72 lines
1.9 KiB
Racket
72 lines
1.9 KiB
Racket
#lang scribble/doc
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@(require "web-server.ss"
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(for-label web-server/lang/soft
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web-server/lang/web))
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@title[]{Soft State}
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@defmodule[web-server/lang/soft]{
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Sometimes you want to reference a large data-structure from a stateless program without the data-structure being serialized
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and increasing the size of the serialization. This module provides support for this scenario.
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@defproc[(soft-state? [v any/c])
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boolean?]{
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Determines if @scheme[v] is a soft state record.
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}
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@defproc[(make-soft-state [thnk (-> any/c)])
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soft-state?]{
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Creates a piece of soft state that is computed by @scheme[thnk]. This value is serializable.
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}
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@defproc[(soft-state-ref [ss soft-state?])
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any/c]{
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Extracts the value associated with @scheme[ss]. If the value is not available (perhaps because of garbage collection, deserialization in an uninitialized process, etc), then the thunk associated with @scheme[ss] is invoked and the value is cached.
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}
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@defform[(soft-state expr ...)]{
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Equivalent to @scheme[(make-soft-state (lambda () expr ...))].
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}
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Here's an example servlet that uses soft state:
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@schememod[
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web-server
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(provide interface-version start)
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(define interface-version 'stateless)
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(define softie
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(soft-state
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(printf "Doing a long computation...~n")
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(sleep 1)))
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(define (start req)
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(soft-state-ref softie)
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(printf "Done~n")
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(start
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(send/suspend
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(lambda (k-url)
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`(html (body (a ([href ,k-url]) "Done")))))))
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]
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When this is run and the link is clicked a few times, the output is:
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@verbatim{
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$ plt-web-server -p 8080
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Doing a long computation...
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Done
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Done
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Done
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Done
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}
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If the server is restarted or the hostname in the URL is changed to a different host with the same code, and the URL is clicked:
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@verbatim{
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^Cuser break
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$ plt-web-server -p 8080
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Doing a long computation...
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Done
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}
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}
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