Dear Derby users,
I would like to understand if anyone thinks that they might be affected
by the following issue. This issue affects customers who do the following:
o Run an embedded Derby application which generates its own Derby
properties on the fly.
o In the same VM, run other JDBC applications. These other applications
could request Connections to DB2, Oracle, Derby, or any other database.
In general, we recommend against generating Derby properties on the fly.
This general problem is described in DERBY-1428. However, the Derby
mainline (slated to become release 10.2) expands this problem, as
described in DERBY-1429. For a workaround, see the Release Note attached
to DERBY-930.
Here is more detail on this issue:
a) With JDBC4, vendors mark their jar files to indicate the names of
jdbc drivers in those jars. During the lifetime of a vm, the very first
request for a Connection causes the DriverManager to look inside all of
the jars and register all indicated jdbc drivers.
b) When our embedded driver registers itself, it also boots the engine,
using whatever derby properties are currently visible. Typically, the
engine stays booted for the lifetime of the vm.
This can cause a Heisenbug in the following scenario:
o The customer runs two applications in the same vm: EmbeddedApp and
OtherApp.
o Before getting its first Connection, the EmbeddedApp hand-crafts its
own derby properties to configure the engine's behavior.
o OtherApp could be an application which uses the Derby client driver or
some other jdbc driver.
o If OtherApp runs before EmbeddedApp, then the engine will boot without
the hand-crafted properties.
o It may not be deterministic whether OtherApp or EmbeddedApp runs
first. Sometimes you get the right engine properties and sometimes you
don't.
It is worth pointing out that this Heisenbug can occur today, pre-JDBC4,
if OtherApp is another embedded Derby application.
JDBC4-driver-autoloading broadens the family of affected scenarios.
I would like to understand how much the family is broadened. Please let
me know if you think this problem will affect you.
Thanks,
-Rick
