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From "Dag H. Wanvik" <Dag.Wan...@Sun.COM>
Subject Re: Multiple engines in same JVM
Date Wed, 20 Jul 2005 21:14:29 GMT

please see comment inlined.

>>>>> "DVC" == David Van Couvering <David.Vancouvering@Sun.COM> wrote:
DVC> Jeremy Boynes wrote:
DVC> > I would like to explore the possibility of running multiple Derby 
DVC> > instances in the same JVM, probably in different classloaders.
DVC> You use the term "Derby instance" but I'm not sure what you mean here.
DVC> Based on a discussion I had with Dan, a Derby "system" is the union of 
DVC> all databases running under the same classloader.  Is that what you 
DVC> mean?  Each system is supposed to have its own configuration.  But I had 
DVC> trouble figuring out how to do this with multiple classloaders (see your 
DVC> system properties discussion below), and nowhere do I see this tested 
DVC> currently.
DVC> I thought Dag, with Kathey's help, was able to configure things such 
DVC> that you could have different Derby properties set for different 
DVC> databases in the same VM.  Dag, how did that work again?

Actually, no. What I did was for my test was to unload Derby (system)
and then re-load it a number of times with different system property
values set (see test lang/errorStream.java).


DVC> > 
DVC> > One use for that is an application server configuration where different 
DVC> > applications may require different versions of Derby or where different 
DVC> > instances may have different requirements e.g. for security.
DVC> > 
DVC> > Two big issues come to mind:
DVC> > 1) use of system properties
DVC> >    I would like to explore ways in which these can be replaced with a
DVC> >    per-instance configuration mechanism where each instance can have
DVC> >    separate properties. This could be as simple as a per-classloader
DVC> >    static property map but ideally something a little more flexible
DVC> >    would be useful
DVC> Well, I would love us moving to JMX.  It would solve a lot of problems 
DVC> around configuration for us, and it would also enable management by nice 
DVC> pretty GUIs, integration into larger systems managed by JMX (e.g. most 
DVC> app servers), etc.
DVC> I recognize this a change with system-wide impact which can not be taken 
DVC> upon lightly.  We'd also have to do it in a way that is pluggable, JMX 
DVC> is not something I expect is part of J2ME...
DVC> > 2) common touch point in DriverManager, especially for use within
DVC> >    stored procedures.
DVC> >    I think we can do something here with a special Driver implementation
DVC> >    that could handle multiple engine instances which I think makes this
DVC> >    related to David Van Couvering's common jar discussion.
DVC> I don't understand this point, could you explain further?
DVC> That said, I do agree if what you are discussing involves multiple 
DVC> classloaders, then I think there is some crossover here and we should 
DVC> talk to make sure I don't design something that will require rework 
DVC> later on to support what you are envisioning here.
DVC> David
DVC> > 
DVC> > Thoughts?
DVC> > -- 
DVC> > Jeremy
Dag H. Wanvik
Sun Microsystems, Web Services, Database Technology Group
Haakon VII gt. 7b, N-7485 Trondheim, Norway
Tel: x43496/+47 73842196, Fax:  +47 73842101

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