Heroku operates in EC2, which is an Amazon service providing virtual machines spread across several datacenters. Rapgenius' nodes, as well as all of Heroku's in

Heroku operates in EC2, which is an Amazon service providing virtual machines spread across several datacenters. Rapgenius' nodes, as well as all of Heroku's internal infrastructure (e.g. routing) run as EC2 virtual machines. EC2 VMs are multitenant environments because you share them with other virtual machines. The disk, CPU, and network are all virtualized, shared resources, with wildly varying latency, throughput, and reliability characteristics. RapGenius doesn't need a hundred nodes to do their routing, but Heroku does, because they're responsible for lots of customers pushing a large volume of traffic. Their routing infrastructure needs to share global state about which dynos are available for which applications, (and which versions of those applications) belong to a given HTTP request, and that routing infrastructure is broadly distributed. I'd phrase their routing TCP load as "nontrivial", in the sense that if I had to build a system like this on physical hardware in, say, Portland, I'd start by buying five floors in the Pittock building. That's not to say Heroku's architecture is optimal, just that their problem is not as easy as it may appear. [edit] As for why companies choose to run in a heavily virtualized environment which has so many drawbacks... it comes down to a tradeoff in time, personnel, and costs. Running in EC2 allows for dramatically shorter lead times in hardware acquisition--and when your customers can increase their traffic by an order of magnitude without warning, I think that comes in handy. It also frees them from running their own power, network, physical security, hardware acquisition pipeline, associated vendor contracts, etc. I suspect both Heroku and RapGenius have weighed these costs carefully, but I don't really know their specific constraints.