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From Bryan Pendleton <bpendleton.de...@gmail.com>
Subject Re: Locking problem
Date Sat, 16 Oct 2010 15:07:08 GMT
> Obviously there must be something wrong with this code, but I cannot
> see what it is. Should I perhaps be closing the (empty) result set to
> commit the transaction?

To commit your transaction, you need to call Connection.commit() (or put
your connection into auto-commit mode). Closing your result sets is
desirable, but by itself it won't commit the transaction.

I think the first thing you should try is to put a Connection.commit()
into your password validation subroutine, and re-analyze the locking
behaviors to see how they change.

You should also close your statements when you no longer need them. Derby
has finalizers for ResultSet, Statement, and Connection objects, but
waiting for the Java garbage collector to collect your JDBC objects can
be very problematic, as they can accumulate quickly and can hold quite
a bit of resources. So it's good to keep in the habit of closing them
as soon as possible.

One thing that programmers often overlook is that you need to commit
your read-only transactions as well as committing your update transactions.
Read-only transactions take locks, too!

Another thing you can do is to configure the transaction's isolation level;
the default isolation level holds a lot of locks, and many applications
find that they can run successfully using a lower level of isolation.

It definitely seems that you've done the overall analysis correctly, and
I think you're getting the hang of reading your derby trace logs. As you
continue to practice reading those logs, you'll become more comfortable
with them, and you'll find that you get more and more information out of them.
But it does take a bit of practice, so keep it up :)

Here's a couple resources that should prove useful:

http://db.apache.org/derby/docs/10.6/devguide/cdevconcepts19173.html
http://db.apache.org/derby/docs/10.6/devguide/cdevconcepts30291.html
http://download.oracle.com/javase/tutorial/jdbc/basics/transactions.html
http://download.oracle.com/javase/1.4.2/docs/api/java/sql/Connection.html

Good luck and let us know about your further experiments!

bryan

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