Basic Replication Concepts

The replication mechanism is described in detail in Chapter 4, Directory Server Replication, in Sun Java System Directory Server Enterprise Edition 6.0 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:

• Master or read-write replica. A read-write database that contains a master copy of the directory data. A master replica can process update requests from directory clients. A topology that contains more than one master is called a multi-master topology.

• Consumer replica. A read-only database that contains a copy of the information in the master replica. A consumer replica can process search requests from directory clients but refers update requests to master replicas.

• Hub replica. A read-only database (like a consumer replica) that is stored on a Directory Server that supplies one or more consumer 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

The previous figure is for illustration purposes only and is not necessarily a recommended topology. Directory Server 6.0 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:

• In multi-master replication, a master replica is mastered on two different Directory Servers. Each server acts as a supplier and a consumer of the other server.

• When the server contains a hub replica, the server receives updates from a supplier and replicates the changes to consumers.

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:

• Respond to update requests from directory clients

• Maintain historical information and a change log

• Initiate replication to consumers

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:

• Respond to read requests

• Refer update requests to the servers that contain a master replica

• Maintain historical information and a change log

• Initiate replication to consumers

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

• Respond to read requests

• Maintain historical information

• Refer update requests to the servers that contain a master replica

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:

• Automatic write failover occurs when one master is inaccessible.

• Updates can be made on a local master in a geographically distributed environment.

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 6.0 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 http://www.slamd.com.

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:

• The suffix to replicate

• The consumer server to which the data is pushed

• The times during which replication can occur

• The bind DN and credentials that the supplier must use to bind to the consumer

• How the connection is secured, SSL or client authentication, for example

• Information about the replication status for this particular agreement

• Information about replication filtering

Replication Priority

In versions of Directory Server prior to Directory Server6.0, 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:

• The identity of the client

• The type of update

• The entry or subtree that was updated

• The attributes changed by the update

For more information, see Prioritized Replication in Sun Java System Directory Server Enterprise Edition 6.0 Reference.