def create(self, req, body, tenant_id):
    # TODO(hub-cap): turn this into middleware
    LOG.info(_LI("Creating a database instance for tenant '%s'"),
             tenant_id)
    LOG.debug("req : '%s'\n\n", strutils.mask_password(req))
    LOG.debug("body : '%s'\n\n", strutils.mask_password(body))
    context = req.environ[wsgi.CONTEXT_KEY]
    context.notification = notification.DBaaSInstanceCreate(context,
                                                            request=req)
    datastore_args = body['instance'].get('datastore', {})
    datastore, datastore_version = (
        datastore_models.get_datastore_version(**datastore_args))
    image_id = datastore_version.image_id
    name = body['instance']['name']
    flavor_ref = body['instance']['flavorRef']
    flavor_id = utils.get_id_from_href(flavor_ref)

    configuration = self._configuration_parse(context, body)
    databases = populate_validated_databases(
        body['instance'].get('databases', []))
    database_names = [database.get('_name', '') for database in databases]
    users = None
    try:
        users = populate_users(body['instance'].get('users', []),
                               database_names)
    except ValueError as ve:
        raise exception.BadRequest(msg=ve)

    if 'volume' in body['instance']:
        volume_info = body['instance']['volume']
        volume_size = int(volume_info['size'])
        volume_type = volume_info.get('type')
    else:
        volume_size = None
        volume_type = None

    if 'restorePoint' in body['instance']:
        backupRef = body['instance']['restorePoint']['backupRef']
        backup_id = utils.get_id_from_href(backupRef)
    else:
        backup_id = None

    availability_zone = body['instance'].get('availability_zone')
    nics = body['instance'].get('nics')

    slave_of_id = body['instance'].get('replica_of',
                                       # also check for older name
                                       body['instance'].get('slave_of'))
    replica_count = body['instance'].get('replica_count')
    modules = body['instance'].get('modules')
    locality = body['instance'].get('locality')
    if locality:
        locality_domain = ['affinity', 'anti-affinity']
        locality_domain_msg = ("Invalid locality '%s'. "
                               "Must be one of ['%s']" %
                               (locality,
                                "', '".join(locality_domain)))
        if locality not in locality_domain:
            raise exception.BadRequest(msg=locality_domain_msg)
        if slave_of_id:
            dupe_locality_msg = (
                'Cannot specify locality when adding replicas to existing '
                'master.')
            raise exception.BadRequest(msg=dupe_locality_msg)

    instance = models.Instance.create(context, name, flavor_id,
                                      image_id, databases, users,
                                      datastore, datastore_version,
                                      volume_size, backup_id,
                                      availability_zone, nics,
                                      configuration, slave_of_id,
                                      replica_count=replica_count,
                                      volume_type=volume_type,
                                      modules=modules,
                                      locality=locality)

    view = views.InstanceDetailView(instance, req=req)
    return wsgi.Result(view.data(), 200)

基本就是四步走，首先是请求上下文，然后创建实例对象，然后操作，最后返回wsgi