Replication

You can share between several directory servers the load of processing requests generated by directory service clients for the same information. This is done by defining a replica, or slave, server to provide an alternative access point to the directory service for clients. A master naming context can have more than one replica naming context. Figure 1-2 shows a master server with two replica servers. Replication is the process by which changes in the master data store are propagated to all the replica naming contexts. You can replicate an entire naming context, a subtree, or a particular entry. You can replicate the entire content of an entry or you can specify a subset of attributes to be replicated.

Figure 1-2 Master and Replica Servers

Using replication has the following advantages:

• It reduces the load on the master server by diverting some traffic to other servers.

• You can store copies of data where it is mostly frequently used, reducing network traffic.

• You can store many copies of the data, but the data is maintained from a central location.

• You need only replicate the data that is required by clients of the replica server, if you know the requirements of those clients precisely enough. You may be able to tailor a replica exactly to the needs of a specific client. By reducing the number of entries replicated, you reduce the network traffic caused by replication updates.

• You could maintain a "public" replica server containing information that is not confidential, allowing greater access to this information than you usually allow for other servers. For example, you could create a server containing the email addresses for the sales and support staff who deal with current products but not the research staff working on future products, and make it available to the sales staff of a partner company.

You could provide the same partial view of directory information with appropriate access controls. However, using a partial replica on a dedicated machine ensures that you are not providing access to your entire network. For extra security, you could connect the replica server to your network only while the replication update is in progress.

The costs of using replication are:

• Additional network traffic caused by replication of data. However, though there may be an overall increase in traffic, more of the traffic will be local, so you can avoid known network bottlenecks for inquiry traffic. Also, you can time the replication updates for when the network is least busy.

• Information retrieved from replicas may be out of date if replication has not happened since an update, so certain applications may always need to query the master data store.

• You cannot modify a replica. All updates must be performed on the master copy of an entry.