I am building a web-application and have a couple of quick questions. From what I learnt, one should not worry about scalability when initially building the app and should only start worrying when the traffic increases. However, this being my first web-application, I am not quite sure if I should take an approach where I design things in an ad-hoc manner and later "fix" them. I have been reading stories about how people start off with an app that gets millions of users in a week or two. Not that I will face the same situation but I can't help but wonder, how do these people do it?

Currently, I bought a shared hosting account on Lunarpages and that got me started in building and testing the application. However, I am interested in learning how to build the same application in a scalable-manner using the cloud, for instance, Amazon's EC2. From my understanding, I can see a couple of components:

• There is a load balancer that first receives requests and then decides where to route each request
• This request is then handled by a server replica that then processes the request and updates (if required) the database and sends back the response to the client
• If a similar request comes in, then a caching mechanism like memcached kicks into picture and returns objects from the cache
• A blackbox that handles database replication

Specifically, I am trying to do the following:

• Setting up a load balancer (my homework revealed that HAProxy is one such load balancer)
• Setting up replication so that databases can be synchronized
• Using memcached
• Configuring Apache to work with multiple web servers
• Partitioning application to use Amazon EC2 and Amazon S3 (my application is something that will need great deal of storage)
• Finally, how can I avoid burning myself when using Amazon services? Because this is just a learning phase, I can probably do with 2-3 servers with a simple load balancer and replication but until I want to avoid paying loads of money accidentally.

I am able to find resources on individual topics but am unable to find something that starts off from the big picture. Can someone please help me get started?