Who owes whom?

A few months after having created a little tool to manage shared expenses, one of my roommates who just doesn't like computers, asked me if she could use the tool to settle accounts after holidays with some friends. I was delighted that she understood that the tool could be extended to that usage and thought that if she preferred to use such a tool rather than a piece of paper, others might also do. Finally, I have an opportunity to make something with my skills that is at least useful, even if it's unsignificant and absolutely not exciting. Geek section The first task was to choose the technology I will use to build who owes whom. I was sure to use GWT but didn't know for the server part. At that time, cloud computing and scalability were in the air. And that was also the time Google AppEngine appeared. I chose that technology rather than traditional JEE and Amazon cloud for one main reason : it is free. Plus it is Google and I like and Google ; plus it is based o