Bound Repeat Work

A request that arrives twice should not do its work twice. The cheapest guard is a key that names the work, so the second arrival is recognised as the same job.

Name the work with a key

Derive the key from the parts that make the request unique, not from the wall clock.

def key(req):
    return f"{req.user}:{req.action}:{req.target}"

Keep the first result

Store the result under the key and return it for later arrivals. Release the key when the work is genuinely done.

A missing key costs a duplicate side effect that nobody notices until it compounds.