2.14 Limiting Requests

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▼ | 2 | Dispatchers

2.1 | General
2.2 | Mapping URLs to Paths
2.3 | Sequencing
2.4 | Timeouts
2.5 | Lifting Procedures
2.6 | Filtering Requests
2.7 | Procedure Invocation upon Request
2.8 | Logging
2.9 | Password Protection
2.10 | Virtual Hosts
2.11 | Serving Files
2.12 | Serving Servlets
2.13 | Statistics
2.14 | Limiting Requests

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#### 2.14`` Limiting Requests

( require web-server/dispatchers/limit)

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
web-server/http
web-server/http/response
( 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
( sequencer:make
( filter:make
#rx"/limited"
( limit:make
5
( lambda (conn req)
(output-response/method
conn
( make-response/full
200 #"Okay"
( current-seconds) TEXT/HTML-MIME-TYPE
empty
( 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)
(output-response/method
conn
( make-response/full 200 #"Okay"
( current-seconds) TEXT/HTML-MIME-TYPE
empty
( list #"<html><body>Unlimited</body></html>"))
( request-method req))))
#:port 8080)
( do-not-return)

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