做个一个没怎么学过python的菜鸟，看源码是最好的学习方式了，现在就从nova入手，主要注释一下 nova/compute/api.py 中的 create_instance函数
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_group,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, create_instance_here=False,scheduler_hints=None):"""Verify all the input parameters regardless of the provisioningstrategy being performed and schedule the instance(s) forcreation."""if not metadata:metadata = {}if not display_description:display_description = ''if not security_group:security_group = 'default'if not instance_type:instance_type = instance_types.get_default_instance_type()if not min_count:min_count = 1if not max_count:max_count = min_countif not metadata:metadata = {}block_device_mapping = block_device_mapping or []#从quota得到具体的实例限制num_instances = quota.allowed_instances(context, max_count,instance_type)if num_instances < min_count:pid = context.project_idif num_instances <= 0:msg = _("Cannot run any more instances of this type.")else:msg = (_("Can only run %s more instances of this type.") %num_instances)LOG.warn(_("Quota exceeded for %(pid)s,"" tried to run %(min_count)s instances. " + msg) % locals())raise exception.QuotaError(code="InstanceLimitExceeded")#检查元数据、注入文件、网络self._check_metadata_properties_quota(context, metadata)self._check_injected_file_quota(context, injected_files)self._check_requested_networks(context, requested_networks)(image_service, image_id) = nova.image.get_image_service(context,image_href)#通过镜像id，，访问镜像的具体服务。得到一个image字典。image = image_service.show(context, image_id)#iamge 是一个字典，返回key='min_ram'对应的值， get是字典的一个方法。#memory_mb：是虚拟机所属内存。if instance_type['memory_mb'] < int(image.get('min_ram') or 0):raise exception.InstanceTypeMemoryTooSmall()#root_gb ：虚拟机根硬盘大小；instance_type代表了创建虚拟机的需求或者说配置，image则是实际上存在镜像的一个东西。if instance_type['root_gb'] < int(image.get('min_disk') or 0):raise exception.InstanceTypeDiskTooSmall()config_drive_id = Noneif config_drive and config_drive is not True:# config_drive is volume idconfig_drive, config_drive_id = None, config_driveos_type = None#properties是字典image的一个属性，image[properties]又是一个字典。if 'properties' in image and 'os_type' in image['properties']:os_type = image['properties']['os_type']architecture = Noneif 'properties' in image and 'arch' in image['properties']:architecture = image['properties']['arch']vm_mode = Noneif 'properties' in image and 'vm_mode' in image['properties']:vm_mode = image['properties']['vm_mode']# If instance doesn't have auto_disk_config overridden by request, use# whatever the image indicatesif auto_disk_config is None:if ('properties' in image and'auto_disk_config' in image['properties']):#bool_from_str:将字符串转化为数组auto_disk_config = utils.bool_from_str(image['properties']['auto_disk_config'])if kernel_id is None:kernel_id = image['properties'].get('kernel_id', None)if ramdisk_id is None:ramdisk_id = image['properties'].get('ramdisk_id', None)# FIXME(sirp): is there a way we can remove null_kernel?# No kernel and ramdisk for raw imagesif kernel_id == str(FLAGS.null_kernel):kernel_id = Noneramdisk_id = NoneLOG.debug(_("Creating a raw instance"))# Make sure we have access to kernel and ramdisk (if not raw)#locals() 返回一个名字/值对的字典LOG.debug(_("Using Kernel=%(kernel_id)s, Ramdisk=%(ramdisk_id)s")% locals())#show具体是干嘛的？？？？if kernel_id:image_service.show(context, kernel_id)if ramdisk_id:image_service.show(context, ramdisk_id)if config_drive_id:image_service.show(context, config_drive_id)#检查是否具有安全防火墙，如果不具有就创建一个默认的安全组。self.ensure_default_security_group(context)#ssh 密钥文件名存在，但是数据不存在。if key_data is None and key_name:key_pair = self.db.key_pair_get(context, context.user_id, key_name)key_data = key_pair['public_key']if reservation_id is None:reservation_id = utils.generate_uid('r')#根设备名root_device_name = block_device.properties_root_device_name(image['properties'])# NOTE(vish): We have a legacy hack to allow admins to specify hosts#via az using az:host. It might be nice to expose an#api to specify specific hosts to force onto, but for#now it just supports this legacy hack.host = None#parition 函数进行分割字符串，如果分割成功，返回tuple,中间的为分隔符；如果找不到，返回1个有用字符串#剩余两个元素为空。#下边主要功能是设置调度集群。if availability_zone:availability_zone, _x, host = availability_zone.partition(':')if not availability_zone:availability_zone = FLAGS.default_schedule_zoneif context.is_admin and host:filter_properties = {'force_hosts': [host]}else:filter_properties = {}filter_properties['scheduler_hints'] = scheduler_hintsbase_options = {'reservation_id': reservation_id,'image_ref': image_href,'kernel_id': kernel_id or '','ramdisk_id': ramdisk_id or '','power_state': power_state.NOSTATE,'vm_state': vm_states.BUILDING, #注意刚开始是创建中状态。'config_drive_id': config_drive_id or '','config_drive': config_drive or '','user_id': context.user_id,'project_id': context.project_id,'launch_time': time.strftime('%Y-%m-%dT%H:%M:%SZ', time.gmtime()),#格式化一个时间字符串。'instance_type_id': instance_type['id'],#虚拟机的套餐类型。'memory_mb': instance_type['memory_mb'],#虚拟机的内存'vcpus': instance_type['vcpus'],#虚拟机的cpu核数。'root_gb': instance_type['root_gb'],#虚机根硬盘大小'ephemeral_gb': instance_type['ephemeral_gb'],'display_name': display_name,'display_description': display_description,'user_data': user_data or '','key_name': key_name,#密钥文件名'key_data': key_data,#密钥数据'locked': False,'metadata': metadata,'access_ip_v4': access_ip_v4,'access_ip_v6': access_ip_v6,'availability_zone': availability_zone,'os_type': os_type,#操作系统类型。'architecture': architecture,'vm_mode': vm_mode,#虚机状态。'root_device_name': root_device_name,#根设备名称。'progress': 0,'auto_disk_config': auto_disk_config}LOG.debug(_("Going to run %s instances…") % num_instances)if create_instance_here:instance = self.create_db_entry_for_new_instance(context, instance_type, image, base_options,security_group, block_device_mapping)# Tells scheduler we created the instance already.base_options['uuid'] = instance['uuid']#cast是单向的不需要等待对方回复即可返回。rpc_method = rpc.castelse:# We need to wait for the scheduler to create the instance# DB entries, because the instance *could* be # created in# a child zone.#我的理解：该实例不一定在这个集群创建，可能去子集群，因为可能有很多个吧#因此需要等待他的创建完成。rpc_method = rpc.call# TODO(comstud): We should use rpc.multicall when we can# retrieve the full instance dictionary from the scheduler.# Otherwise, we could exceed the AMQP max message size limit.# This would require the schedulers' schedule_run_instances# methods to return an iterator vs a list.#这里应该是调度器发消息了，让调度器去通知具体节点的nova-compute进程去创建虚机。instances = self._schedule_run_instance(rpc_method,context, base_options,instance_type,availability_zone, injected_files,admin_password, image,num_instances, requested_networks,block_device_mapping, security_group,filter_properties)#这里创建完毕，返回instance实例。if create_instance_here:return ([instance], reservation_id)return (instances, reservation_id)
下边这个是给调度器发送instance创建消息的函数：_schedule_run_instance
以前我是个爱仰望天空的人，苍蓝的天空总是给我求生的勇气，
