I'm carrying out the development of a Disaster Recovery Plan (DRP) for my Customer who run various UC & Collaboration applications. In particular, I am looking for clarification on the roles of the servers in a Unity Connection Cluster and their capabilities in a Disaster Recovery (DR) scenario.
Firstly, the following documnet provides some details:
However, I am still a little confused as to what the impact on the Cluster will be if the Publisher server is lost.
I understand the terms Publisher & Subscriber and what that usually means in terms of database management on all UC applications.
I understand that best practice is for the Publisher server to hold the "Primary" status in normal working conditions within the cluster.
I understand that the Subscriber Server can hold the "Primary" status as would happen if the Publisher server was offline, for instance in a DR scenario.
It is here that I am struggling to understand the impact of losing the Publisher server. The documentation states the following in terms of the "Primary" server role:
Publishes the database and message store, which are both replicated to the other server in the cluster.
Receives replicated data from the other server when that server is able to share its data.
Displays and accepts changes to the administrative interfaces (such as Connection Administration). This data is replicated to the other server in the cluster.
The documentation states the following in terms of the "Secondary" server role:
Receives replicated data from the server with Primary status. Data includes the database and message store.
Replicates data to the server with Primary status.
Displays and accepts changes to the administrative interfaces (such as Connection Administration). The data is replicated to the server with Primary status.
The following is also about the Publisher server:
"About the Connection Publisher Server
The first Cisco Unity Connection server that is configured in the cluster is the publisher server. The Cluster Management page in Cisco Unity Connection Serviceability identifies the publisher server.
The publisher server assumes responsibility for publishing the database and message store when the cluster is functioning normally.
When the publisher server does not have Primary status (for example, when the administrator manually changes the status of the other server to Primary, which automatically changes the status of the publisher server to Secondary), the other server assumes responsibility for publishing the database and message store.
The publisher server cannot be removed from the cluster."
Now here I get to my point...
Why, if the Subscriber can be the "Primary" server and therefore be able to publish the database and message store, be able to have configuration changes made in its Connection Admin GUI, Answer calls etc, is it necessary for the Publisher server to be part of the cluster? The documentation clearly states "the publisher server cannot be removed from the cluster"...
What role is the Publisher server doing that is so important that it should always be present, if the Subscriber server with Primary status can effectively provide all services for voicemail users as well as system administration ?
In the context of my DRP planning, I would have to make provision to rebuild the Publisher in a DR scenario where that server would be lost. This could mean my customer having to purchase new UCS hardware to cater for this rebuild and I'm wondering if this is really necessary if the Subscriber server running in the Primary role can appear to do everything ?
So clarity would be appreciated as no documentation really explains to me why with Unity Connection, the Publisher server is so vital like it is with CUCM.
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