Kristian Waagan wrote:
Kristian Waagan wrote:
...
The wiki page is at:
http://wiki.apache.org/db-derby/InMemoryBackEndPrimer
Thanks for pulling this spec together, Kristian. Some comments:
Proper Delete Mechanism
I agree that option (1) looks fine: the mechanism for deleting a
database then looks like the mechanisms for creating and halting
databases. I think it's ok to restrict this privilege to the DBO. For
most use-cases that will probably be fine. There might be an oddball
case of a System Administrator needing to delete an in-memory database
without bouncing the VM. This sounds like something we could add when we
add system privileges.
Automatic database persistence on JVM shutdown
I think we should say something about how the user experiences this feature:
1) It makes sense to me that snapshot-on-exit is an attribute of a
database which is set when the database is initially booted. Can you
change the setting and if so, who can do this and how?
2) It would be nice to say a few words about how you reboot one of these
snapshots. Is this done via the restoreFrom connection attribute? Do you
need to specify snapshot-on-exit when you reboot one of these in-memory
databases or is it assumed that if you reboot one of these in-memory
databases then you want the original exit behavior to continue in force?
Automatic database persistence on database shutdown
Similar questions about this feature. In addition, it becomes possible
for a database to have two states: the in-memory version and the
snapshot created when the database was shutdown.
3) If you reboot the database, do you get the version that's still in
memory? In the meantime, someone could have booted the snapshot as a
persistent database, fiddled with it, and re-written the snapshot.
4) Can you reboot the database with a restoreFrom setting? Does this
override the version in memory or does this raise an error?
I think there's definitely an opportunity to share code between
snapshot-on-exit and snapshot-on-shutdown. But they look like separate
features to me. My gut feeling is that snapshot-on-shutdown is more
important than snapshot-on-exit and has fewer problems. I think that
most people who need to persist an in-memory database could write their
application in such a way that they could gracefully park the database
before bringing down the vm. The more I think about it, the more that
shutdown-on-exit seems to refer to an undefined state of the database
and cannot be guaranteed to work properly.
Thanks,
-Rick
