First of all, what is a "bad vm"? Second: smoketests should show if the engine is up and serving requests not under load. Third: you have the fallback of reve

First of all, what is a "bad vm"? Second: smoketests should show if the engine is up and serving requests not under load. Third: you have the fallback of reversing the flow. You will always lose requests. All you can do is minimize the damage. EDIT: Want to clarify what I mean by lose requests since someone is downvoting. In the process of administrating a site with that many requests and a moving infrastructure you can plan as much as you want and try to employ hedging strategies all you want. You will STILL run into problems once in a while that will drop messages. You don't have to lose requests to handing load-balancing off to another handler and you can reload apps without losing requests if the app server is properly coded.