Advanced replication overview

The basic relationship in advanced replication is that of a master server and its replica server. The master server can contain a directory or a subtree of a directory. The master is writable, and means it can receive updates from clients for a given subtree. The replica server contains a copy of the directory or a copy of part of the directory of the master server. The replica is read only; it cannot be directly updated by clients. Instead it refers client requests to the master server, that performs the updates and then replicates them to the replica server.

A master server can have several replicas. Each replica can contain a copy of the masters entire directory, or a subtree of the directory. In the following example, Replica 2 contains a copy of the complete directory of the Master Server, Replica 1, and Replica 3 each contain a copy of a subtree in the Master Server directory.

The relationship between two servers can also be described in terms of roles, either supplier or consumer. In the previous example, the Master Server is a supplier to each of the replicas. Each replica in turn is a consumer of the Master Server.

Forwarding (cascading) replication is a topology that has multiple tiers of servers. A master server replicates to a set of read-only (forwarding) servers that in turn replicate to other servers. Such a topology offloads replication work from the master server. In the example of this type of topology, the master server is a supplier to the two forwarding servers. The forwarding servers serve two roles. They are consumers of the master server and suppliers to the replica servers associated with them. The replica servers are consumers of their respective forwarding servers. For example:

There can be several servers acting as masters for directory information, with each master responsible for updating other master servers and replica servers. This is referred to as peer replication. Peer replication can improve performance, availability, and reliability. Performance is improved by providing a local server to handle updates in a widely distributed network. Availability and reliability are improved by providing a backup master server ready to take over immediately if the primary master fails. Peer master servers replicate all client updates to the replicas and to the other peer masters, but do not replicate updates received from other master servers.

Figure 3 is an example of peer-to-peer replication:

Gateway replication is a more complex adaptation of peer-to-peer replication that extends replication capabilities across networks. The most notable difference is that a gateway server does replicate changes received from other peer servers through the gateway.

A gateway server must be a master server, that is, writable. It acts as a peer server within its own replication site. That is, it can receive and replicate client updates and receive updates from the other peer-master servers within the replication site. It does not replicate the updates received from the other peer-masters to any servers within its own site.

Within the gateway network, the gateway server acts as a two-way forwarding server. In one instance, the peers in its replication site act as the suppliers to the gateway server and the other gateway servers are its consumers. In the other instance, the situation is reversed. The other gateway servers act as suppliers to the gateway server and the other servers within its own replication site are the consumers.

Gateway replication uses gateway servers to collect and distribute replication information effectively across a replicating network. The primary benefit of gateway replication is the reduction of network traffic. For example: