Sun Java System Directory Server Enterprise Edition 6.2 Deployment Planning Guide

Basic Replication Concepts

The replication mechanism is described in detail in Chapter 4, Directory Server Replication, in Sun Java System Directory Server Enterprise Edition 6.2 Reference. The following section provides basic information that you need to understand before reviewing the sample topologies described later in this chapter.

Master, Consumer, and Hub Replicas

A database that participates in replication is defined as a replica.

Directory Server distinguishes between three kinds of replicas:

The following figure illustrates the role of each of these replicas in a replication topology.

Figure 10–1 Role of Replicas in a Replication Topology

Figure shows the flow of replication traffic and LDAP

Note –

The previous figure is for illustration purposes only and is not necessarily a recommended topology. Directory Server6.x supports an unlimited number of masters in a multi-master topology. A master-only topology is recommended in most cases.

Suppliers and Consumers

A Directory Server that replicates to other servers is called a supplier. A Directory Server that is updated by other servers is called a consumer. The supplier replays all updates on the consumer through specially designed LDAP v3 extended operations. In terms of performance, a supplier is therefore likely to be a demanding client application for the consumer.

A server can be both a supplier and a consumer, as in the following situations:

A server that plays the role of a consumer only is called a dedicated consumer.

For a master replica, the server must do the following:

The server that contains the master replica is responsible for recording any changes made to the master replica and for replicating these changes to consumers.

For a hub replica, the server must do the following:

For a consumer replica, the server must do the following:

Multi-Master Replication

In a multi-master replication configuration, data can be updated simultaneously in different locations. Each master maintains a change log for its replica. The changes that occur on each master are replicated to the other servers.

Multi-master configurations have the following advantages:

Multi-master replication uses a loose consistency replication model. This means that the same entries may be modified simultaneously on different servers. When updates are sent between the two servers, any conflicting changes must be resolved. Various attributes of a WAN, such as latency, can increase the chance of replication conflicts. Conflict resolution generally occurs automatically. A number of conflict rules determine which change takes precedence. In some cases conflicts must be resolved manually. For more information, see Solving Common Replication Conflicts in Sun Java System Directory Server Enterprise Edition 6.2 Administration Guide.

The number of masters that are supported in a multi-master topology is theoretically unlimited. The number of consumers and hubs is also theoretically unlimited. However, the number of consumers to which a single supplier can replicate depends on the capacity of the supplier server. You can use the SLAMD Distributed Load Generation Engine (SLAMD) to assess the capacity of the supplier server. For information about SLAMD, and to download the SLAMD software, see

Unit of Replication

The smallest unit of replication is the suffix. The replication mechanism requires one suffix to correspond to one database. You cannot replicate a suffix, or namespace, that is distributed over two or more databases using custom distribution logic. The unit of replication applies to both consumers and suppliers, which means that you cannot replicate two suffixes to a consumer that holds only one suffix.

Change Log

Every server that acts as a supplier maintains a change log. A change log is a record that describes the modifications that have occurred on a master replica. The supplier replays these modifications to its consumers. When an entry is modified, renamed, added, or deleted, a change record that describes the LDAP operation is recorded in the change log.

Replication Agreement

Directory Server uses replication agreements to define how replication occurs between two servers. A replication agreement describes replication between one supplier and one consumer.

A replication agreement identifies the following:

Replication Priority

In versions of Directory Server prior to Directory Server 6.x, updates were replicated in chronological order. In this version of the product, updates can be prioritized for replication. Priority is a boolean feature, it is on or off. There are no levels of priority. In a queue of updates waiting to be replicated, updates with priority are replicated before updates without priority. In a queue of updates waiting to be replicated, updates with priority are replicated before updates without priority.

The priority rules are configured according to the following parameters:

For more information, see Prioritized Replication in Sun Java System Directory Server Enterprise Edition 6.2 Reference.