new_contents: """
     Copyright 2015 The Trustees of Princeton University
  
     Licensed under the Apache License, Version 2.0 (the "License");
     you may not use this file except in compliance with the License.
     You may obtain a copy of the License at
  
         http://www.apache.org/licenses/LICENSE-2.0
  
     Unless required by applicable law or agreed to in writing, software
     distributed under the License is distributed on an "AS IS" BASIS,
     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     See the License for the specific language governing permissions and
     limitations under the License.
  """
  
  import random
  
  from google.appengine.api import memcache
  from google.appengine.ext import ndb
  
  import backends as backend
  
  UUID_KEY_TEMPLATE = 'uuid-{}-{}'
  
  
  class GeneralUUID(ndb.Model):
      """
      Shards for each named counter.
      """
      uuid = ndb.StringProperty(default="")
  
  def __uuid_key_names( uuids, namespace ):
     key_strings = [UUID_KEY_TEMPLATE.format(uuid, namespace) for uuid in uuids]
     return key_strings
     
     
  @ndb.tasklet
  def get_uuids_async( uuids, namespace, use_memcache=True ):
     """
     Get all UUIDs under a namespace
     """
     do_cache = False
     all_key_names = __uuid_key_names( uuids, namespace )
     all_keys = [ndb.Key( GeneralUUID, key_name ) for key_name in all_key_names]
     uuid_data = yield ndb.get_multi_async( all_keys, use_cache=False, use_memcache=use_memcache )
     
     # NOTE: uuid_data is ordered by uuids
     raise ndb.Return( [uuid.uuid if uuid is not None else None for uuid in uuid_data] )
  
  
  def get_uuids( uuids, namespace, use_memcache=True ):
     uuid_data_fut = get_uuids_async( uuids, namespace, use_memcache )
     return uuid_data_fut.get_result()
  
  @ndb.tasklet
  def put_uuids_async( uuids, namespace ):
     """
     Put UUIDs to a namespace 
     """
     all_key_names = __uuid_key_names( uuids, namespace )
     all_keys = [ndb.Key( GeneralUUID, key_name ) for key_name in all_key_names]
     
     uuid_data = [ GeneralUUID( key=key, uuid=uuid ) for (key, uuid) in zip( all_keys, uuids ) ]
     
     put_keys = yield ndb.put_multi_async( uuid_data )
     
     raise ndb.Return( put_keys )
  
subject: Store namespaced UUIDs, i.e., to prevent replay attacks on JSON RPC calls.