才疏學淺，若有不當之處敬請雅正, 本文適合剛剛開始看Nova原始碼的人，先把整個流程串一下，後面再詳細介紹一下。 Nova 接收使用者請求的入口：nova/api/compute/servers.py nova/api/compute/servers.py

class ServersController(wsgi.Controller):
"""The Server API base controller class for the OpenStack API."""

.........................

def create(self, req, body):
"""Creates a new server for a given user."""

................................

(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,
                **create_kwargs)

..................................... 跳轉到nova/compute/api.py

class API(base.Base):

"""API for interacting with the compute manager."""

............................................................

def create(self,------------):
"""Provision instances, sending instance information to the
 
    scheduler.  The scheduler will determine where the instance(s)
    go and will handle creating the DB entries.

............................................................

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)

跳轉到本檔案的_create_instance（）函式

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):
"""Verify all the input parameters regardless of the provisioning
    strategy being performed and schedule the instance(s) for
    creation.
    """

。。。。。。。。。。。。。。。。。。。。。。。。。。。

self.compute_task_api.build_instances(context,
instances=instances, image=boot_meta,
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,
legacy_bdm=False)

return (instances, reservation_id)

跳轉到 nova/conductor/api.py

class ComputeTaskAPI(object):
"""ComputeTask API that queues up compute tasks for nova-conductor."""

............................................

def build_instances(self, context, instances, image, filter_properties,
        admin_password, injected_files, requested_networks,
        security_groups, block_device_mapping, legacy_bdm=True):
self.conductor_compute_rpcapi.build_instances(context,
instances=instances, image=image,
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,
legacy_bdm=legacy_bdm)

跳轉到 nova/conductor/rpcapi.py

class ComputeTaskAPI(object):
"""Client side of the conductor 'compute' namespaced RPC API

..........................

def build_instances(self, context, instances, image, filter_properties,
        admin_password, injected_files, requested_networks,
        security_groups, block_device_mapping, legacy_bdm=True):

        cctxt = self.client.prepare(version=version)

        cctxt.cast(context, 'build_instances', **kw)

跳轉到 nova/conductor/manager.py

class ComputeTaskManager(base.Base):

"""Namespace for compute methods.
This class presents an rpc API for nova-conductor under the 'compute_task'
namespace.  The methods here are compute operations that are invoked
by the API service.  These methods see the operation to completion, which
may involve coordinating activities on multiple compute nodes.
"""

.............

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

...............................

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'])

跳轉到 nova/compute/rpcapi.py

class ComputeAPI(object):
'''Client side of the compute rpc API.

..........................................

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):

    version = '4.0'
cctxt = self.router.by_host(ctxt, host).prepare(
server=host, version=version)
cctxt.cast(ctxt, 'build_and_run_instance', 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)

跳轉到 nova/compute/manager.py

class ComputeManager(manager.Manager):

"""Manages the running instances from creation to destruction."""

....................................

def build_and_run_instance(self, context, instance, 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):

def _locked_do_build_and_run_instance(*args, **kwargs):

    #NOTE(danms): We grab the semaphore with the instance uuid

        #locked because we could wait in line to build this instance
        # for a while and we want to make sure that nothing else tries
        # to do anything with this instance while we wait.

with self._build_semaphore:

        self._do_build_and_run_instance(*args, **kwargs)

utils.spawn_n(_locked_do_build_and_run_instance,
              context, instance, image, request_spec,
              filter_properties, admin_password, injected_files,
              requested_networks, security_groups,
              block_device_mapping, node, limits)

跳轉到本類的_do_build_and_run_instance（）方法

def _do_build_and_run_instance(self, context, instance, image,
request_spec, filter_properties, admin_password, injected_files,
        requested_networks, security_groups, block_device_mapping,
        node=None, limits=None):

self._build_and_run_instance(context, instance, image,
        decoded_files, admin_password, requested_networks,
        security_groups, block_device_mapping, node, limits,
        filter_properties)

跳轉到本類中_build_and_run_instance() 方法

def _build_and_run_instance(self, context, instance, image, injected_files,
        admin_password, requested_networks, security_groups,
        block_device_mapping, node, limits, filter_properties):

with timeutils.StopWatch() as timer:
self.driver.spawn(context, instance, image_meta,
                      injected_files, admin_password,
network_info=network_info,
block_device_info=block_device_info)

然後呼叫具體的驅動來建立虛擬機器