Cloud versus Client-Server

In many ways, the basic style of development for cloud-based software is similar to programming for client-server computing. Both are based on the idea that you don’t really run programs on your own computer. Your computer provides a window into an application, but it doesn’t run the application itself. Instead of running the program on your computer, all you do on your own computer is run some kind of user interface. The real program is running somewhere else on a computer called a server. You use the server because, for whatever reason, the resources necessary to run the program aren’t available on your local computer: it’s cheaper, faster, or more convenient to run the program somewhere else where the necessary resources are easy to obtain.

The big difference between cloud and client-server development is in what you know: in traditional client-server systems, you might have a specific computer that is your server, and that’s where your stuff is running. The computer may not be sitting on your desk in front of you, but you know where it is. For example, when I was in college, one of the first big computers I used was a VAX 11/780 nicknamed “Gold.” Gold lived in the Rutgers university computing lab in Hill Center. I used Gold pretty much daily for at least a year before I actually got to see it. The data center had at least thirty other computers: several DEC 20s, a couple of Pyramids, an S/390, and a bunch of Suns. But of those machines, I specifically used Gold. Every program that I wrote, I wrote specifically to run on Gold, and that’s the only place that I could run it.

In the cloud, you aren’t confined to a specific server. You have computing resources—that is, someone is renting you a certain amount of computation on some collection of computers somewhere. You don’t know where they are; you don’t know what kind of computers they are. You could have two massive machines with 32 processors each and 64 gigabytes of memory; or they could be 64 dinky little single-processor machines with 2 gigabytes of memory. The computers where you run your program could have great big disks of their own, or they could be diskless machines accessing storage on dedicated storage servers. To you, as a user of the cloud, that doesn’t matter. You’ ve got the resources you pay for, and where they are makes no difference as long as you get what you need.

Source of Information on:  Pragmatic - Code in the Cloud Programming Google App Engine 2011