My reply was too big with the attachments, so I moved my response
into the wiki:
http://wiki.objectstyle.org/confluence/display/WO/
Programming__WebObjects-EOF-Using+EOF-Problems
The last one, "Strange Locking Problems" ... And for the lazy:
It would appear that there is, in our opinion, some bugs related to
optimistic locking within a single EOF stack. Essentially what it
boils down to is that it appears that the update database operation
that is created as a result of a call to .saveChanges() is backed by
the EODatabaseContext snapshot and NOT the "working" snapshot inside
in the EO in the editing context it came from. What this means is
that while changes are not merged until you .unlock() and .lock()
under normal circumstances, because the underlying snapshot that EOF
diffs your changes against on save is the DBC snapshot, it's
effectively inadvertently "merged" on commit. That is to say that if
another EC makes changes and saves, then you make different changes
and save, you will blow away their changes with no sign of an
optimistic locking exception because your snapshot IS their snapshot
now (meaning, it looks like just you are overwriting their changes,
versus the reality of the situation that you are actually conflicting
with their changes). After discussing this some, we believe that if
the update operation used a version of the EO's backing snapshot
instead that these weird behaviors would be fixed and it would behave
exactly like a normal conflicting update if you were in two EOF
stacks. The current behavior smells of bug, but I'm curious if anyone
a dissenting opinion on the topic. It's certainly really complicated
and nasty down in that code, so it's possible there's some crazy
justifiable reason for it.
Go to the wiki for the diagrams.
ms
On Aug 2, 2007, at 1:12 AM, Chuck Hill wrote:
The problem comes in when the modifications are made and saved
after your editing context has been locked in the RR loop. I
tricked ;-) Mike into looking at this today with. Looks like
tigers lurk here. Maybe Mike will comment.
Chuck
On Aug 1, 2007, at 2:34 PM, Pierre Bernard wrote:
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 concurrencyssue her
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
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Webobjects-dev mailing list (Webobjects-dev@lists.apple.com)
Help/Unsubscribe/Update your Subscription:
http://lists.apple.com/mailman/options/webobjects-dev/kenlists%
40anderhome.com
This email sent to [EMAIL PROTECTED]
Miguel Arroz
http://www.terminalapp.net
http://www.ipragma.com
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Webobjects-dev mailing list (Webobjects-dev@lists.apple.com)
Help/Unsubscribe/Update your Subscription:
http://lists.apple.com/mailman/options/webobjects-dev/chill%
40global-village.net
This email sent to [EMAIL PROTECTED]
--
Practical WebObjects - for developers who want to increase their
overall knowledge of WebObjects or who are trying to solve
specific problems.
http://www.global-village.net/products/practical_webobjects
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Webobjects-dev mailing list (Webobjects-dev@lists.apple.com)
Help/Unsubscribe/Update your Subscription:
http://lists.apple.com/mailman/options/webobjects-dev/webobjects-
lists%40houdah.com
This email sent to [EMAIL PROTECTED]
- - -
Houdah Software s. à r. l.
http://www.houdah.com
HoudahGeo: One-stop photo geocoding
HoudahSpot: Powerful Spotlight frontend
--
Practical WebObjects - for developers who want to increase their
overall knowledge of WebObjects or who are trying to solve specific
problems.
http://www.global-village.net/products/practical_webobjects
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Webobjects-dev mailing list (Webobjects-dev@lists.apple.com)
Help/Unsubscribe/Update your Subscription:
http://lists.apple.com/mailman/options/webobjects-dev/mschrag%
40mdimension.com
This email sent to [EMAIL PROTECTED]
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Webobjects-dev mailing list (Webobjects-dev@lists.apple.com)
Help/Unsubscribe/Update your Subscription:
http://lists.apple.com/mailman/options/webobjects-dev/archive%40mail-archive.com
This email sent to [EMAIL PROTECTED]
