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From Jeremy Boynes <jboy...@gluecode.com>
Subject Re: multiple systems in same VM?
Date Thu, 07 Oct 2004 02:15:39 GMT
Daniel John Debrunner wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> Khin, Gerald wrote:
> 
>>Is it planned to support multiple systems in the same VM? Think of two
>>or more java libraries from different companies each using derby as
>>embedded database. I guess that they will hamper each other.
> 
> 
> Technically derby supports a single system, which can comprise of
> multiple databases, per class loader in a single JVM. Typically the
> Derby libraries are on the class path and thus are loaded by the system
> class loader leading to a single system per JVM. Database instances can
> only be loaded by one Derby system.
> 
> Derby also supports (or will support once the first release with new
> features is available) "soft-upgrade", allowing an application to use a
> newer version of the engine without upgrade. Thus if two Derby
> applications are running in a single JVM, one intended to run against
> 10.0 and one against 10.5, then the JVM can use the single 10.5 Derby
> engine for both databases and applications.
> 
> Since Derby is just a jar file in embedded mode, it is like two
> applications trying to use two different versions of a jar.
> 

For us in Geronimo land, that is not an uncommon situation. For example, 
we could be hosting two different applications that need two different 
versions of Derby (e.g. 10.0 and 11.0 with a requirement for different 
physical disk layouts). We do all the classloader management to allow this.

I posted earlier about providing an alternative to using system 
properties as a step in this direction. Any thoughts or comments would 
be appreciated.

--
Jeremy


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