On 10.09.2012 20:07, Bergquist, Brett wrote:
From the stack traces of the Derby engine, it appears that something
causes the utility to fail after the database was frozen and neither
the shutdown hook nor the try/finally unfroze the database. So after
that point, the database was effectively locked up. The system was
still operating and connections were being made trying to access the
database exhausting all of the connections.
So if none of your unfreeze attempt worked here, what happened to that
process/VM? You say "failed", did it hang, did it complete normally,
albeit with no effect, or was it killed off?
Curious, since If we were to implement an automatic unfreeze at
connection close, if the method you used didn't work, an automatic
unfreeze might fail too (if there is a bug in the code that prevents
unfreeze from doing its thing). If it hangs without unfreezing it might
be interesting to see the VM state at that point, e.g. via jstack.
So I was thinking that maybe the database engine should have some sort
of protection if this were to happen. Maybe the database engine
should automatically unfreeze the database if the connection that
freezes the database terminates/closes. Or maybe a timer to be added
to the freeze command to automatically unfreeze the database after the
fact.
I am thinking this because I was told on a previous emailing when
trying to build this utility totally from a script point of view using
IJ to freeze the database, SH to perform the ZFS snapshot and IJ to
unfreeze the database that it was not expected that the
freeze/unfreeze would be done from separate connections. I fact I ran
into a problem with the utility at that point where the IJ connection
to unfreeze could not be created because the database was frozen.
So I guess is there ever a use case that would require a database to
be frozen and not unfrozen before the connection is closed/lost?
