Openstack liberty源码分析 之 云主机的启动过程2

接上一篇： Openstack liberty源码分析 之 云主机的启动过程1

nova-conductor

nova-api通过rpc发送启动云主机请求后，nova-conductor会收到该请求，根据路由映射，该请求会递交给
nova/conductor/manager.py.ComputeTaskManager.build_instances处理，如下（函数说明见注释）：

def build_instances(self, context, instances, image,    
    filter_properties,
    admin_password, injected_files, requested_networks,
    security_groups, block_device_mapping=None,   
    legacy_bdm=True):

   # TODO(ndipanov): Remove block_device_mapping and  
   # legacy_bdm in version 2.0 of the RPC API.

   #为nova-scheduler生成请求参数
   request_spec = scheduler_utils.build_request_spec(context, image,
                                                          instances)
   # TODO(danms): Remove this in version 2.0 of the RPC API
   if (requested_networks and
                not isinstance(requested_networks,
                               objects.NetworkRequestList)):
        requested_networks = objects.NetworkRequestList(
                objects=[objects.NetworkRequest.from_tuple(t)
                         for t in requested_networks])
   # TODO(melwitt): Remove this in version 2.0 of the RPC API
   flavor = filter_properties.get('instance_type')
   if flavor and not isinstance(flavor, objects.Flavor):
       """
       Code downstream may expect extra_specs to be populated 
       since it is receiving an object, so lookup the flavor to 
       ensure
       """
       flavor = objects.Flavor.get_by_id(context, flavor['id'])
       filter_properties = dict(filter_properties, 
                                   instance_type=flavor)

   try:
       '''
       如果指定了group信息并且支持相应的filter，就添加group过滤信息到
       filter_properties
       '''
       scheduler_utils.setup_instance_group(context, request_spec,
                                                 filter_properties)
       # check retry policy. Rather ugly use of instances[0]...
       # but if we've exceeded max retries... then we really only
       # have a single instance.
       scheduler_utils.populate_retry(filter_properties,
                instances[0].uuid)
       """
       发送同步消息给nova-scheduler，选取用于创建云主机的主机
       接口调用如下：SchedulerClient -> SchedulerQueryClient -
       > SchedulerAPI
       与主机过滤相关的`filter_prorerties`参数是一个字典，内容如下：

       {
            u'instance_type':   
Flavor(created_at=None,deleted=False,deleted_at=None,disabled=False,ephemeral_gb=0,extra_specs={},flavorid='2',id=5,is_public=True,memory_mb=2048,name='m1.small',projects=[],root_gb=20,rxtx_factor=1.0,swap=0,updated_at=None,vcpu_weight=0,vcpus=1), 
           'retry': {'num_attempts': 1, 'hosts': []}, 
           u'scheduler_hints': {}
       }

       启动命令中没有指定`--hints`选项，所以`scheduler_hints`为空，而
       `instance_type`是一个Flavor实例与命令行中的`--flavor 2`相关
       """
       hosts = self.scheduler_client.select_destinations(context,
                    request_spec, filter_properties)
  except Exception as exc:
       updates = {
  
  'vm_state': vm_states.ERROR, 'task_state': None}
       for instance in instances:
           self._set_vm_state_and_notify(
                    context, instance.uuid, 'build_instances', 
                    updates,
                    exc, request_spec)
       return

 #依次发送启动instance的rpc请求到host，由nova-compute完成instance的启动过程
 for (instance, host) in itertools.izip(instances, hosts):
      try:
          instance.refresh()
      except (exception.InstanceNotFound,
                    exception.InstanceInfoCacheNotFound):
          LOG.debug('Instance deleted during build', 
          instance=instance)
          continue

     local_filter_props = copy.deepcopy(filter_properties)   
     scheduler_utils.populate_filter_properties(local_filter_props,
                                                            host)
     # The block_device_mapping passed from the api doesn't contain
     # instance specific information
     bdms = objects.BlockDeviceMappingList.get_by_instance_uuid(
                    context, instance.uuid)

     self.compute_rpcapi.build_and_run_instance(context,
                        instance=instance, host=host['host'], 
                        image=image,
                        request_spec=request_spec,
                        filter_properties=local_filter_props,
                        admin_password=admin_password,
                        injected_files=injected_files,
                        requested_networks=requested_networks,
                        security_groups=security_groups,
                        block_device_mapping=bdms, 
                        node=host['nodename'],
                        limits=host['limits'])

从上面的源码可以看出，build_instances方法主要实现过滤参数的组装，然后通过客户端api发送rpc请求到scheduler完成host的选取，最后发送rpc请求到选取的host上，由nova-compute完成云主机的启动。下面来看看nova-scheduler是如何选取主机的。

nova-scheduler

scheduler 收到客户端发来的select_destination请求消息后，