Jeff Dean, Head of Google's School of Infrastructure Wizardry—instrumental in many of Google's key systems: ad serving, BigTable; search, MapReduce, ProtocolBuffers—advocates evaluating different designs using back-of-the-envelope calculations. He gives the full story in this Stanford video presentation.

Back-of-the-envelope calculations are estimates you create using a combination of thought experiments and common performance numbers to a get a good feel for which designs will meet your requirements. Dr. Dean thinks an important skill for every software engineer is the ability to estimate the performance of alternative systems, using back of the envelope calculations, without having to build them.

He gives a great example of the process in the video, but first...