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Postel's Law (Robustness Principle)

"Be conservative in what you send, be liberal in what you accept" - this asymmetric tolerance enables interoperability between independently-evolving systems while maintaining forward compatibility.

Tension Addressed

Stability vs. Evolution

Recognizable As

Ignoring unknown fields in responses, accepting multiple date formats, permissive parsers with strict emitters, graceful degradation when optional fields missing

Canonical Sources

Postel, RFC 761 (1980); Fielding, REST dissertation (2000)

Transfers To

Protocol design, Parser design, Data migration, Backward compatibility strategies, Microservices integration

Postel's Law has critics (it can mask bugs, encourage sloppiness) but remains essential for evolving systems. The alternative - strict bilateral contracts - creates brittle coupling that breaks under change.