I have a client that just wants 0 scheduled downtime with 0 data loss with our SharePoint 2013 environment.
Our plans included a load balanced 3 tiered farm (2 WFE, 2 App servers, 2 SQL servers (AlwaysOn)). The initial thought was that we sever the connections between the two parts (move all services and DBs from web1, app1, sql1 to web2, app2, sql2 and remove the capability for the two halves to communicate with each other), patch one side of the environment, bring it back online while taking the other down for patching.
While that would work for OS patching, it would be a total disaster for SharePoint upgrades since one database would have new data and the other database would be in a new version, we'd either lose the data and have a "happy farm" or we'd lose the update and have an "angry farm."
My other thought was to utilize a Hot COOP style farm where we acquiesce the current production to the COOP production, update the current production then let current production take over again, I thought this would be far less dangerous than trying to split up a farm, but we still have the same database issues.
Are there any products or services that can allow this to happen?