XM Principle 4: Contract-First Design

The initial design decision of the WIKISPEED car was that it should consist of eight modules – body, chassis, motor, suspension, interior, etc. Before Team WIKISPEED even started to design individual components, they designed the interfaces between those modules.

Joe did not know what suspension would be used on his car, but he was able identify the external parameters and boundary conditions of the suspension. After researching the subject, he found that that if the suspension mounting could withstand 8 gees, it would more than meet all the necessary requirements, even for Formula One racing applications. So the team identified a suitably sized block of aluminum that could carry that load. Any suspension that could be attached to that block could be used on the WIKISPEED car without modification to the rest of the car.

So when designing a solution,

• Design the interfaces based on outside parameters, e.g. load factors, or communication and power requirements.
• Only architect the connections up front, not the individual components.
• Leave room to grow, i.e. over engineer these interfaces, because changing these fundamental contracts may be expensive.

P.S. Be sure to check out the Wrapper pattern to ensure independence between your design and the design of any component suppliers.

Next: Iterate the Design

The 10 Principles of Extreme Manufacturing