In a cascading replication configuration, a server acting as a hub receives updates from a server acting as a supplier. The hub replays those updates to consumers. The following figure illustrates a cascading replication configuration.
Cascading replication is useful in the following scenarios:
When there are a lot of consumers.
Because the masters in a replication topology handle all update traffic, it could put them under a heavy load to support replication traffic to the consumers. You can off-load replication traffic to several hubs that can each service replication updates to a subset of the consumers.
To reduce connection costs by using a local hub in geographically distributed environments.
The following figure shows cascading replication to a large number of consumers.
In Figure 11–6, hubs 1 and 2 relay replication updates to consumers 1 through 10, leaving the master replicas with more resources to process directory updates.
The masters and the hubs maintain a change log. However, only the masters can process directory modification requests from clients. The hubs contains a Replication Manager entry for each master that sends updates to them. Consumers 1 through 10 contain Replication Manager entries for hubs 1 and 2.
The consumers and hubs can process search requests received from clients, but cannot process modification requests. The consumers and hubs refer modification requests to the masters.