Another drawback for using Task Queue is its limitation. Max number of active queues is 100. Max invocations per second per queue is 50. So Max jobs per second 100*50 = 500. If my app has 10000 users and for every one I need a transaction -> so it is not enough resources to accomplish that. I have also read about an existence of distributed transacions: http://www.google.com/events/io/2009/sessions/DesignDistributedTransactionLayerAppEngine.html It seems it is very hard to implement and there is not any robust java lib for that. So now my choice is to use 'deferred completion' approach described here: http://blog.notdot.net/2009/9/Distributed-Transactions-on-App-Engine. For rolling forward I don't want to use a queue or the cron because of their limitations. I prefer to make rolling forward failed transaction just during a user request. Thus I can get round limitations and synchronization issues: e.g. a queue hasn't rolled forward failed transaction but my app needs to do some work based on a failed transaction.
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