In CM 4.X, I seem to remember that if the Publisher went offline that the IP phones would not be able to use call forwarding (besides the obvious issue of not being able to make database changes at all.) In CM 6.X, I've seen that this is not the case; when a Publisher is offline phones can still to call forward. This is a welcome difference since, unless the Publisher is used to host phone directories) there will be no noticeable service interruption. Can someone help me understand if I am seeing this correctly? Have there been changes to the way database modifications are made between versions of CM? I've asked TAC this question in the past and the answer was kind of wishy-washy.
You are most correct my friend :)
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;
Hope this helps!