def autoretry_datastore_timeouts(attempts=5.0, interval=0.1, exponent=2.0):
"""
Copyright (C) 2009 twitter.com/rcb
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======================================================================
This function wraps the AppEngine Datastore API to autoretry
datastore timeouts at the lowest accessible level.
The benefits of this approach are:
1. Small Footprint: Does not monkey with Model internals
which may break in future releases.
2. Max Performance: Retrying at this lowest level means
serialization and key formatting is not
needlessly repeated on each retry.
At initialization time, execute this:
>>> autoretry_datastore_timeouts()
Should only be called once, subsequent calls have no effect.
>>> autoretry_datastore_timeouts() # no effect
Default (5) attempts: .1, .2, .4, .8, 1.6 seconds
Parameters can each be specified as floats.
:param attempts: maximum number of times to retry.
:param interval: base seconds to sleep between retries.
:param exponent: rate of exponential back-off.
"""
import time, logging
from google.appengine.api import apiproxy_stub_map
from google.appengine.runtime import apiproxy_errors
from google.appengine.datastore import datastore_pb
attempts = float(attempts)
interval = float(interval)
exponent = float(exponent)
wrapped = apiproxy_stub_map.MakeSyncCall
errors = {datastore_pb.Error.TIMEOUT:'Timeout',
datastore_pb.Error.CONCURRENT_TRANSACTION:'TransactionFailedError'}
def wrapper(*args, **kwargs):
count = 0.0
while True:
try:
return wrapped(*args, **kwargs)
except apiproxy_errors.ApplicationError, err:
errno = err.application_error
if errno not in errors: raise
sleep = (exponent ** count) * interval
count += 1.0
if count > attempts: raise
msg = "Datastore %s: retry #%d in %s seconds.\n%s"
vals = ''
if count == 1.0:
vals = '\n'.join([str(a) for a in args])
logging.warning(msg % (errors[errno], count, sleep, vals))
time.sleep(sleep)
setattr(wrapper, '_autoretry_datastore_timeouts', False)
if getattr(wrapped, '_autoretry_datastore_timeouts', True):
apiproxy_stub_map.MakeSyncCall = wrapper