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Exception in saveChanges(): rowDiffsForAttributes - snapshot does not contain value
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Exception in saveChanges(): rowDiffsForAttributes - snapshot does not contain value


  • Subject: Exception in saveChanges(): rowDiffsForAttributes - snapshot does not contain value
  • From: Chuck Hill <email@hidden>
  • Date: Wed, 21 Feb 2007 15:04:02 -0800

Hi,

Some of you will recognize this extremely fun exception:

java.lang.IllegalStateException: rowDiffsForAttributes: snapshot in com.webobjects.eoaccess.EODatabaseOperation {
_dbSnapshot = {};
...
this = "<com.foo.bar.DownloadLog a849b0 _EOIntegralKeyGlobalID [DownloadLog (java.lang.Long)0]>"; }";
_globalID = _EOIntegralKeyGlobalID[DownloadLog (java.lang.Long)0];
_databaseOperator = "EODatabaseUpdateOperator"; }
does not contain value for attribute named downloadDate with snapshot key: downloadDate


The stack trace is:
at com.webobjects.eoaccess.EODatabaseOperation.rowDiffsForAttributes (EODatabaseOperation.java:338)
at com.webobjects.eoaccess.EODatabaseContext.createAdaptorOperationsForData baseOperationAttributes(EODatabaseContext.java:5373)
at com.webobjects.eoaccess.EODatabaseContext.createAdaptorOperationsForData baseOperation(EODatabaseContext.java:5548)
at com.webobjects.eoaccess.EODatabaseContext.performChanges (EODatabaseContext.java:6365)
at com.webobjects.eocontrol.EOObjectStoreCoordinator.saveChangesInEditingCo ntext(EOObjectStoreCoordinator.java:415)
at com.webobjects.eocontrol.EOEditingContext.saveChanges (EOEditingContext.java:3226)
at net.global_village.eofextensions.ForgetfulEC.saveChanges (ForgetfulEC.java:54)
at net.global_village.eofvalidation.EOEditingContext.saveChanges (EOEditingContext.java:126)
at net.global_village.eofvalidation.NotifyingEditingContext.saveChanges (NotifyingEditingContext.java:159)



The exception is correct, the snapshot _dbSnapshot = {}; does not contain any values, let alone the one it is looking for. I am not completely sure of how this happens. It is not consistent so I believe it to be caused by concurrent threads sending EOF notifications interacting with the locked / unlocked state of editing contexts.


The situation that I have that causes this exception is this: I have a page that starts an import process. It spins off a thread that handles the actual import. This thread has its own EC. The first thing it does is to create a new DownloadLog to record the start / status of the import process. While it is processing, it periodically unlocks and re-locks the EC and does a refreshing fetch on the download log so that it can see changes made in other editing contexts.

Once this thread is started, and concurrent with its processing, the app goes to a page that monitors all the download logs. This was built using the AjaxGrid and refreshes every 30 seconds. At each refresh it does a refreshing fetch. Of course, its EC is unlocked between requests. It has a link to abort the import by writing to the download log, though that is not being used when this exception occurs.

Periodically the download processing thread will throw the above exception while saving an update to the download log. My guess is that the refreshing fetch is invalidating the snapshot that the EC was referring to and it gets a null / empty dictionary for it as it is still locked.

I fixed this (or at least I can't reproduce it anymore) by doing ec.unlock(); ec.lock(); immediately before ec.saveChanges();. My guess is that this allows the snapshot to get updated. I suspect there is still a race condition in there where the a refreshing fetch is done on the download load while saveChanges() is in progress.

Does anyone know anything more about this or can you shed light on what is happening and how you worked around it?


Cheers, Chuck



--

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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