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  # Copyright (c) 2010 Openstack, LLC.
  # Copyright 2010 United States Government as represented by the
  # Administrator of the National Aeronautics and Space Administration.
  # All Rights Reserved.
  #
  #    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.
  
  """
  Scheduler that allows routing some calls to one driver and others to another.
  """
  
  from nova import flags
  from nova import utils
  from nova.scheduler import driver
  
  
  FLAGS = flags.FLAGS
  flags.DEFINE_string('compute_scheduler_driver',
                      'nova.scheduler.chance.ChanceScheduler',
                      'Driver to use for scheduling compute calls')
  flags.DEFINE_string('volume_scheduler_driver',
                      'nova.scheduler.chance.ChanceScheduler',
                      'Driver to use for scheduling volume calls')
  
  
  # A mapping of methods to topics so we can figure out which driver to use.
  _METHOD_MAP = {'run_instance': 'compute',
                 'start_instance': 'compute',
                 'create_volume': 'volume'}
  
  
  class MultiScheduler(driver.Scheduler):
      """A scheduler that holds multiple sub-schedulers.
  
      This exists to allow flag-driven composibility of schedulers, allowing
      third parties to integrate custom schedulers more easily.
  
      """
  
      def __init__(self):
          super(MultiScheduler, self).__init__()
          compute_driver = utils.import_object(FLAGS.compute_scheduler_driver)
          volume_driver = utils.import_object(FLAGS.volume_scheduler_driver)
  
          self.drivers = {'compute': compute_driver,
                          'volume': volume_driver}
  
      def __getattr__(self, key):
          if not key.startswith('schedule_'):
              raise AttributeError(key)
          method = key[len('schedule_'):]
          if method not in _METHOD_MAP:
              raise AttributeError(key)
          return getattr(self.drivers[_METHOD_MAP[method]], key)
  
      def set_zone_manager(self, zone_manager):
          for k, v in self.drivers.iteritems():
              v.set_zone_manager(zone_manager)
  
      def schedule(self, context, topic, *_args, **_kwargs):
          return self.drivers[topic].schedule(context, topic, *_args, **_kwargs)
  
subject: Allow different schedulers for compute and volume.