What should be synced to a secondary server?

[iapyeh]

My company currently has a productive service system which is based on the rpki.net.It works excellently. Now we are requested to have a secondary server as a spare. In case the major productive server encountered hardware issues.The secondary server could take over the service.There would be only one server on duty at the same time.

Since I am not the developer of the master system. I am looking for instructions or guidelines about what should be synced from the master server to the secondary server? (critical folders, files, Postgresql?)

Suppose that we already built the secondary server by the VM image of the master server. So the syncing would only need for those "delta" in the productive master server. ( I was told that it is impossible, the only solution is to duplicate the VM image of the master server to be the secondary one periodically. I am looking for reconfirmation or disapproval about this statement)

Thanks in advance for your help and suggestions.