Upgrading Multi-Server Configurations

[sproggit]

I have a pair of Technitium DNS Servers set up for a smallish network, with both machines running v14.3.

I've seen the notice for availability of 15.2 and would like to consider an upgrade, but although I've read through the available documentation, I haven't yet found any references to multi-server configurations.

My specific setup provides both authoritative resolution for my local network, plus forwarding services for general internet resolution, which I suspect is pretty common. With a pair of servers, I have one set up as a Primary and one for a Secondary for my local domain (although for reasons I have yet to fathom, my Secondary server keeps dropping out and giving me a "Notify Failed" status message.

Putting that to one side, I'd like to ask if there are any specific actions or sequences of events to be obeyed in a situation where a Primary and a Secondary server are to be upgraded. Is there a "right" and "wrong" sequence? I rather suspect it makes more sense to upgrade the Secondary first, since if that works and a problem with the Primary manifests, I should be able to take the existing primary offline and promote the Secondary to becoming a new Primary...

Unfortunately, the documentation simply doesn't cover this.

Suggestions on a correct upgrade methodology would be gratefully appreciated. Any ideas on how to address "Notify Failed" would also be appreciated.

Thank you!