Code AnalysisHard 60s

Idempotency key stored after the effect

The effect happens before the key is recorded. What is the race?

Starter code

def charge(req):
    if exists_key(req.idem): return cached_result(req.idem)
    result = gateway.charge(req)   # side effect
    store_key(req.idem, result)    # recorded AFTER

system-design idempotency concurrency

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