103 lines
3.4 KiB
Plaintext
103 lines
3.4 KiB
Plaintext
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#lang scribble/manual
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@require[racket/include]
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@include{prelude.inc}
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@title{Background}
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@section{Network programming in the small}
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Even the simplest of programs can be seen as a network. This program
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sends a message containing @racket[3] and @racket[4] to @racket[+],
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and awaits a reply message, which contains @racket[7]:
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@interaction[(+ 3 4)]
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Subroutines are network services. Here, when the message @racket[3] is
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sent to @racket[add-four-to], it delegates to @racket[+]:
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@interaction[(define (add-four-to n) (+ n 4))
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(add-four-to 3)]
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@note{In π-calculus encodings of programs, replication and state are
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made explicit.} Immutable programs (of which stateless programs are a
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special case) are easily able to be @emph{replicated}; programs using
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mutable state, however, must be fixed in place. In fact, strictly
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speaking, it is the @emph{state} that must be fixed in place, not the
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program @emph{manipulating} the state. In the following program, the
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mutable @racket[total] variable must not be replicated:
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@interaction[(define accumulate
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(let ((total 0))
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(lambda (n)
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(set! total (+ total n))
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total)))
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(accumulate 3)
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(accumulate 4)
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(accumulate 0)]
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It would be a mistake to simply replace each occurrence of
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@racket[accumulate] with its definition, since three separate
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@racket[total] locations would be created. However, there is no
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problem replicating the @racket[lambda] term, so long as
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@racket[total] always refers to the same location:
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@interaction[(define total 0)
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((lambda (n) (set! total (+ total n)) total) 3)
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((lambda (n) (set! total (+ total n)) total) 4)
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((lambda (n) (set! total (+ total n)) total) 0)]
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Programs raise exceptions to signal partial failure:
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@interaction[(define accumulate
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(let ((total 0))
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(lambda (n)
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(when (negative? n)
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(error 'accumulate "n must be non-negative!"))
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(set! total (+ total n))
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total)))
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(accumulate 3)
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(accumulate -2)]
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Programs can handle exceptions to @emph{contain} partial failure:
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@interaction[(with-handlers ([exn:fail? (lambda (e) 'ok)])
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(error 'oh-dear))]
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Partial failures interact with shared, mutable state in unpredictable
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ways, especially in larger programs that combine stateful subprograms.
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Programmers often ignore issues of security and trust between
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subprograms within a larger program. Most programming languages
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entirely lack any means of securely isolating subprograms from each
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other, leading to predictable failures. Even memory-safe languages
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such as Racket, Java and .NET only offer weak techniques for securely
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composing mutually suspicious subprograms. Techniques such as avoiding
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@emph{ambient authority} and using @emph{object capabilities} to
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compose subprograms
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TODO ^
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@section{Network programming in the large}
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Programs which engage in I/O are very obviously part of a network:
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@racketblock[(define username (read-line))
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(printf "Hello, ~a!\n" username)]
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But look closely! There is a difference here between the kind of communication
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This program is more obviously composed of a number of moving pieces:
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@interaction[(let ((b (box 3)))
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(set-box! b (+ (unbox b) 4))
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(unbox b))]
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@interaction[(define (add-three-to b)
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(set-box! b (+ (unbox b) 3)))
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(define (add-four-to b)
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(set-box! b (+ (unbox b) 4)))
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(define my-box (box 0))
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(add-three-to my-box)
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(add-four-to my-box)
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(unbox my-box)]
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