Re: Weird Behaviour...
Re: Weird Behaviour...
- Subject: Re: Weird Behaviour...
- From: Calven Eggert <email@hidden>
- Date: Wed, 07 Dec 2011 15:06:14 -0500
It might be helpful to know that I've discovered that my application that was having this problem allows the user to click on a record in a list to go to an edit page. On the edit page there is a save button and when clicked the page remains displayed. My other WO apps do not have this behaviour, the save button returns the user to the list page and so far I have not found any issues with data not being saved in the database.
This all seems to fit in with how the bug is described earlier from Phillippe and Chuck's additional comment. (In my testing I was editing a record hitting save and editing again immediately on the same page. After approximately 7 saves in a row the problem arose. In any case, this is now resolved with the suggested fix.)
On 2011-12-07, at 11:01 AM, Ramsey Gurley wrote:
>
> On Dec 6, 2011, at 3:43 PM, Chuck Hill wrote:
>
>>
>> On 2011-12-06, at 2:31 PM, Lachlan Deck wrote:
>>
>>> On 07/12/2011, at 9:23 AM, Philippe Rabier wrote:
>>>
>>>> I can feel how uncomfortable you are.
>>>>
>>>> What makes me confused is to never see this bug before. It's hard to believe that nobody saw the errors if there are error. But on another side, I (and all the team) worked on many applications so following the explanation, I don't know why we don't see this issue much more often.
>>>
>>> I suspect the more prevalent use of concurrent requests these days has exposed this bug.
>>
>> It is worth carefully reading over the reproduction steps:
>>
>>> * How it fails: A request is handled by WorkerThread0. By the end of the request the eo has been modified but not saved, so the EOObserverCenter remembers
>>> * that WorkerThread0's most recent object is that eo. Fifteen more requests are handled by WorkerThreads 1-15 in sequence. One of these requests completes
>>> * the modification of the eo and calls saveChanges on the ec. At this point the ec tells the EOObserverCenter to forget about its most recent object, but
>>> * it's being set to null in WorkerThread14 or whatever, not WorkerThread0.
>>> *
>>> * The next request will wrap around to to be handled by WorkerThread0. This request modifies an attribute on the eo, but since the EOObserverCenter still
>>> * thinks WorkerThread0 has already noticed the eo, it ignores the willChange and the ec doesn't grab a snapshot.
>>> *
>>> * Later in the processing of this request, a different object gets changed, willChange gets called and the ec grabs a snapshot of the second object. Then,
>>> * a change gets made to the original eo, willChange gets called, and since the EOObserverCenter was paying attention to the second object, it goes ahead
>>> * and notifies the ec about the first object.
>>> *
>>> * At this point the ec grabs a snapshot of the first object, but it's too late -- the object has already been modified, the ec didn't know about the
>>> * previous change, so when saveChanges gets called the previous changes don't get saved to the database. And now your object graph no longer matches the
>>> * database, and your app is borked.
>>
>>
>> Note that it has to be the _same_ eo (same GID) and the same thread and it has to be in a modified but unsaved state.
>
> In EOObserverCenter I see:
>
> eo == threadInfo._lastWillChangeObject
>
> Same ec, multiple threads.. oy!
>
> It stands to reason that when the ec is saved, any eo in that ec should be removed from the threadInfos. It sounds like the _ThreadInfo should set up an observer on the ec's EditingContextDidSaveChangesNotification. Once saved, any threadInfo with an eo in that ec should set _lastWillChangeObject to null.
>
> I wonder what about nested ECs? Let's say the parent ec is saved, but the nested ec has changes to an EO already. What then?
>
>
>> My guess is that unless your users are doing highly concurrent editing of the same data that this will rarely affect you. That is why I found it fixing Calven's problem so readily a surprise.
>>
>>
>>
>>>> And a question regarding the workaround: is there any drawback to call the EOObserver in the dispatchRequest method like suggested?
>>>
>>> For multiple active worker threads, a good question...
>>>
>>> Lachlan Deck
>>> email@hidden
>>>
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>>
>> --
>> Chuck Hill Senior Consultant / VP Development
>>
>> 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
>>
>>
>>
>>
>>
>>
>>
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