Sun Java System Directory Server Enterprise Edition 6.0 Reference

Introduction to Replication

This introduction to replication addresses the following topics:

Types of Replica

A database that participates in replication is called a replica. There are three kinds of replica:

A single instance of Directory Server can be configured to manage several replicas.

A replica can act as a supplier of updates, or a consumer of updates, or both.

A replica can be promoted or demoted to change its behavior with respect to other replicas. Dedicated consumers can be promoted to hubs, and hubs can be promoted to masters. Masters can be demoted to hubs, and hubs can be demoted to dedicated consumers.

A server that contains a consumer replica only is called a dedicated consumer.

Unit of Replication

The smallest logical unit of replication is a suffix, also known as a naming context. The term suffix arises from the way the base DN for the naming context is a suffix for all DNs in that context. For example, the suffix dc=example,dc=com contains all directory entries in the naming context.

The replication mechanism requires one suffix to correspond to one database. The unit of replication applies to both suppliers and consumers. Therefore, two suffixes on a master replica cannot be replicated to one suffix on a consumer replica, and vice versa.

Replica Identity

Master replicas require a unique replica identifier that is a 16-bit integer between 1 and 65534. Consumer and hub replicas all have the replica ID of 65535. The replica ID identifies the replica on which changes are made.

If multiple suffixes are configured on one master, you can use the same replica ID for each suffix on the master. In this way, when a change is made on that replica ID, it is possible to identify the server on which change was made.

Replication Agreements

Replication agreements define the relationships between a supplier and a consumer. The replication agreement is configured on the supplier. A replication agreement contains the following replication parameters:

Replication Authentication

Before a master can update a consumer, the consumer authenticates the master by using a special entry called the Replication Manager entry. The master uses the Replication Manager entry to bind to the consumer.

The Replication Manager entry has a special user profile that bypasses all access control rules defined on the consumer server. The special user profile is only valid in the context of replication.

The Replication Manager entry has the following characteristics.

The Replication Manager entry is created by default when you configure replication through the browser-based interfaceDirectory Service Control Center. You can also create your own Replication Manager entry. For information about how to create a Replication Manager entry, see Using a Non-Default Replication Manager in Sun Java System Directory Server Enterprise Edition 6.0 Administration Guide.

Authentication can be performed in the following ways for SSL with replication.

Replication Change Log

All modifications received by a master replica are recorded in a change log. A change log is maintained on all master replicas and hub replicas.

In earlier versions of Directory Server, the change log was accessible over LDAP. In this version of the product, the change log is not accessible over LDAP but is stored in its own database. If your application needs to read the change log, use the retro change log plug-in for backward compatibility. For more information about the retro change log plug-in, see Replication and the Retro Change Log Plug-In.

Change Sequence Number

Each change to a master replica is identified by a change sequence number, CSN. The CSN is generated by the master server and is not visible to the client application. The CSN contains the timestamp, a sequence number, the replica ID, and a subsequence number. The change log is ordered by the CSN.

Replica Update Vector

The replica update vector, RUV, identifies the state of each replica in a topology. Stored on the supplier and on the consumer, the RUV is used to establish which changes need to be replicated. The RUV stores the URL of the supplier, the ID of the supplier, the minimum CSN, and the maximum CSN.

RUVs can be read through nsds50ruv(5dsconf) and nsds50ruv(5dsconf) attributes.

Deleted Entries: Tombstones

Directory entries deleted on one replica are maintained by Directory Server until no longer needed for replication. Such deleted entries are called tombstones, as they have objectclass: nsTombstone. In rare cases, you might need to remove tombstones manually over LDAP.

Tombstones visible only to Directory Manager. Furthermore, tombstones show up only in a search with filter (objectclass=nsTombstone). The following ldapsearch command returns tombstone entries under dc=example,dc=com.

$ ldapsearch -D "cn=Directory Manager" -b dc=example,dc=com "(objectclass=nsTombstone)"

Consumer Initialization and Incremental Updates

During consumer initialization, or total update, all data is physically copied from a master to a consumer. When you have created a replication agreement, the consumer defined by that agreement must be initialized. When a consumer has been initialized, the master can begin to replay, or replicate, update operations to the consumer. Under normal circumstances, the consumer should not require further initialization. However, if the data on a master is restored from a backup, it might be necessary to re-initialize the consumers that depend on that master.

In a multi-master replication topology, the default behavior of a read-write replica that has been re-initialized from a backup or from an LDIF file, is to refuse client update requests. By default, the replica remains in read-only mode until it is configured to accept updates again. You set the suffix property repl-accept-client-update-enabled to on using the dsconf set-suffix-prop command when the oldest updates are on the read-only replica.

When a consumer has been initialized, replication updates are sent to the consumer when the modifications are made on the supplier. These updates are called incremental updates. A consumer can be incrementally updated by several suppliers at once, provided that the updates originate from different replica IDs.

The binary copy feature can be used to clone master replicas or consumer replicas by using the binary backup files of one server to restore another server. For information about how to use binary copy for replication, see Initializing a Replicated Suffix by Using Binary Copy in Sun Java System Directory Server Enterprise Edition 6.0 Administration Guide.

Referrals and Replication

When a consumer receives a request to modify data, it does not forward the request to the server that contains the master replica. Instead, it returns to the client a list of the URLs of the masters that can satisfy the request. These URLs are called referrals.

The replication mechanism automatically configures consumers to return referrals for all known masters in the replication topology. However, you can also add your own referrals and overwrite the referrals set automatically by the server. The ability to control referrals helps enables you to perform the following tasks:

Directory Proxy Server is able to follow referrals.