David W. Van Couvering wrote:
Jeremy Boynes wrote:
[snip]
Daniel John Debrunner wrote:
The issue is that today this is fully supported because the client and
engine do not share code.
Some of the code sharing approaches regress Derby in this area by not
supporting this, or require class path ordering for it to be supported.
Some of the others support this by defining compatibility contracts
and eliminate the need for classpath ordering by not duplicating classes.
Can't you have the situation where common 10.2 and common 10.3 are both
included in the classpath (by accident, as Dan brings up)? Wouldn't you
end up with order dependencies then?
To what extent do we need to cater for accidents? But, yes if you do end
up with both then the (actual) order would determine which one you ended
up with. In many cases the user would not notice due to the API
compatibility between 10.2 and 10.3 (with 10.3 being a pure superset by
the rules defined). The key thing is that there is no overlap in
functionality between jars (so you are not getting 10.2 (containing
client + common) merged with 10.3 (containing engine + common)).
[snip]
Let's recharacterize this a little. What we are contemplating with
code sharing is extracting common functionality out into a library. By
saying that we are not willing to accept any solution where a
component depends on a library we are shutting ourselves off from
using any external library or any functionality not provided by Derby
itself. This dooms us forever to reinvent any functionality that could
be provided by other projects.
For example, there are libraries out there that support bytecode
generation, JMX for management, high-performance concurrency on Java
1.4, regexp processing to support SQL patterns, ... By saying we are
not prepared to incorporate them but instead need our own versions
that can be morphed for client and server we dramatically reduce the
functionality that can be made available to users.
So let me ask this: do our users want more functionality faster by
allowing the use of libraries, or a completely standalone solution
with tight control over the entire implementation?
You make a compelling argument here. I already would like to use stuff
in Jakarta Commons (I haven't brought it up yet, one thing at a time).
It seems a good Apache Java citizen should make use of what's in Jakarta
Commons rather than build stuff themselves. And I think Jeremy's right
that we will run into this same configuration situation with these guys.
Jeremy, how *do* the users of commons avoid accidentally using a version
they are not compatible with (e.g. a consumer depends on new features
that aren't available in an older version of the common jar file)?
This can be a problem - Java's variant of DLL hell. The solution
generally relies on two factors:
1) upward compatibility between versions - because they are small-ish
components then they tend to do one thing and not suffer from API
scope creep. You just use the latest version and add it to the
classpath once
2) multi-classloader loading mechanisms, which are commonplace in
open source projects, even command line utilities like Ant and Maven
not just the application servers
It is kind of self-fulfilling - when you adopt a modular structure then
you expect to be loading modules with libraries at different version
levels and so use a classloading architecture that fits.
There is also a cultural thing here. For example, suppose I am writing a
database because I happen to think writing database stuff is fun (yes, I
do realize that doing this for fun makes one appear somewhat odd). I
come up with the idea that converting the query plan to executable code
is a neat idea and so need to generate that code. I can implement a code
generator myself, or look for one out there that is already written and
is available under a suitable license. Given my main interest is in
databases, using someone else's code to generate byte code save me a lot
of effort (especially when the class format changes). If it doesn't do
what I want, I could hack my own but it is generally easier to donate
those changes back so that someone else can maintain them. After all, my
interest is in databases not in byte code generation.
Basically, a volunteer community scales better by incorporating each
other's code rather than trying to do it all.
--
Jeremy
