Re: [moca] Locations?

[Peter Sewell <Peter.Sewell@xxxxxxxxxxxx>]

>m> I would be interested to hear opinions on what "is" the
>m> essence of locations.  my current and simple-minded take
>m> is that locations are units of failures, locations are
>m> partial computations that can fail independently.
>
>I agree with this view, depending on the setting, because we
>also use it (calling it "site") in this sense in our
>forthcoming CONCUR paper.
>
>But it has also been used as "unit of migration" (Dist Join).
>In Nomadic Pi and Dpi, locations have yet another twist,
>but I rather let the inventors comment on this. :-)


ok then :-) There are (at least) four different notions, none of which
necessarily coincide, that have been used in various combinations. One
can have:

- units of failure (machines or runtime system instances for process
failure, and/or units within which communication is guaranteed
reliable if the endpoints do not fail)

- units of migration (or, more generally, units of running computation
that can asynchronously be marshalled up)

- units of trust (both large administrative domains, and small
critical regions from which a particular secret should not escape. As
Sanjiva observes, in interesting cases the latter cut across other
structure.)

- units of synchronisation (global in the case of pi and djoin,
partitioned in many other calculi)

together with some finer variations, eg units of namespace control 
(the body of which can control the names of sub-locations, as in seal
and box-pi).  I wrote a very brief summary of some of the possibilities
in my `applied pi' tutorial, now out-of-date (Ch. 2 of
http://www.cl.cam.ac.uk/users/pes20/index.html#APPPI).  For each,
the possibilities and operational semantics are easy enough (though
operational _reasoning_ may not be, of course), but combining them
smoothly still seems to be a hard pragmatic question.

Peter



  
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