Software Architect's Handbook

Applying Postel's Law to APIs

A helpful design guideline with regards to message contracts is the Robustness Principle, which is also known as Postel's Law. The idea behind Postel's Law is that you should be conservative in what you do and liberal in what you accept from others. This principle was originally proposed when designing the TCP protocol, but it is applicable to message contracts. We should be conservative in what we send but liberal in what we accept. In other words, the data that is sent out of our system should be kept to the minimum that is necessary. A message sender must conform to the message contract, and reducing the amount of data that an API exposes lessens the chances that a breaking change may be necessary for the ...