJ replication can be used over a Wide Area Network.

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