When I did the first pass at this architecture about 6 years ago, we had a few servers in a rack at a local datacenter as well as virtual servers at Rackspace and AWS.
Since then, I haven't shed the habit of designing for the general case (this is also known as avoiding vendor lock-in :)
Since then, I haven't shed the habit of designing for the general case (this is also known as avoiding vendor lock-in :)