接触Openstack也有一段时间了，因为工作需要着重阅读了Glance、Nova、Cinder模块源码并通过搭建的devstack测试环境调试学习相关操作的执行流程。现准备陆续将相关的学习成果和心得在博客中分享出来，希望对读者有所帮助。这是第一篇 - 云主机启动过程源码分析 - 介绍云主机启动时，接口调用在nova-api、nova-conductor、nova-sechduler及nova-compute等组件上的流转过程。
既可以通过Dashboard启动云主机，也可以通过Nova CLI命令行实现，下面的命令启动了一台类型为2（--flavor 2）名为vm2的云主机，使用的镜像是：226bc6e5-60d7-4a2c-bf0d-a568a1e26e00 ，--debug参数用打开调试信息：
nova --debug boot --flavor 2 --image 226bc6e5-60d7-4a2c-bf0d-a568a1e26e00 vm2nova-api
启动云主机过程中，nova-api 主要完成输入参数的验证和装配，之后根据输入参数创建云主机实例对象、记录数据库条目，最后将请求转发给nova-conductor 进行后续处理。简单分析如下：
根据nova-api启动过程中建立的路由映射以及调试信息的佐证，如下：
我们知道启动云主机的请求经过novaclient的处理后，发送给了nova-api，并由nova/api/openstack/servers.py.ServersController.create 处理，从代码上看该方法的处理过程比较简单，主要是完成：用户参数的转换、policy验证，方法声明如下：
@wsgi.response(202)
@extensions.expected_errors((400, 403, 409, 413))
@validation.schema(schema_server_create_v20, '2.0', '2.0')
@validation.schema(schema_server_create, '2.1')
def create（self，req，body）：来看看body参数的内容：
body参数是该次请求的请求体，显示的是待启动云主机的配置参数，包含有：云主机名、image uuid，云主机类型等，很明显这就是之前在nova boot 命令行中指定的参数，之后根据extension 扩展方法转换用户参数，完成参数解析，policy认证等，之后将请求转发给nova/compute/api.py.API.create 。
API.create方法虽然参数很多，但启动云主机必须的参数只有instance_type及image_href ，分别指定主机类型及image uuid，处理逻辑也非常的简单：执行网络及块设备policy检查(如果有)，接着将参数原封不动的传递给_create_instance 方法，该方法主要完成如下工作：
- 验证输入参数
- 获取镜像元信息
- 生成主机配置并创建主机实例对象
- 在数据库生成主机信息
下面具体来看看该方法的实现（说明见红色注释部分）：
//nova/compute/api.py.API
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,
reservation_id=None, scheduler_hints=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.
"""
# Normalize and setup some parameters
if reservation_id is None:
reservation_id = utils.generate_uid('r')
security_groups = security_groups or ['default']
min_count = min_count or 1
max_count = max_count or min_count
block_device_mapping = block_device_mapping or []
if not instance_type:
instance_type = flavors.get_default_flavor()
#根据uuid获取image的metadata信息
if image_href:
image_id, boot_meta = self._get_image(context, image_href)
else:
image_id = None
boot_meta = self._get_bdm_image_metadata(
context, block_device_mapping, legacy_bdm)
self._check_auto_disk_config(image=boot_meta,
auto_disk_config=auto_disk_config)
handle_az = self._handle_availability_zone
availability_zone, forced_host, forced_node = handle_az(context,
availability_zone)
if not self.skip_policy_check and (forced_host or forced_node):
check_policy(context, 'create:forced_host', {})
#根据输入参数，生成主机配置
base_options, max_net_count = self._validate_and_build_base_options(
context,
instance_type, boot_meta, image_href, image_id, kernel_id,
ramdisk_id, display_name, display_description,
key_name, key_data, security_groups, availability_zone,
forced_host, user_data, metadata, access_ip_v4,
access_ip_v6, requested_networks, config_drive,
auto_disk_config, reservation_id, max_count)
# max_net_count is the maximum number of instances requested by the
# user adjusted for any network quota constraints, including
# consideration of connections to each requested network
if max_net_count == 0:
raise exception.PortLimitExceeded()
elif max_net_count < max_count:
LOG.debug("max count reduced from %(max_count)d to "
"%(max_net_count)d due to network port quota",
{'max_count': max_count,
'max_net_count': max_net_count})
max_count = max_net_count
#归总用户输入(--block-device-mapping参数)、镜像属性中指定、flavor配置中指定的块设备映射
block_device_mapping = self._check_and_transform_bdm(context,
base_options, instance_type, boot_meta, min_count, max_count,
block_device_mapping, legacy_bdm)
# We can't do this check earlier because we need bdms from all sources
# to have been merged in order to get the root bdm.
self._checks_for_create_and_rebuild(context, image_id, boot_meta,
instance_type, metadata, injected_files,
block_device_mapping.root_bdm())
instance_group = self._get_requested_instance_group(context,
scheduler_hints, check_server_group_quota)
#创建云主机实例对象，并生成数据库条目
instances = self._provision_instances(context, instance_type,
min_count, max_count, base_options, boot_meta, security_groups,
block_device_mapping, shutdown_terminate,
instance_group, check_server_group_quota)
#scheduler需要用的过滤选项
filter_properties = self._build_filter_properties(context,
scheduler_hints, forced_host,
forced_node, instance_type)
#更新数据库中实例的启动状态
for instance in instances:
self._record_action_start(context, instance,
instance_actions.CREATE)
#调用conductor api，通过conductor rpc将请求转发给conductor manager
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)接着上面的分析，nova/conductor/api.py.ComputeTaskAPI.build_instance 直接将请求转发给nova/conductor/rpcapi.py.ComputeTaskAPI.build_instance，来看看该函数的实现（请根据注解阅读）：
//nova/conductor/rpcapi.py.ComputeTaskAPI
def build_instances(self, context, instances, image,
filter_properties,
admin_password, injected_files, requested_networks,
security_groups, block_device_mapping, legacy_bdm=True):
'''
该函数的逻辑清晰简单：根据rpc client的版本，调整云主机参数：主机过滤参数，网络参数，块设备映射参数，最后通过一个异步rpc调用，将请求转发给conductor-manager
'''
image_p = jsonutils.to_primitive(image)
version = '1.10'
if not self.client.can_send_version(version):
version = '1.9'
if 'instance_type' in filter_properties:
flavor = filter_properties['instance_type']
flavor_p = objects_base.obj_to_primitive(flavor)
filter_properties = dict(filter_properties,
instance_type=flavor_p)
kw = {'instances': instances, 'image': image_p,
'filter_properties': filter_properties,
'admin_password': admin_password,
'injected_files': injected_files,
'requested_networks': requested_networks,
'security_groups': security_groups}
if not self.client.can_send_version(version):
version = '1.8'
kw['requested_networks'] =
kw['requested_networks'].as_tuples()
if not self.client.can_send_version('1.7'):
version = '1.5'
bdm_p =
objects_base.obj_to_primitive(block_device_mapping)
kw.update({'block_device_mapping': bdm_p,
'legacy_bdm': legacy_bdm})
#返回_CallContext对象
cctxt = self.client.prepare(version=version)
#发送异步rpc消息到消息队列，conductor-manager会收到该消息
cctxt.cast(context, 'build_instances', **kw)至此，nova-api的任务就完成了，下一篇文章分析conductor-manage的处理过程。