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2.14 Limiting Requests

The web-server/dispatchers/limit module provides a wrapper dispatcher that limits how many requests are serviced at once.


(make limit inner [#:over-limit over-limit])  dispatcher/c

  limit : number?
  inner : dispatcher/c
  over-limit : (symbols 'block 'kill-new 'kill-old) = 'block
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:
#lang racket
(require web-server/web-server
         (prefix-in limit: web-server/dispatchers/limit)
         (prefix-in filter: web-server/dispatchers/dispatch-filter)
         (prefix-in sequencer: web-server/dispatchers/dispatch-sequencer))
(serve #:dispatch
          (lambda (conn req)
             200 #"Okay"
             (current-seconds) TEXT/HTML-MIME-TYPE
             (list (string->bytes/utf-8
                    (format "hello world ~a"
                           (sort (build-list 100000 (λ x (random 1000)))
            (request-method req)))
         #:over-limit 'block))
       (lambda (conn req)
          (response/full 200 #"Okay"
                         (current-seconds) TEXT/HTML-MIME-TYPE
                         (list #"<html><body>Unlimited</body></html>"))
          (request-method req))))
      #:port 8080)