Re: Distributed system portability

["Marin David Condic" <marin.condic.auntie.spam@pacemicro.com>]

If you are using streams to communicate (perhaps via TCP/IP) between two
processes, you've got some questionable portability problems. The ARM
doesn't give you any guarantees about the representation of things in a
stream - or even that stream elements will be 8 bit bytes, for that matter.
So it is conceivable that what you compile on a PC and what you compile on a
Sun using the same company's compiler may not actually be able to
communicate via this route. You don't get any guarantees that the same
compiler on the same machine in two different releases is going to keep the
representations the same, so you can't even count on that.

That said, one can bring up the practical side of things. It is unlikely
that a compiler is going to change representations of various things
dramatically from one version (or target) to the next without some good
reasons. Its just a whole lot of work to have to change representations or
maintain different representations and who needs that? It also has a habit
of upsetting customers who didn't think much about the non-portability of
this sort of thing. So a practical answer is to determine if the
representations are compatible across the platforms of interest and, if so,
carefully code to that. Isolate the translation from internal to external
form down at a low level so that it is easy to fix. You can take precautions
by overriding the 'Read & 'Write (or 'Input and 'Output) operations so that
you get precise control, but this is possibly creating a bunch of work and
may not fix all your troubles anyway. (Remember, a stream element size is
not guaranteed either, so you've got to work around that as well.)

Now if you go about using the Distributed Systems Annex, that's a different
ball of wax and I'm afraid I've never made use of it to any serious extend &
can't help you there. I seem to recall rumors that it followed some
conventions for doing RPCs that might border on a "standard" and hence they
might be a better way of guaranteeing portability. It all depends on what
your system's requirements are. Can you use RPC's or do you have to blip raw
bytes back & forth?

Now if you have to interface to other systems that might be blipping data up
and down the hose in a clearly defined format, you have just kicked the
complexity up a notch. (*Bam*! :-) There is no easy way to do this except to
roll-your-own WRT getting data into a byte array. If its a fairly fixed set
of messages in the catalog, you at least have the help of representation
clauses on records and Unchecked_Conversion - and that's better than you get
with C, so you're ahead of the game there. If the catalog is large, then
you're hosed because there is no really good way of getting there
automagically from the 'Read, 'Write, 'Input & 'Output operations. You're
going to have to do a lot of work to guarantee representation of every
record type you want to send/receive.

MDC
--
Marin David Condic
Senior Software Engineer
Pace Micro Technology Americas    www.pacemicro.com
Enabling the digital revolution
e-Mail:    marin.condic@pacemicro.com
Web:      http://www.mcondic.com/


"Darren New" <dnew@san.rr.com> wrote in message
news:3B644557.C599A03C@san.rr.com...
> Is it generally the case that a distributed system compiled on two
> different architectures can talk to itself? In other words, if I write a
> distributed system with GNAT, and I compile one portion on Windows and
> another portion on Linux, are these two systems likely to talk to each
> other? If not, are the stream formats likely to be compatible? If not,
> is there a way to redefine the low-level stream operators for things
> like Integer to ensure that the same format is used at both ends? Or do
> I need to write translation routines to some architecture-independent
> format?
>
> I'm basically thinking of something where the clients would be
> distributed, talking between themselves peer-to-peer. Does Ada's
> portability mean I don't need to write a
> translate-to-and-from-network-representation layer, because I can use
> streams?
>
> --
> Darren New / Senior MTS & Free Radical / Invisible Worlds Inc.
>        San Diego, CA, USA (PST).  Cryptokeys on demand.
>           Only a WIMP puts wallpaper on his desktop.