2.14 Limiting Requests
Returns a dispatcher that defers to inner for work, but will forward a maximum of limit requests concurrently.
If there are no additional spaces inside the limit and a new request is received, the over-limit option determines what is done.
The default ('block) causes the new request to block until an old request is finished being handled.
If over-limit is 'kill-new, then the new request handler is killed—a form of load-shedding.
If over-limit is 'kill-old, then the oldest request handler is killed—prioritizing new connections over old.
(This setting is a little dangerous because requests might never finish if there is constant load.)
Consider this example: