Postel's law

Synonyms: robustness, reliability principle, trustworthiness principle

Be liberal in what you accept, be conservative in what you send. What comes into a system can be messy, non-compliant, unconventional. But the output needs to be clear and understandable (guidelines, principles, design patterns).


Illustration by Vago


«KISS principle»


Postel’s Law isn’t perfect.That fundamental flaw is on the argument of flexibility. In engineering, reducing their tolerance for variability streamlined work processes and made work a lot more efficient. 


Any implementation is required to replicate the aberrant behavior, or it is not interoperable. This is both a consequence of applying the robustness principle, and a product of a natural reluctance to avoid fatal error conditions. Ensuring interoperability in this environment is often referred to as aiming to be "bug for bug compatible":

  1. Defective implementation
  2. Bug compatible
  3. Dangerous
  4. Hazardous
  5. Unsafe
  6. Risky


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