Say you want to upgrade your laptop's RAM, what's the first thing you do if you have a new RAM stick in your hand? Turn off the laptop, install the new RAM, and restart. Pretty easy? Now imagine you're a system engineer at Amazon AWS. You are asked to upgrade the RAM of a physical machine, but it got some virtual machines running on it, what would you do? If you stop the server even for a second, the users will notice immediately. How to upgrade the RAM without shutting down the services? The Ze