做个一个没怎么学过python的菜鸟。看源代码是最好的学习方式了，如今就从nova入手，主要凝视一下 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 provisioning
        strategy being performed and schedule the instance(s) for
        creation."""

        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 = 1
        if not max_count:
            max_count = min_count
        if not metadata:
            metadata = {}

        block_device_mapping = block_device_mapping or []

		#从quota得到详细的实例限制
        num_instances = quota.allowed_instances(context, max_count,
                                                instance_type)
        if num_instances < min_count:
            pid = context.project_id
            if 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。訪问镜像的详细服务。
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'相应的值， 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 = None
        if config_drive and config_drive is not True:
            # config_drive is volume id
            config_drive, config_drive_id = None, config_drive

        os_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 = None
        if 'properties' in image and 'arch' in image['properties']:
            architecture = image['properties']['arch']
        vm_mode = None
        if '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 indicates
        if 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 images
        if kernel_id == str(FLAGS.null_kernel):
            kernel_id = None
            ramdisk_id = None
            LOG.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详细是干嘛的？？？？
        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。假设不具有就创建一个默认的安全组。
        self.ensure_default_security_group(context)

		#ssh 密钥文件名称存在。可是数据不存在。

        if key_data is None and key_name:
            key_pair = self.db.key_pair_get(context, context.user_id, key_name)
            key_data = http://www.mamicode.com/key_pair['public_key']>

            '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,#密钥文件名称
            '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是单向的不须要等待对方回复就可以返回。
            rpc_method = rpc.cast
        else:
            # 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。由于可能有非常多个吧
			#因此须要等待他的创建完毕。

            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，让调度器去通知详细节点的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

	#create_instance 调用，发送消息给调度器
    def _schedule_run_instance(self,
            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):
        """Send a run_instance request to the schedulers for processing."""
		
		#pid不是进程id，是项目id.....
        pid = context.project_id
        uid = context.user_id

        LOG.debug(_("Sending create to scheduler for %(pid)s/%(uid)s's") %
                locals())

        request_spec = {
            'image': utils.to_primitive(image),
            'instance_properties': base_options,
            'instance_type': instance_type,
            'num_instances': num_instances,
            'block_device_mapping': block_device_mapping,
            'security_group': security_group,
        }

        return rpc_method(context,
                FLAGS.scheduler_topic,
                {"method": "run_instance",
                 "args": {"topic": FLAGS.compute_topic,
                          "request_spec": request_spec,
                          "admin_password": admin_password,
                          "injected_files": injected_files,
                          "requested_networks": requested_networks,
                          "is_first_time": True,
                          "filter_properties": filter_properties}})

加油！！。！