Hi!
I'm trying to understand what's the best way to do something here.
Imagine that I need to get a object from the database, modify some
attribute based on itself and save it again.
So, we have the method:
public void incrementIt() {
if( aIsEven() ) { // 1
setB( b() + 1 ); // 2
}
setA( a() + 1 ); // 3
editingContext.saveChanges(); // 4
}
Well, it's easy to solve the problem of update conflicts between
two different instances of the app. Just tick the OL lock for a and b
fields, catch the evil expression, refault the object, and
recalculate (and retry to save). That's "easy".
Now, my problem is inside the same instance! Imagine that this
method runs at the same time and we have the following run order, for
threads X and Y, with the same object in two different contexts (and
imagine a = 3):
X 1
Y 1
X 3
X 4
Y 3
Y 4
This will produce wrong results, but it won't cause any locking
exception. Why?
1) Both threads get the object with a = 3.
2) Both threads do not run line 2 because 3 is not even.
3) The thread X increments a, and saves it. When saving, the
object at thread Y will have it's 'a' attribute updated, assuming
both objects are in the same coordinator.
4) The thread Y increments a again, and saves it. N optimistic
locking exception will be thrown, because the coordinator snapshot
was updated in the last commit, so the SELECT FOR UPDATE will run OK.
This will cause a to be 5, but b did not increment as it should.
The problem is that, when saving, we are basing OL on the row
snapshot (that is updated during the process) and not to the original
value of the object when it was loaded into the editing context.
Well, this may be solved using the "classic" Java "syncronized"
stuff, and locks and all that stuff. But this is a bit stupid. I
already solved the problem with OL for the, theoretically, more
difficult case of managing several app instances. Do I have to solve
it all over again, in a different way, to deal with multiple updates
on the same instance? Isn't there a way to use OL just like I'm
already doing?
Yours
Miguel Arroz
Miguel Arroz
http://www.terminalapp.net
http://www.ipragma.com
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