[google-appengine] Need help understanding transactions in tasklets

[Rob Curtis]

Hi,

I have some code that queues up a task inside *_post_put_hook*. 
The task retrieves the key and fetches the entity. However sometimes the 
worker fails because the object for that key hasn't been created yet, but 
will succeed when it next runs.
Note that we're retrieving the object by key, so I expect the data to be 
consistent.

I want to understand WHY the object isn't available. I'm only calling the 
enqueue on commit, but it seems this isn't always working.
Below is the example code which does the committing of the entity.


@ndb.synctasklet
def log_usage(self):

    @ndb.transactional_tasklet(xg=True)

    def _txn():   

       # some other stuff happens here, but trying to make this example as 
simple as possible.
       yield Log.insert_document_log_async() 



    yield _txn()



class Log(ndb.Expando):

     @classmethod

@ndb.tasklet

def insert_document_log_async(cls):

    log = cls()

    logging.debug("insert document log in transaction: 
{}".format(ndb.in_transaction()))

    yield log.put_async()



@ndb.synctasklet

def _post_put_hook(self, future):


    @ndb.synctasklet

    def _callback_on_commit():

        key = future.get_result()

        yield SqlTaskHelper.enqueue_syncronise_sql_model_async(key)


    logging.debug("_post_put_hook In transaction: 
{}".format(ndb.in_transaction()))

    ndb.get_context().call_on_commit(lambda: _callback_on_commit())





   1. *log_usage *is called. 
   2. When calling *insert_document_log_async* logging indicates that we're 
   in a transaction (insert document log in transaction: *True*). 
   3. But the post put hook* _post_put_hook* In transaction: *False *(so 
   call_on_commit is executed immediately). The task runs shortly after an 
   occasional fails.


Can someone please shed some light on where the problem is?

Thanks
Rob

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