David Van Couvering wrote:
Jeremy Boynes wrote:
I would like to explore the possibility of running multiple Derby
instances in the same JVM, probably in different classloaders.
You use the term "Derby instance" but I'm not sure what you mean here.
I believe instance == system, my bad.
Based on a discussion I had with Dan, a Derby "system" is the union of
all databases running under the same classloader. Is that what you
mean? Each system is supposed to have its own configuration. But I had
trouble figuring out how to do this with multiple classloaders (see your
system properties discussion below), and nowhere do I see this tested
currently.
AIUI there are two levels of configuration: system and database
* system configuration comes from system properties and
classloader resources
* database configuration overrides based on values from derby.properties
in the database directory
To locate the database directory (to get the derby.properties) you
either need an absolute path for the database name, or the value of the
derby.system.base system property
I thought Dag, with Kathey's help, was able to configure things such
that you could have different Derby properties set for different
databases in the same VM. Dag, how did that work again?
One use for that is an application server configuration where
different applications may require different versions of Derby or
where different instances may have different requirements e.g. for
security.
Two big issues come to mind:
1) use of system properties
I would like to explore ways in which these can be replaced with a
per-instance configuration mechanism where each instance can have
separate properties. This could be as simple as a per-classloader
static property map but ideally something a little more flexible
would be useful
Well, I would love us moving to JMX. It would solve a lot of problems
around configuration for us, and it would also enable management by nice
pretty GUIs, integration into larger systems managed by JMX (e.g. most
app servers), etc.
I recognize this a change with system-wide impact which can not be taken
upon lightly. We'd also have to do it in a way that is pluggable, JMX
is not something I expect is part of J2ME...
I would like to see JMX instrumentation too but think it needs to be
layered on over the top of the component model rather than used as the
basis for it. We have MX4J available to support pre-Java5 JREs and it
may be possible to run that under a J2ME VM as well.
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2) common touch point in DriverManager, especially for use within
stored procedures.
I think we can do something here with a special Driver implementation
that could handle multiple engine instances which I think makes this
related to David Van Couvering's common jar discussion.
I don't understand this point, could you explain further?
The SQL standard Java binding says that a Java stored procedure can
retrieve a connection to its database by obtaining a connection to the
standard URL "jdbc:default:connection" from DriverManager. This sucks.
Especially in J2ME where you don't have a DriverManager.
Derby supports this by registering a Driver that understands that URL
and which then retrieves the Connection from a thread local. We need to
make sure this implementation still works when multiple Drivers get
registered for the same URL from different engines. It should (I think)
if the internal Driver is registered from the engine's classloader.
I have also wondered about providing an injection mechanism for stored
procedure implementations where the engine would inject a DataSource
into the class or instance that could be used instead of DriverManager
to obtain the connection. This is obviously an extension to the spec but
might be friendlier.
--
Jeremy
