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message order

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svn: r13042
This commit is contained in:
Matthias Felleisen 2009-01-08 23:37:55 +00:00
parent 2821275df7
commit c58702123f
1 changed files with 14 additions and 7 deletions
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collects/teachpack/2htdp/scribblings/universe.scrbl
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@ -1110,13 +1110,20 @@ The first step in designing a @tech{universe} is to understand the
throughout a system. We know that the @tech{universe} doesn't exist until
the server starts and the @tech{world}s are joining. Because of the nature
of computers and networks, however, we may assume little else. Our network
connections ensure that if some @tech{world} sends two messages in some
order, they arrive in the same order at the server. In contrast, it is
generally impossible to ensure whether one world joins before another or
whether a message from one world gets to the server before another world's
message gets there. It is therefore the designer's task to establish a
protocol that enforces a certain order onto a universe and this activity
is called @emph{protocol design}.
connections ensure that if some @tech{world} or the @tech{server} sends
two messages to the @emph{same} place in some order, they arrive in the
same order (if they arrive at all). In contrast, if two distinct
@tech{world} programs send one message each, the network does not
guarantee the order of arrival at the server; similarly, if the
@tech{server} is asked to send some messages to several distinct
@tech{world} programs, they may arrive at those worlds in the order sent
or in the some other order. In the same vein, it is impossible to ensure
that one world joins before another. Worst, when someone removes the
connection (cable, wireless) between a computer that runs a @tech{world}
program and the rest of the network or if some network cable is cut,
messages don't go anywhere. Due to this vagaries, it is therefore the
designer's task to establish a protocol that enforces a certain order onto
a universe and this activity is called @emph{protocol design}.
From the perspective of the @tech{universe}, the design of a protocol is
about the design of data representations for tracking universe information