[racket] web-server and comet-like requests?

The Web Server will kill connections after a relatively short timeout.
You'll get an exception when you finally try to use the connection.
This isn't very nice for COMET. You'll have to reset the timeout on
the connection manually.

Here's how:

- make a sequence dispatcher
- put a timeout dispatcher before the servlet dispatcher

Look at

web-server/dispatchers/dispatch-sequencer
web-server/dispatchers/dispatch-timeout
dispatch/servlet
serve/launch/wait

Jay

2011/2/21 Danny Yoo <dyoo at cs.wpi.edu>:
> I'm trying to do a comet-like push for a web-server application.  Say,
> for example, that I'm doing something like this:
>
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> #lang racket
>
> ;; Comet demonstration
>
> (require web-server/servlet web-server/servlet-env)
>
> (define ch (make-channel))
>
> (void
>  (thread (lambda ()
>          (define (start req)
>            (cond
>              ;; Server-side sync for a program
>              [(exists-binding? 'comet (request-bindings req))
>               (handle-comet req)]
>              [else
>               (handle-default req)]))
>          (serve/servlet start
>                         #:banner? #f
>                         #:launch-browser? #f
>                         #:quit? #f
>                         #:port 8080
>                         #:servlet-path "/compile"))))
>
> (define (handle-comet req)
>  (let ([v (sync ch)])
>    (response/full 200 #"Okay"
>                   (current-seconds)
>                   #"text/plain; charset=utf-8"
>                   empty
>                   (list #"" (string->bytes/utf-8 (format "~s" v))))))
>
>
> (define (handle-default req)
>  '(html (body (p "hello world"))))
>
> (define (send-to-client v)
>  (channel-put ch v))
> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
>
> Here's the normal order of things:
>
>    1.  I run the above in DrRacket, which starts up the web server.
>
>    2.  I switch to my web browser and visit the page:
> http://localhost:8080/compile?comet=t, where the browser waits to get
> a response.
>
>    3.  I switch back to DrRacket, and call (send-to-client "hello world").
>
>    4.  Finally, I go back to the web browser, which should
> triumphantly show the words "hello world" on screen.
>
> In the normal situation, this seems to work.  The problem, of course,
> is what happens when things aren't perfect.  I don't know the
> semantics of what web-server will do if the connection drops in the
> middle of a sync, as in handle-comet.  What happens?  Do I get a
> catchable exception?  This situation is common in Comet applications,
> because the connection eventually times out and needs to be
> re-established.
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-- 
Jay McCarthy <jay at cs.byu.edu>
Assistant Professor / Brigham Young University
http://faculty.cs.byu.edu/~jay

"The glory of God is Intelligence" - D&C 93