Perry,

> There must be others out there who have multi-database
> applications without multiple sets of source.

I guess I fall in this category; two alternatives come to mind: a) use
an OPF like tiOPF (open source) or InstantObjects (commercial) and start
developing applications that are *truly* storage-independent (and, as a
bonus, truly object-oriented), or b) wrap the different technologies in
a single set of components yourself.
I have done experimentations with both things and I must say that the
results are encouraging. If you find a storage layer that does not work,
you ditch it and recode another (I have developed an IBO layer for
Instant Objects in a couple hours, for example). The OPF approach,
inevitably, incurs some performance penalties. The other approach can be
made as interface-compatible with the BDE as one needs, and performance
is good. My solution, in particular, is based on the ClientDataSet, and
I have "drivers" for the BDE, dbX, dbGo... It's not as OO as an OPF, but
performance is very close to the underlying data access components.
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_/\/ando