Comparison
Hyrum's lawvsUpstream drift
Hyrum's law
you changed the order of a JSON response and a customer's integration broke, and nowhere did you promise an order.
With enough users, every observable behaviour of your system will be depended on by somebody, regardless of what you documented. Response ordering, timing, error message wording, the number of decimal places — all of it becomes contract by use. It is the reason internal-only rarely stays internal-only and the reason a deprecation plan is a negotiation about actual usage rather than about what the specification said.
Full entry →Upstream drift
the source team changed a status code's meaning, the pipeline kept running, and your funnel report has been wrong since Tuesday.
The producing system changing behaviour — semantics, encoding, defaults, volume — without changing anything your pipeline validates. It is worse than a schema break precisely because nothing fails: the types still match and the numbers are merely wrong. The defences are semantic tests rather than structural ones, and a contract with the producer that says which changes require notice.
Full entry →