== free-key-123 allows 5 per minute; requests alternate between instance A and B
sent to A -> 200  remaining=4  (served by A)
sent to B -> 200  remaining=3  (served by B)
sent to A -> 200  remaining=2  (served by A)
sent to B -> 200  remaining=1  (served by B)
sent to A -> 200  remaining=0  (served by A)
sent to B -> 429  remaining=0  (served by -)
sent to A -> 429  remaining=0  (served by -)
allowed in total: 5 (not 10)
== one bucket in Redis, written by both instances
$ redis-cli --scan
rl:global
rl:key:c31ab6f47fba3609
== 200 concurrent requests for pro-key-456 (20 per minute), half to A and half to B
200 sent: 20 allowed, 180 rejected with 429
== instance B's clock is 30 s ahead of A's (user carol: 10 per minute, one token per 6 s)
A, bucket empty -> 429
B sees 30 s of refill that never happened on A's clock: 5 of 8 allowed