openstack-nova源码虚拟机创建流程
nova-api接收到消息后，调用nova\api\openstack\compute\servers.py 中ServersController类的create（）方法，
部分代码：
try:
inst_type = flavors.get_flavor_by_flavor_id(
flavor_id, ctxt=context, read_deleted="no")
supports_multiattach = common.supports_multiattach_volume(req)
(instances, resv_id) = self.compute_api.create(context,
inst_type,
image_uuid,
display_name=name,
display_description=description,
availability_zone=availability_zone,
forced_host=host, forced_node=node,
metadata=server_dict.get('metadata', {}),
admin_password=password,
requested_networks=requested_networks,
check_server_group_quota=True,
supports_multiattach=supports_multiattach,
**create_kwargs)
这个方法最后调用了.compute_api.create（）方法，这个实际调用的是nova.compute.api.API.create()，下面为这个方法的部分代码：
def create(self, context, instance_type,
image_href, kernel_id=None, ramdisk_id=None,
min_count=None, max_count=None,
display_name=None, display_description=None,
key_name=None, key_data=None, security_groups=None,
availability_zone=None, forced_host=None, forced_node=None,
user_data=None, metadata=None, injected_files=None,
admin_password=None, block_device_mapping=None,
access_ip_v4=None, access_ip_v6=None, requested_networks=None,
config_drive=None, auto_disk_config=None, scheduler_hints=None,
legacy_bdm=True, shutdown_terminate=False,
check_server_group_quota=False, tags=None,
supports_multiattach=False, trusted_certs=None):
（省略中间部分代码）
return self._create_instance(
context, instance_type,
image_href, kernel_id, ramdisk_id,
min_count, max_count,
display_name, display_description,
key_name, key_data, security_groups,
availability_zone, user_data, metadata,
injected_files, admin_password,
access_ip_v4, access_ip_v6,
requested_networks, config_drive,
block_device_mapping, auto_disk_config,
filter_properties=filter_properties,
legacy_bdm=legacy_bdm,
shutdown_terminate=shutdown_terminate,
check_server_group_quota=check_server_group_quota,
tags=tags, supports_multiattach=supports_multiattach,
trusted_certs=trusted_certs)
这个方法最后调用了该类中的_create_instance方法，这个方法最后调用了
self.compute_task_api.schedule_and_build_instances方法
部分代码如下：
def _create_instance(self, context, instance_type,
image_href, kernel_id, ramdisk_id,
min_count, max_count,
display_name, display_description,
key_name, key_data, security_groups,
availability_zone, user_data, metadata, injected_files,
admin_password, access_ip_v4, access_ip_v6,
requested_networks, config_drive,
block_device_mapping, auto_disk_config, filter_properties,
reservation_id=None, legacy_bdm=True, shutdown_terminate=False,
check_server_group_quota=False, tags=None,
supports_multiattach=False, trusted_certs=None):
（中间部分代码省略）
else:
self.compute_task_api.schedule_and_build_instances(
context,
build_requests=build_requests,
request_spec=request_specs,
image=boot_meta,
admin_password=admin_password,
injected_files=injected_files,
requested_networks=requested_networks,
block_device_mapping=block_device_mapping,
tags=tags)
return instances, reservation_id
而这个schedule_and_build_instances方法实际调用的为nova/conductor/rpcapi.py下的ComputeTaskAPI类中的schedule_and_build_instances方法，此方法定义如下：
def schedule_and_build_instances(self, context, build_requests,
request_specs,
image, admin_password, injected_files,
requested_networks,
block_device_mapping,
tags=None):
version = '1.17'
kw = {'build_requests': build_requests,
'request_specs': request_specs,
'image': jsonutils.to_primitive(image),
'admin_password': admin_password,
'injected_files': injected_files,
'requested_networks': requested_networks,
'block_device_mapping': block_device_mapping,
'tags': tags}
if not self.client.can_send_version(version):
version = '1.16'
del kw['tags']
cctxt = self.client.prepare(version=version)
cctxt.cast(context, 'schedule_and_build_instances', **kw)
这里的schedule_and_build_instances调用了消息队列客户端的方法cast方法，这个cast是异步调用了rpc server端的schedule_and_build_instances方法。实际这个最后调用了
nova/conductor/manager.py 中的ComputeTaskManager类的schedule_and_build_instances方法，部分代码如下：
def schedule_and_build_instances(self, context, build_requests,
request_specs, image,
admin_password, injected_files,
requested_networks, block_device_mapping,
tags=None):
（中间部分代码省略）
try:
host_lists = self._schedule_instances(context, request_specs[0],
instance_uuids, return_alternates=True)
except Exception as exc:
LOG.exception('Failed to schedule instances')
self._bury_in_cell0(context, request_specs[0], exc,
build_requests=build_requests,
block_device_mapping=block_device_mapping)
return
...........................................................
with obj_target_cell(instance, cell) as cctxt:
self.compute_rpcapi.build_and_run_instance(
cctxt, instance=instance, image=image,
request_spec=request_spec,
filter_properties=filter_props,
admin_password=admin_password,
injected_files=injected_files,
requested_networks=requested_networks,
security_groups=legacy_secgroups,
block_device_mapping=instance_bdms,
host=host.service_host, node=host.nodename,
limits=host.limits, host_list=host_list)
这个方法主要执行了两个函数，_schedule_instances和 build_and_run_instance，前者调用self._schedule_instances()筛选合适的计算节点。这个方法实际上通过rpc调用了nova-schedule的select_destinations方法。这个select_destinations方法使用了driver的筛选方法，也就是说这个driver是可以用户自行配置的，配置文件默认的filter_scheduler 。后者调用的是nova\compute\rpcapi.py中的ComputeAPI类的build_and_run_instance方法，部分代码如下：
def build_and_run_instance(self, ctxt, instance, host, image, request_spec,
filter_properties, admin_password=None, injected_files=None,
requested_networks=None, security_groups=None,
block_device_mapping=None, node=None, limits=None,
host_list=None):
# NOTE(edleafe): compute nodes can only use the dict form of limits.
if isinstance(limits, objects.SchedulerLimits):
limits = limits.to_dict()
kwargs = {"instance": instance,
"image": image,
"request_spec": request_spec,
"filter_properties": filter_properties,
"admin_password": admin_password,
"injected_files": injected_files,
"requested_networks": requested_networks,
"security_groups": security_groups,
"block_device_mapping": block_device_mapping,
"node": node,
"limits": limits,
"host_list": host_list,
}
client = self.router.client(ctxt)
version = '5.0'
cctxt = client.prepare(server=host, version=version)
cctxt.cast(ctxt, 'build_and_run_instance', **kwargs)
这个方法最后通过rpc调用，nova\compute\manager.py 中的ComputeManager类的build_and_run_instance方法，最后调用libvirt创建虚拟机。