store: redis, in-progress ttl: 2 s, handler takes 3.2 s
request A (t=0 s)                  -> HTTP 201  replayed=-    retry-after=-    {"id":1,"customer":"cus_42","amount":1999,"currency":"INR"}
request B (t=2.5 s, same key)      -> HTTP 201  replayed=-    retry-after=-    {"id":2,"customer":"cus_42","amount":1999,"currency":"INR"}
payments in the table: 2
requests that finished after their lock expired: 2
what the store holds afterwards: nothing (EXISTS idem:k-8 = 0)