Replication Change Numbers

Change numbers uniquely identify changes that are made on an LDAP directory server. Change numbers also provide a consistent ordering of changes. The change number order is used to resolve conflicts and to determine the order in which forwarded changes should be replayed.

A change number consists of the following elements:

• Time stamp, in milliseconds. Time stamps are generated using the system clock. The change number is also generated such that each change number is always greater than all the change numbers that have already been processed by the server. Constantly increasing change numbers guarantees that operations that depend on previous operations are consistently replayed in the correct order. An example of an operation that depends on a previous operation is a modify operation that directly follows the add operation for that entry.

• Sequence number. A sequential number, incremented for each change that occurs within the same millisecond.

• Replica identifier. A unique integer identifier that is assigned to each replica in a topology. (A replication topology is the set of all replicas of a given data set. For example, the replication topology for example.com might be all copies of the dc=example,dc=com suffix across a directory service.)

  The replica identifier ensures that two different servers do not assign the same identifier to two different changes. In a future directory server release, an algorithm might be used to assign replica identifiers automatically.