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# 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.

"""
Managers are responsible for a certain aspect of the sytem.  It is a logical
grouping of code relating to a portion of the system.  In general other
components should be using the manager to make changes to the components that
it is responsible for.

For example, other components that need to deal with volumes in some way,
should do so by calling methods on the VolumeManager instead of directly
changing fields in the database.  This allows us to keep all of the code
relating to volumes in the same place.

We have adopted a basic strategy of Smart managers and dumb data, which means
rather than attaching methods to data objects, components should call manager
methods that act on the data.

Methods on managers that can be executed locally should be called directly. If
a particular method must execute on a remote host, this should be done via rpc
to the service that wraps the manager

Managers should be responsible for most of the db access, and
non-implementation specific data.  Anything implementation specific that can't
be generalized should be done by the Driver.

In general, we prefer to have one manager with multiple drivers for different
implementations, but sometimes it makes sense to have multiple managers.  You
can think of it this way: Abstract different overall strategies at the manager
level(FlatNetwork vs VlanNetwork), and different implementations at the driver
level(LinuxNetDriver vs CiscoNetDriver).

Managers will often provide methods for initial setup of a host or periodic
tasksto a wrapping service.

This module provides Manager, a base class for managers.
"""

from nova import utils
from nova import flags

from twisted.internet import defer

FLAGS = flags.FLAGS
flags.DEFINE_string('db_driver', 'nova.db.api',
                    'driver to use for volume creation')


class Manager(object):
    """DB driver is injected in the init method"""
    def __init__(self, host=None, db_driver=None):
        if not host:
            host = FLAGS.host
        self.host = host
        if not db_driver:
            db_driver = FLAGS.db_driver
        self.db = utils.import_object(db_driver)  # pylint: disable-msg=C0103

    @defer.inlineCallbacks
    def periodic_tasks(self, context=None):
        """Tasks to be run at a periodic interval"""
        yield

    def init_host(self):
        """Do any initialization that needs to be run if this is a standalone
        service. Child classes should override this method."""
        pass
