Re: Concurrency question
Re: Concurrency question
- Subject: Re: Concurrency question
- From: Mike Schrag <email@hidden>
- Date: Thu, 2 Aug 2007 09:59:16 -0400
I've never used it, but it's also MAYBE possible that turning on the
pessimistic locking strategy might be a fix, too? The potential
downfalls of that are huge, though.
ms
On Aug 2, 2007, at 8:41 AM, Ken Anderson wrote:
This is pretty amazing Mike.
I'm having this exact problem right now, and have been banging my
head against the wall, saying "THIS IS NOT POSSIBLE!".
Apparently... it is.
Now that the problem is identified, is there a reasonable solution?
Ken
On Aug 2, 2007, at 7:58 AM, Mike Schrag wrote:
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
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http://www.terminalapp.net
http://www.ipragma.com
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