Programming language will not matter much - it's more a matter or server configuration. You will need a seperate data server (any flavor of SQL will do) so that it can be accessed by as many web servers as you need. You will need a hardware or software solution to route web requests to available servers (Network Load Balancing), Data replication between your primary and backup data servers, and of course redundant data, power, air conditioning, etc.

Check out http://www.west-wind.com/presentations/loadbalancing/loadbalancing.htm and http://en.wikipedia.org/wiki/Load_balancing_(computing)

Michael

>Hi,
>
>I didn't know where to place this question.... it's a doubt I have more than a question.
>
>If you plan to design or develop a website, or a social network, or any kind of web application that must deal with several concurrent users (let's say millons of concurrent users, like Facebook) , must be up all of the time , must deal with lot of email exchange and manage sms traffic maybe.
>
>How and where do you start from?
>There are a lot of issues coming to my mind... from choosing the programming languages (php, javascript, html, java, etc ) , choosing the best DBMS to handle that kind of traffic, designing server clusters, hosting the application, etc etc...
>
>What would be the best approach and what advises should be on mind to face this kind of project?
>
>Thanks