The company I'm at hasn't patched their production servers for a while, neither SQL 2012 or Windows 2012. When I say "for a while" I mean, well, actually "never" - no patches or updates since the installation in late 2012. They are using synchronized AGs with auto-failover for redundancy so we can upgrade one node at a time.
The plan is to disable auto-failover, update the second node, failover the databases then update the first node.
Question: How do you recommend approaching this? Apply 3 years of DB and OS patches all at once to the second node, then failover to that node? How much out of sync can the nodes be and still expect a trouble-free failover of AG databases? Is it better to do the Windows update first, or the SQL CU?
The goal is to minimize downtime and the number of failover/failbacks.