I would like to add a few comments to this very useful discussion:
(1) The question was raised: why do we want to share code in the first place? For me, the big issue is the brittleness of code cloning. Over time, cloned code diverges--I don't know of any bullet-proof process for preventing this. Over time, the assumptions in the cloned code will diverge. If you are lucky, this will surface as exceptions. If you're not lucky, this will surface silently in very tricky bugs. This issue just gets bigger as the Derby developer community grows and is particularly important for our under-tested network layer.
(2) A possible solution was raised: instead of cloning the code in the source tree, clone it during the build process. I have written similar kinds of ant-based code pre-processors and can give some advice if we adopt this approach. Done right, it's a pleasant afternoon's work. However, I have reservations about this approach. It seems to me that it subverts the meaning of "common" code. To me "common" means "behaves the same way." It's not just a matter of preserving APIs that the compiler can check. It comes down to actually doing the same thing internally. As an example, consider a pair of encoding/decoding methods. You want the internal encoding/decoding logic to be the same everywhere. I don't think that build-time cloning saves us from the problem of having to do a run-time compatibility check.
(3) Concerning the packaging of common code: I'm a big fan of duplicating the classes in the client and server jars. I would like to keep the "out of box" experience simple for users who are new to Derby or even new to Java. Our "0 administration" story is arguably our most compelling differentiator.
(4) Concerning the specifics of David's proposal: I think that checkCommonCompatibility() would be easier to use if the second argument were firstCompatibleVersion rather than firstIncompatibleVersion. It seemed to me that the secondIncompatibleVersion number should be less than the firstIncompatibleVersionNumber and I always get muddled when 2 < 1.
Cheers, -Rick
