Lately I've been trying to improve how our web application employs transactions
(we don't use them enough). While doing this I became aware of this rather
general problem. It seems as though database transactions cannot be mixed with
exceptions unless great vigilance is practiced. I'm wondering if AspectJ can
help me in some way.
Consider this pseudo code:
function doOperationX()
{
connection = get_connection()
connection.start_transaction()
try
{
updateA(connection)
try
{
updateB(connection)
}
catch (C_DoesNotExist)
{
//ignore (non fatal)
}
connection.commit()
}
finally
{
connection.end_transaction() //rolls back uncommitted
connection.close()
}
}
function updateA(connection)
{
connection.execute("UPDATE a SET ...")
}
function updateB(connection)
{
connection.execute("UPDATE b SET ...")
updateC(connection)
}
function updateC(connection)
{
result = connection.execute("SELECT c.id FROM c INNER JOIN b ON ...")
if (result.next())
connection.execute("UPDATE c SET ...")
else
throw C_DoesNotExist()
}
Don't study the pseudo SQL too closely, the problem isn't there. Read the code
like this:
1) operationX requires updating of A and, optionally, B.
2) Internally, updating B always requires that C also be updated. All or
nothing.
3) Sometimes, C cannot be updated because the database is not in the proper
state at this time. (Not an invalid state, mind you)
4) operationX knows that C might not be updatable, so it catches the exception
and ignores it.
The problem is:
updateB() leaves the database in an inconsistent state if an exception is
thrown
out of updateC(). The inconsistency is that table B has been updated but D
has
not been updated accordingly.
Yet, it's only a problem because operationX caught the exception. Had it let
the exception continue up, the finally block would have rolled back the entire
transaction quite nicely.
How can I avoid this problem? The only proper solution that I can see is a
prolific use of transaction checkpoints (aka savepoints) so that a partial
rollback can occur if needed:
function updateB(connection)
{
success = false
savepoint = connection.setSavepoint()
try
{
connection.execute("UPDATE b SET ...")
updateD(connection)
success = true
}
finally
{
if (not success)
connection.rollback(savepoint)
}
}
But this hurts performance considerably and requires programmer vigilance and
lots of extra code! The programmer must remember that:
**If your function does more than one update it MUST use a savepoint!**
That's tough pill to swallow (for me). I try to avoid programming models that
require programmer vigilance. All it takes is one groggy Monday...
What do you think? Am I missing a better solution? Can AspectJ help?
Videx Inc. 1105 N. E. Circle Blvd. Corvallis OR 97330 (541) 758-0521
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