When an entry is modified, a change record describing the LDAP operation that was performed is recorded in the changelog.

The changelog size is maintained with two attributes, nsslapd-changelogmaxageor nsslapd-changelogmaxentries. These attributes trim the old changelogs to keep the changelog size reasonable.

6.1.1.5 Replication agreement

Directory Servers use replication agreements to define replication. A replication agreement describes replication between a single supplier and a single consumer. The agreement is configured on the supplier server. It identifies:

•The database to replicate.

•The consumer server to which the data is pushed.

•The times that replication can occur.

•The DN that the supplier server must use to bind (called the supplier bind DN).

•How the connection is secured (TLS/SSL, Start TLS, client authentication, SASL, or simple authentication).

•Any attributes that will not be replicated (see “Replicated selected attributes with fractional replication”).

6.1.2Data consistency

Consistency refers to how closely the contents of replicated databases match each other at a given point in time. Part of the configuration for replication between servers is to schedule updates. The supplier server always determines when consumer servers need to be updated and initiates replication.

Directory Server offers the option of keeping replicas always synchronized or of scheduling updates for a particular time of day or day in the week.

The advantage of keeping replicas constantly synchronized is that it provides better data consistency. The cost is the network traffic resulting from the frequent update operations. This solution is the best option when:

•There is a reliable, high-speed connection between servers.

•The client requests serviced by the directory service are mainly search, read, and compare operations, with relatively few update operations.

If it is all right to a lower level of data consistency, choose the frequency of updates that best suits the use patterns of the network or lowers the affect on network traffic. There are several situations where having scheduled updates instead of constant updates is the best solution:

•There are unreliable or intermittently available network connections.

•The client requests serviced by the directory service are mainly update operations.

•Communication costs have to be lowered.

In the case of multi-master replication, the replicas on each supplier are said to be loosely consistent, because at any given time, there can be differences in the data stored on each supplier. This is true, even if the replicas are constantly synchronized, for two reasons:

•There is a latency in the propagation of update operations between suppliers.

•The supplier that serviced the update operation does not wait for the second supplier to validate it before returning an "operation successful" message to the client.

6.2Common replication scenarios

Decide how the updates flow from server to server and how the servers interact when propagating updates. There are the four basic scenarios and a few strategies for deciding the method

6.2 Common replication scenarios 75