Scalability of servers

Hi Experts,
What makes a webapp scalable?  Can someone give me a basic algorithm of a scalable server?  (i.e., have the ability to throw more hardware at a problem to spread out the load).

Thanks!
Mike
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wls3Commented:
Scalability is a potentially complex issue.  A web application is typically scalable in two ways: 1) scaling out and 2) scaling up.  Scaling out means adding more machines to host an application.  In IIS this would be going from a web application (1 application on 1 CPU) to a web farm (1 application on multiple PC's/servers).  Scaling up would meaning adding more cores to a single machine.  Typically this would be going from a web application (1 application on 1 CPU) to a web garden (1 application on 2 or more CPU's).   That is one dimension of scalability.  Another dimension would be modifications to the NIC or general network in which your server resides.  To increase bandwidth would increase to the total amount of traffic that could access your site in a given period of time.  This will have limitations as there are application/web server limitations as well.  One of the keys to building truly scalable projects is the capacity of an application to grow from a small application to one with more complex resources smoothly and easily.  In situations like this, design will also play a key role.  For example, building a web site with the intention of increasing over time to a high-volume site will allow for the migration plan from low to medium to high volume to have clear milestones and a plan to accomplish those intermediate steps.  For instance, if you know that your site will start with basic database access, but, over time will grow to include additional interfaces (web forms and web services) then start by coding your application with serialization from the outset, thoroughly normalized databases and a stable, extendable domain model.  In short, have a clear plan and know, generally, what you are going to do along the way.  Scalability is a whole world unto itself, and, could certainly suck up a lot of time.  One of the better books I looked at when I was getting into the idea of such things was Richard Kiessigs Ultra-Fast ASP.NET: Build Ultra-Fast and Ultra-Scalable web sites using ASP.NET and SQL Server.  As an additional reference, check out: http://highscalability.com/.
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