[plt-scheme] RE: Shared resources for servlets

Here's a quck draft (probably would need a bit of debugging, I have no DrScheme on this machine):

(module lock mzscheme
  (define read/write-object
    (class
      (define (read) 'todo)
      (define (write to-write) 'todo)))
 
  (define lockable-object
    (class
      (define reading 0)
      (define wrining? #f)
 
      (init-field obj)
 
      (define (open-read)
        (if writing?
            (begin (sleep) (open-read))
            (set! reading (+ reading 1))))
 
      (define (close-read)
        (set! reading (- reading 1)))
 
      (define (open-write)
        (if (or writing? (> reading 0))
            (begin (sleep) (open-write))
            (set! writing? #t)))

      (define (close-write)
        (set! writing? #f))
 
      (define (read)
        ; returns current value.
        ; Copies it first so it doesn't change when writing later on
        (open-read)
        (let ([r (send obj read)])
          (close-read)
          r)))
 
      (define (write to-write)
        (open-write)
        (send obj write to-write)
        (close-write)))))
 
Now all you have to do is fill in (read) and (write to-write) in read/write-object.
(define lo (instantiate lockable-object () [obj 'insert-read/write-object-here]))
(send lo read)
(send lo write 'value)
 
 
-----Original Message----- 
From: Keith Frost [mailto:keithf at amnis.com] 
Sent: ג 09/09/2003 22:40 
To: Dor Kleiman 
Cc: plt-scheme at list.cs.brown.edu 
Subject: Re: Shared resources for servlets



	Dor Kleiman wrote:
	
	> I'm not sure that would work.
	> Consider this:
	> 3 readers are reading. A writer comes and blocks. Then a reader comes
	> and joins in, blocking the writer even longer. This could block
	> indefinitely, which won't be very good, would it?
	
	Well, this is a simple model.  You point out that it has the problem
	that writers can be denied service indefinitely, if there is a sufficient
	volume of readers.  This is true.  One can devise a more complex
	model, such as one where (for example) a writer sends a request,
	and new readers are blocked until that request is satisfied.  But first,
	I just want to know how to implement the simple model!
	
	Keith
	
	
	

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