69 lines
3.7 KiB
Racket
69 lines
3.7 KiB
Racket
#lang scribble/doc
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@(require "web-server.ss")
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@title[#:tag "considerations"]{Usage Considerations}
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A servlet has the following process performed on it automatically:
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@itemize[
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@item{All uses of @scheme[letrec] are removed and replaced with equivalent uses of
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@scheme[let] and imperative features. (@filepath{lang/elim-letrec.ss})}
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@item{The program is converted into ANF (Administrative Normal Form),
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making all continuations explicit. (@filepath{lang/anormal.ss})}
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@item{All continuations (and other continuations marks) are recorded in the
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continuation marks of the expression
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they are the continuation of. (@filepath{lang/elim-callcc.ss})}
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@item{All calls to external modules are identified and marked.
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(@filepath{lang/elim-callcc.ss})}
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@item{All uses of @scheme[call/cc] are removed and replaced with
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equivalent gathering of the continuations through the continuation-marks.
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(@filepath{lang/elim-callcc.ss})}
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@item{The program is defunctionalized with a serializable data-structure for each
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anonymous lambda. (@filepath{lang/defun.ss})}
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]
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This process allows the continuations captured by your servlet to be serialized.
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This means they may be stored on the client's browser or the server's disk.
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Thus, your servlet has no cost to the server other than execution. This is
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very attractive if you've used Scheme servlets and had memory problems.
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This process IS defined on all of PLT Scheme and occurs AFTER macro-expansion,
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so you are free to use all interesting features of PLT Scheme. However, there
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are some considerations you must make.
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First, this process drastically changes the structure of your program. It
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will create an immense number of lambdas and structures your program
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did not normally contain. The performance implication of this has not been
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studied with PLT Scheme. However, it is theoretically a benefit. The main
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implications would be due to optimizations MzScheme attempts to perform
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that will no longer apply. Ideally, your program should be optimized first.
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Second, the defunctionalization process is sensitive to the syntactic structure
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of your program. Therefore, if you change your program in a trivial way, for example,
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changing a constant, then all serialized continuations will be obsolete and will
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error when deserialization is attempted. This is a feature, not a bug!
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Third, the values in the lexical scope of your continuations must be serializable
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for the continuations itself to be serializable. This means that you must use
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@scheme[define-serializable-struct] rather than @scheme[define-struct], and take
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care to use modules that do the same. Similarly, you may not use @scheme[parameterize],
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because parameterizations are not serializable.
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Fourth, and related, this process only runs on your code, not on the code you
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@scheme[require]. Thus, your continuations---to be capturable---must not
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be in the context of another module. For example, the following will not work:
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@schemeblock[
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(define requests
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(map (lambda (rg) (send/suspend/url rg))
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response-generators))
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]
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because @scheme[map] is not transformed by the process. However, if you defined
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your own @scheme[map] function, there would be no problem.
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Fifth, the store is NOT serialized. If you rely on the store you will
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be taking huge risks. You will be assuming that the serialized continuation
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is invoked before the server is restarted or the memory is garbage collected.
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This process is derived from the paper
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@href-link["http://www.cs.brown.edu/~sk/Publications/Papers/Published/pcmkf-cont-from-gen-stack-insp/" "Continuations from Generalized Stack Inspection"].
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We thank Greg Pettyjohn for his initial implementation of this algorithm.
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