instance create:
nova/api/servers.create -- self.compute_api.create -- self.compute_task_api.build_instances-- self.conductor_compute_rpcapi.build_instances(conductorp/api.py) -- cctxt.cast(context, 'build_instances', **kw)从nova api到conductor的调用为异步conductor manager 里面:1. hosts = self._schedule_instances:-- self.scheduler_client.select_destinations -- self.scheduler_rpcapi.select_destinations -- cctxt.call(ctxt, 'select_destinations'从conductor调用select_destinations到scheduler采用同步所以，当FilterScheduler有异常：
reason = _('There are not enough hosts available.')
raise exception.NoValidHost(reason=reason)scheduler manager 中利用@messaging.expected_exceptions(exception.NoValidHost)
把该异常传回给conductor,conductor在这里捕获该异常并把状态设置为ERROR：
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 出错为：
nova.scheduler.utils [^[[01;36mreq-7ceeb23a-1d2e-401b-bbb7-2d8058715e82 ^[[00;36madmin admin^[[01;33m] ^[[01;35m^[[01;33mFailed to compute_task_build_instances: No valid host was found. There are not enough hosts available.Traceback (most recent call last): File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/server.py", line 150, in inner return func(*args, **kwargs) File "/opt/stack/nova/nova/scheduler/manager.py", line 71, in select_destinations filter_properties) File "/opt/stack/nova/nova/scheduler/filter_scheduler.py", line 81, in select_destinations raise exception.NoValidHost(reason=reason)NoValidHost: No valid host was found. There are not enough hosts available.^[[00m2016-01-07 02:26:32.496 ^[[01;33mWARNING nova.scheduler.utils [^[[01;36mreq-7ceeb23a-1d2e-401b-bbb7-2d8058715e82 ^[[00;36madmin admin^[[01;33m] ^[[01;35m[instance: 47b93a3e-b7d1-428f-9b7c-edd80c0735db] ^[[01;33mSetting instance to ERROR state.^[[00m
2. self.compute_rpcapi.build_and_run_instance:
cctxt.cast(ctxt, 'build_and_run_instance -- utils.spawn_n -- eventlet.spawn_n从conductor调用到compute为异步调用
其他：
conductor/rpcapi/ComputeTaskAPI中，对conductor的远程调用
migrate_server为同步
其他build_instances／unshelve_instance／rebuild_instance均为异步
