Re: Concurrency question
Re: Concurrency question
- Subject: Re: Concurrency question
- From: Pierre Bernard <email@hidden>
- Date: Wed, 1 Aug 2007 23:34:14 +0200
You can simulate OL, by listening to merge notifications. If it
affects a modified object you can later on refuse to save.
Pierre
On Aug 1, 2007, at 7:18 PM, Chuck Hill wrote:
On Aug 1, 2007, at 5:00 AM, Miguel Arroz wrote:
Hi!
The contexts are locked. The problem is that it's not the same
context - it's a different context per thread, with local copies
of the same objects.
Synchronizing solves the sample problem, but as my real problem
is much more complex than this example, it starts to get a
little... ugly. Also, I still did not full understand why, the
objects in the thread that runs in second are not updated with the
data saved by the thread that run first. I suspect that the cause
of this is that the objects only receive the notifications to
update themselves after finishing the R-R. Can anyone confirm this?
Yes, if you are locking properly (and I know you are), the changes
only get merged at the end of the RR loop. You can do it yourself
by unlocking and relocking the EC.
About being solving 2 different problems or not, it depends on
the level os abstraction you use to look at it. :) From my point
of view, I'm solving one problem - the problem of concurrent
updates. My app has now enough info to solve the problem (OL
fields, and how to retry). It would work *if* the UPDATE WHERE
data was fetched from the original data in the context, and not in
the row data.
It gets more complex than this. One quick fix is to have each EC
in its own EOF stack. But that can be rather memory expensive.
Chuck
On 2007/08/01, at 03:03, Ken Anderson wrote:
Miguel,
Is your editing context locked before calling incrementIt() ? I
would think that would solve your concurrency issue here.
If not, just synchronizing the method should solve the problem.
It may seem inelegant, but you really ARE solving 2 different
problems...
Ken
On Jul 31, 2007, at 7:41 PM, Miguel Arroz wrote:
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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Miguel Arroz
http://www.terminalapp.net
http://www.ipragma.com
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