Building a Scalable Website

SecondTeacher is a website which will have very definate peaks in loads that are easy to predict. This is a great advantage in building the website but it also poses some challenges.
First we want to run the site efficently and on a budget that doesn't comprimise quality and user experience. We know the site will be busiest between 3pm and 9pm when the majority of students do their homework. In planning for peak load of say 20,000 concurrent users we can evaluate how much each machine can comfortably manage, imagine it's 1,000. We will then need 20 machines load balanced to operate the site. Correct ....?
Well no, we can do alot better than that and we also have a few concerns in relation to bandwidth usage. 20,000 people watching a 500kps video concurrently will need a hell of a lot of bandwidth if our 1,000 users per machine are all watching video then we would be pushing 500mps. Most hosters will only give you a 100mps burst capable pipe at a reasonable price so suddenly at best our machine can only handle 200 users meaning we now need 100 machines to cope....this is getting expensive.
Well our solution was up in the clouds.
First off we pushed all images and video to Amazon cloudfront which allows us to put all these objects on the edge locations close to the customer and also they are served from there meaning they do now clog up our bandwidth. This means we are now only in effect serving html and the silverlight controls now bandwidth is no longer a limiting factor. Now we're back to 1,000 per machine if anyone is keeping count.
Next we implemented our servers in the Amazon EC2 cloud meaning we could using there elastic computing model expand and contract our resources as we wanted to and only pay for the resources we use. This means that at 4pm on a school day we can run 20 machines but at 2am we only need run 1 machine to keep the service available for the desperate or slow students doing they're homework at this time. Our peak hours are only 30 hours per week the rest of the time 1 machine is enough. This reduces our resource costs to an average of just over 3 machines compared to the 20 we'd need to run if we went the traditional route.
But this is not all about cost, there are big advantages too in using the the Amazon cloud 1) Amazon engineers working 24/7 to keep the cloud up 2) Multiple Redundant storage and backup built in 3) No more concerns about hardware issues, if a machine goes down just start a new one. 4) Fully scalable e.g. if all our Christmases come at once and 100,000 try to access SecondTeacher in the morning instead of a technical disaster I can just fire up another 100 instances and the user experience is the same as ever.
I'm banking on the cloud to become the way of the future and I'm not alone.
Check out Amazon webservices for yourself Amazon Web Sevices