Configuring the Red Hat Directory Server CSN behavior

Solution Unverified - Updated -

Environment

Red Hat Directory Server 9.1.2 and later
Red Hat Directory Server 10.1 and later
Red Hat Enterprise Linux 6.9 and later
Red Hat Enterprise Linux 7.3 and later
Red Hat Identity Management 4.4 and later

Issue

Replication fails because of a missing change sequence number (CSN) in the local change log database.

Resolution

The 389-ds-base package, provided in the RHBA-2016-25950 erratum, adds the nsds5ReplicaIgnoreMissingChange Directory Server configuration parameter to the replication configuration entry. The parameter supports the following values:

  • never or off: (Default) No alternative CSN is used. A missing CSN causes the replication to fail.
  • once or on: A missing CSN is ignored once and an alternative CSN is used. If Directory Server is unable to also locate the alternative CSN, replication fails.
  • always: A missing CSN is always ignored and the entry never replicated.

To set the nsds5ReplicaIgnoreMissingChange parameter on a Directory Server instance, use:

# ldapmodify -D "cn=Directory Manager" -W -x -p 389 -h server.example.com

dn: cn=*agreement_name*,cn=cn=replica,cn=*suffixDN*,cn=mapping tree,cn=config
changetype: modify
replace: nsds5ReplicaIgnoreMissingChange
nsds5ReplicaIgnoreMissingChange: once

This setting does not require re-initializing the replication agreement or restarting the server to take effect.

Root Cause

In a Red Hat Directory Server replication environment, the supplier selects a CSN to before sending an update to the replica. If the supplier is unable to locate the CSN in the local change log database, the update cannot be sent and the supplier retries the process later. If the consumer was updated in the meantime by a different supplier, the CSN can be different and replication succeeds. However, if the supplier continuously fails to locate the CSN in the change log, replicating the entry fails.

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