Prior to 6.x its always been understood you never make DB changes on a Subscriber, only the Publisher. From 6.x onwards there is this statement in the SRND:
"Unified CM 6.x and later releases, subscriber servers in the cluster read their local database. Database modifications can occur in both the local database as well as the publisher database, depending on the type of changes."
Does anyone know what is meant by "type" of changes. Unless there is any detail about what can/can't be done I'm inclined to play safe and continue to advise all changes must be done on the Publisher. I'm trying to understand what would be safe to change under Publisher failure scenarios.
Pre - 6.x
The configuration database is stored on a publisher server, and a read-only copy is replicated to the subscriber members of the cluster. Changes made on the publisher are communicated to the subscriber databases, ensuring that the configuration is consistent across the members of the cluster, as well as facilitating spatial redundancy of the database.
The publisher server is the only server that has read and write access to the configuration database. When configuration changes are made, other members of the cluster have a read-only copy of the database.
From this CCM SRND;
This changes with the release of CCM 6.x;
In prior versions of Cisco Unified Communications Manager, subscriber servers in the cluster use the publisher database for READ/WRITE access, and only use the local database for READ access when the publisher database cannot be reached. With Cisco Unified Communications Manager Release 6.0, subscriber servers in the cluster READ the local database. DB WRITES happens in both the local database as well as the publisher database, depending on the type of data. DBMS (IDS) replication is used to synchronize the databases on the nodes of the cluster. When recovering from a failover conditions such as loss of WAN connectivity for extended period of time, the Cisco Unified Communications Manager databases need to be synchronized with any changes that may have been made during the outage. This process happens automatically when database connectivity gets restored. This process may take longer over low bandwidth and/or higher delay links.
Database modifications for CallProcessing
User Facing features can be made on subscribers. These include updates for:
Call Forward All (CFA)
Message Waiting Indication (MWI)
Do Not Disturb Enable/Disable (DND)
Extension Mobility Login (EM)
Monitor (for future use, currently no updates at the user level)
Hunt Group Logout
CTI CAPF status for end users and application users
Credential hacking and authentication
From this 6.x SRND;
This capability only applies to the "User Facing" features listed above. For example, in all previous versions of CCM if the Publisher was down no CFWDALL changes could be made on the user phones. Now these phones can set or unset CFWDALL due this new Subscriber ability to do these type of Database Modifications. Same with EM and the others that were listed. It's not like having two Publisher servers but is a Huge improvement in the overall cluster redundancy :)
With Unified CM 6.x, database modifications for user-facing call processing features are made on the subscriber servers to which the IP phones are registered. The subscriber servers then replicate these database modifications to all the other servers in the cluster, thus providing redundancy for the user-facing features.
(See âCall processing user-facing feature replicationâ in Figure 8-2.)
From this 6.x SRND;
Hope this helps and makes sense!