Design problems (was Re: Why don't large companies use Ada?)

[paj@gec-mrc.co.uk (Paul Johnson)]

Robert I. Eachus (eachus@spectre.mitre.org) wrote:
> In article <5994@gec-mrc.co.uk> paj@gec-mrc.co.uk (Paul Johnson) writes:

>  > We were also using a library versioning system, and of course this
>  > basic stuff was in a library.  So when you changed it, everything
>  > else had to be recompiled.  What with all the other book-keeping
>  > associated with making a change, it held up the project for a full
>  > day.

>    This is Ada trying to tell you something, learn to listen.

Yeah.  I know.  But I was working as contract labour on that project
so my opinions counted for zilch.

This wasn't just the dimension libraries.  All the data structure
libraries suffered from this.  The analysis document contained Data
Flow Diagrams and the bubbles communicated through types defined in a
Data Dictionary.  So all the data types were implemented in low-level
library packages for access by the procedures that implemented the
bubbles.  Every time a data type needed to be added or modified, a
low-level package had to be checked out, and this triggered a
recompile of everything above.  C and C++ suffer from similar problems
in "include" files.

If they had taken an "object based" approach to low level design
(packaging up data structures with the relevant operations) then
things would have been different.  But structured analysis doesn't
group things like that, so they didn't either.

To be fair, the management knew this was a problem as well, but they
were well into the lifecycle by then (this was the Version 2 release),
so the design was pretty much cast in concrete.

We were also working with two compilers (development platform and
target platform), one of which had a bug that caused it to reject the
dimension libraries.  So one version was coded with subtypes and one
with ranges.  Most of the time we managed to produce code that worked
with both, but not always.  So then some arbitrary module halfway up
the hierarchy had to have a version forked off and separately
maintained.  We had a paper-based change-control system running, and
we certainly needed it.

This was the first Ada project by this team.  They commented that
next time they used Ada they would know how to do it right.  But by
then something else would be flavour-of-the-year, and they would be
back at the bottom of the learning curve again.

I will say that, despite the problems, we produced code on-time and
on-budget.  It was expensive code, but we knew ahead of time how much
it would cost.  Code was reviewed before being checked back into the
development version, so the bug rate was fairly low.  It was just that
some bugs were very expensive.

Paul.

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