I'm trying to figure out and diagnose an odd problem, and my best guess so
far is that a datastore transaction successfully stored data* and* also
returned an error message.
(I happen to be using Python, but this might be a general datastore issue)
The code goes about like this:
txn(key):
x = db.get(key)
x.foo += 1
x.put()
logging.info('go!')
# transactionally enqueue something to a task-queue
return
foo(key):
while True:
try:
db.run_in_transaction(txn, key)
except Exception, e:
logging.exception('ouch')
time.sleep(10.0) # wait 10 seconds
continue # fast inline retry
else:
return
What I'm seeing is that there was one logged exception:
"The datastore operation timed out, or the data was temporarily
unavailable."
The 'go!' line is logged twice, each time through the transaction, x.foo is
incremented twice and the task-queue task runs once.
What really bugs me is that x.foo added 2 where I wanted to add 1.
What really bugs me after that is that it seems that the datastore part of
the transaction seems to have run twice while the task-queue part of the
transaction seems to have run once.
Another thing that bugs me, but I'm kinda coming to terms with it, is that a
transaction can complete successfully and the API still returns an error. I
suppose this was always possible if all the actions happened on the
datastore server machine but then the connection to the appengine instance
died and never got notified that the transaction happened. The data may be
safe and consistent, but it makes my logical flow of programming around the
operation messier.
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