#/nova/virt/libvirt/driver.py:LibvirtDriver
    # NOTE(ilyaalekseyev): Implementation like in multinics
    # for xenapi(tr3buchet)
    def spawn(self, context, instance, image_meta, injected_files,
              admin_password, network_info=None, block_device_info=None):
        disk_info = blockinfo.get_disk_info(CONF.libvirt.virt_type,
                                            instance,
                                            image_meta,
                                            block_device_info)
        self._create_image(context, instance,
                           disk_info['mapping'],
                           network_info=network_info,
                           block_device_info=block_device_info,
                           files=injected_files,
                           admin_pass=admin_password)
        xml = self._get_guest_xml(context, instance, network_info,
                                  disk_info, image_meta,
                                  block_device_info=block_device_info,
                                  write_to_disk=True)
        self._create_domain_and_network(context, xml, instance, network_info,
                                        disk_info,
                                        block_device_info=block_device_info)
        LOG.debug("Instance is running", instance=instance)

        def _wait_for_boot():
            """Called at an interval until the VM is running."""
            state = self.get_info(instance).state

            if state == power_state.RUNNING:
                LOG.info(_LI("Instance spawned successfully."),
                         instance=instance)
                raise loopingcall.LoopingCallDone()

        timer = loopingcall.FixedIntervalLoopingCall(_wait_for_boot)
        timer.start(interval=0.5).wait()
 

我們這裡主要分析xml形成過程中的tap裝置的寫入以及_create_domain_and_network函式中與network有關的操作。

#/nova/virt/libvirt/driver.py:LibvirtDriver
    def _get_guest_xml(self, context, instance, network_info, disk_info,
                       image_meta, rescue=None,
                       block_device_info=None, write_to_disk=False):
        # NOTE(danms): Stringifying a NetworkInfo will take a lock. Do
        # this ahead of time so that we don't acquire it while also
        # holding the logging lock.
        network_info_str = str(network_info)
        msg = ('Start _get_guest_xml '
               'network_info=%(network_info)s '
               'disk_info=%(disk_info)s '
               'image_meta=%(image_meta)s rescue=%(rescue)s '
               'block_device_info=%(block_device_info)s' %
               {'network_info': network_info_str, 'disk_info': disk_info,
                'image_meta': image_meta, 'rescue': rescue,
                'block_device_info': block_device_info})
        # NOTE(mriedem): block_device_info can contain auth_password so we
        # need to sanitize the password in the message.
        LOG.debug(strutils.mask_password(msg), instance=instance)
        conf = self._get_guest_config(instance, network_info, image_meta,
                                      disk_info, rescue, block_device_info,
                                      context)
        xml = conf.to_xml()

        if write_to_disk:
            instance_dir = libvirt_utils.get_instance_path(instance)
            xml_path = os.path.join(instance_dir, 'libvirt.xml')
            libvirt_utils.write_to_file(xml_path, xml)

        LOG.debug('End _get_guest_xml xml=%(xml)s',
                  {'xml': xml}, instance=instance)
        return xml

#/nova/virt/libvirt/driver.py:LibvirtDriver
    def _get_guest_config(self, instance, network_info, image_meta,
                          disk_info, rescue=None, block_device_info=None,
                          context=None):
        """Get config data for parameters.

        :param rescue: optional dictionary that should contain the key
            'ramdisk_id' if a ramdisk is needed for the rescue image and
            'kernel_id' if a kernel is needed for the rescue image.
        """
        flavor = instance.flavor
        inst_path = libvirt_utils.get_instance_path(instance)
        disk_mapping = disk_info['mapping']
        img_meta_prop = image_meta.get('properties', {}) if image_meta else {}

        virt_type = CONF.libvirt.virt_type

              ... ... ...

        for config in storage_configs:
            guest.add_device(config)

        for vif in network_info:
            config = self.vif_driver.get_config(
                instance, vif, image_meta,
                flavor, virt_type)
            guest.add_device(config)

        ... ... ...

        # Memory balloon device only support 'qemu/kvm' and 'xen' hypervisor
        if (virt_type in ('xen', 'qemu', 'kvm') and
                CONF.libvirt.mem_stats_period_seconds > 0):
            balloon = vconfig.LibvirtConfigMemoryBalloon()
            if virt_type in ('qemu', 'kvm'):
                balloon.model = 'virtio'
            else:
                balloon.model = 'xen'
            balloon.period = CONF.libvirt.mem_stats_period_seconds
            guest.add_device(balloon)

        return guest
 

這裡tap字元設定寫到xml中的程式碼為self.vif_driver.get_config函式。

#/nova/virt/libvirt/vif.py:LibvirtGenericVIFDriver
    def get_config(self, instance, vif, image_meta,
                   inst_type, virt_type):
        vif_type = vif['type']

        LOG.debug('vif_type=%(vif_type)s instance=%(instance)s '
                  'vif=%(vif)s virt_type%(virt_type)s',
                  {'vif_type': vif_type, 'instance': instance,
                   'vif': vif, 'virt_type': virt_type})

        if vif_type is None:
            raise exception.NovaException(
                _("vif_type parameter must be present "
                  "for this vif_driver implementation"))
        vif_slug = self._normalize_vif_type(vif_type)
        func = getattr(self, 'get_config_%s' % vif_slug, None)
        if not func:
            raise exception.NovaException(
                _("Unexpected vif_type=%s") % vif_type)
        return func(instance, vif, image_meta,
                    inst_type, virt_type)
 

這裡vif_type為bridge，因為neutron採用的mechanism driver為linuxbridge，所以get_config中獲取的func為get_config_bridge函式。

#/nova/virt/libvirt/vif.py:LibvirtGenericVIFDriver
    def get_config_bridge(self, instance, vif, image_meta,
                          inst_type, virt_type):
        """Get VIF configurations for bridge type."""
        conf = self.get_base_config(instance, vif, image_meta,
                                    inst_type, virt_type)

        designer.set_vif_host_backend_bridge_config(
            conf, self.get_bridge_name(vif),
            self.get_vif_devname(vif))

        mac_id = vif['address'].replace(':', '')
        name = "nova-instance-" + instance.name + "-" + mac_id
        if self.get_firewall_required(vif):
            conf.filtername = name
        designer.set_vif_bandwidth_config(conf, inst_type)

        return conf

#/nova/virt/libvirt/vif.py:LibvirtGenericVIFDriver
    def get_vif_devname(self, vif):
        if 'devname' in vif:
            return vif['devname']
        return ("nic" + vif['id'])[:network_model.NIC_NAME_LEN]

#/nova/virt/libvirt/vif.py:LibvirtGenericVIFDriver
    def get_bridge_name(self, vif):
        return vif['network']['bridge']

這裡get_config_bridge函式利用neutron返回的port資訊，構建vconfig.LibvirtConfigGuestInterface()物件所需的資訊，最終形成的與network有關的xml資訊為：

目前tap裝置的xml資訊已經形成(但還未實際被建立)，那麼tap裝置的實際建立應該在_create_domain_and_network函式中。

#/nova/virt/libvirt/driver.py:LibvirtDriver
    def _create_domain_and_network(self, context, xml, instance, network_info,
                                   disk_info, block_device_info=None,
                                   power_on=True, reboot=False,
                                   vifs_already_plugged=False):

        """Do required network setup and create domain."""
        block_device_mapping = driver.block_device_info_get_mapping(
            block_device_info)
        image_meta = utils.get_image_from_system_metadata(
            instance.system_metadata)

        for vol in block_device_mapping:
            connection_info = vol['connection_info']

            if (not reboot and 'data' in connection_info and
                    'volume_id' in connection_info['data']):
                volume_id = connection_info['data']['volume_id']
                encryption = encryptors.get_encryption_metadata(
                    context, self._volume_api, volume_id, connection_info)

                if encryption:
                    encryptor = self._get_volume_encryptor(connection_info,
                                                           encryption)
                    encryptor.attach_volume(context, **encryption)

        timeout = CONF.vif_plugging_timeout
        if (self._conn_supports_start_paused and
            utils.is_neutron() and not
            vifs_already_plugged and power_on and timeout):
            events = self._get_neutron_events(network_info)
        else:
            events = []

        launch_flags = events and libvirt.VIR_DOMAIN_START_PAUSED or 0
        domain = None
        try:
            with self.virtapi.wait_for_instance_event(
                    instance, events, deadline=timeout,
                    error_callback=self._neutron_failed_callback):
                self.plug_vifs(instance, network_info)
                self.firewall_driver.setup_basic_filtering(instance,
                                                           network_info)
                self.firewall_driver.prepare_instance_filter(instance,
                                                             network_info)
                with self._lxc_disk_handler(instance, image_meta,
                                            block_device_info, disk_info):
                    domain = self._create_domain(
                        xml, instance=instance,
                        launch_flags=launch_flags,
                        power_on=power_on)

                self.firewall_driver.apply_instance_filter(instance,
                                                           network_info)
        except exception.VirtualInterfaceCreateException:
            # Neutron reported failure and we didn't swallow it, so
            # bail here
            with excutils.save_and_reraise_exception():
                if domain:
                    domain.destroy()
                self.cleanup(context, instance, network_info=network_info,
                             block_device_info=block_device_info)
        except eventlet.timeout.Timeout:
            # We never heard from Neutron
            LOG.warn(_LW('Timeout waiting for vif plugging callback for '
                         'instance %(uuid)s'), {'uuid': instance.uuid})
            if CONF.vif_plugging_is_fatal:
                if domain:
                    domain.destroy()
                self.cleanup(context, instance, network_info=network_info,
                             block_device_info=block_device_info)
                raise exception.VirtualInterfaceCreateException()

        # Resume only if domain has been paused
        if launch_flags & libvirt.VIR_DOMAIN_START_PAUSED:
            domain.resume()
        return domain

對於_create_domain_and_network函式，我們主要分析nova等待event 事件 (與neutron服務相關，即nova-event-callback機制，其blueprints連結為：https://blueprints.launchpad.net/neutron/+spec/nova-event-callback)

其程式碼流程為：在_create_domain_and_network函式的self.virtapi.wait_for_instance_event程式碼會建立一個定時器，等待neutron傳遞過來event事件，那麼怎麼才會觸發neutron傳送一個event給nova呢？

當nova呼叫底層libvirt介面建立VM時，將會根據xml資訊去建立，此時會建立一個真實的tap裝置(不是建立port資訊到neutron資料庫)，而有nova-compute服務的host的mechanism driver對應的agent會定時的檢測device的狀態(update或delete等)，對於mechanismdriver為linuxbridge的agent的預設定時檢測時間為2s，當neutron-linuxbridge-agent服務(因為本環境採用的mechanism driver為linuxbridge)檢測到有新的tap裝置被增加，則會對該port設定相應的security group rule(利用linux系統自帶的iptables去設定)，然後更新neutron資料庫中相應的表資訊。

而neutron資料庫中的ports表在neutron-server服務啟動時建立Ml2Plugin物件時，便被sqlalchemy自帶的event進行監聽，當ports表status欄位被set時，neutron便會為nova準備event事件(並未傳送)，待port表被after_update或after_insert後，neutron則會將準備好的event事件傳送給nova。

neutron傳送event事件給nova採用HTTP請求方式，且HTTP請求傳送到nova-api的extension resource上，最終通過rpc呼叫nova-compute將收到event事件，當nova-compute收到event事件後，表示port相關資訊準備完畢，nova結束等待。此時待VM處於running狀態，表示VM建立完成。

首先，nova側獲取需要等待的event事件

#/nova/virt/libvirt/driver.py:LibvirtDriver
    def _get_neutron_events(self, network_info):
        # NOTE(danms): We need to collect any VIFs that are currently
        # down that we expect a down->up event for. Anything that is
        # already up will not undergo that transition, and for
        # anything that might be stale (cache-wise) assume it's
        # already up so we don't block on it.
        return [('network-vif-plugged', vif['id'])
                for vif in network_info if vif.get('active', True) is False]

這裡，遍歷從neutron中獲得的network_info的vif中key為active的值，如果有任何一個vif的active的值為False，則需等待network-vif-pluggend事件。

下面看看nova側的self.virtapi.wait_for_instance_event等待event的程式碼。

#/nova/compute/manager.py:ComputeVirtAPI
    @contextlib.contextmanager
    def wait_for_instance_event(self, instance, event_names, deadline=300,
                                error_callback=None):
        """Plan to wait for some events, run some code, then wait.

        This context manager will first create plans to wait for the
        provided event_names, yield, and then wait for all the scheduled
        events to complete.

        Note that this uses an eventlet.timeout.Timeout to bound the
        operation, so callers should be prepared to catch that
        failure and handle that situation appropriately.

        If the event is not received by the specified timeout deadline,
        eventlet.timeout.Timeout is raised.

        If the event is received but did not have a 'completed'
        status, a NovaException is raised.  If an error_callback is
        provided, instead of raising an exception as detailed above
        for the failure case, the callback will be called with the
        event_name and instance, and can return True to continue
        waiting for the rest of the events, False to stop processing,
        or raise an exception which will bubble up to the waiter.

        :param instance: The instance for which an event is expected
        :param event_names: A list of event names. Each element can be a
                            string event name or tuple of strings to
                            indicate (name, tag).
        :param deadline: Maximum number of seconds we should wait for all
                         of the specified events to arrive.
        :param error_callback: A function to be called if an event arrives

        """

        if error_callback is None:
            error_callback = self._default_error_callback
        events = {}
        for event_name in event_names:
            if isinstance(event_name, tuple):
                name, tag = event_name
                event_name = objects.InstanceExternalEvent.make_key(
                    name, tag)
            try:
                events[event_name] = (
                    self._compute.instance_events.prepare_for_instance_event(
                        instance, event_name))
            except exception.NovaException:
                error_callback(event_name, instance)
                # NOTE(danms): Don't wait for any of the events. They
                # should all be canceled and fired immediately below,
                # but don't stick around if not.
                deadline = 0
        yield
        with eventlet.timeout.Timeout(deadline):
            for event_name, event in events.items():
                actual_event = event.wait()
                if actual_event.status == 'completed':
                    continue
                decision = error_callback(event_name, instance)
                if decision is False:
                    break

在wait_for_instance_event函式被執行with開始時，將首先執行wait_for_instance_event函式yield之前的程式碼(具體檢視python的@contextlib.contextmanager特性)，而yield之前的程式碼即為構造等待的event事件。待wait_for_instance_event函式被執行with內部的程式碼結束時，執行wait_for_instance_event函式yield之後的程式碼，即等待neutron傳送nova所需的event事件。其等待時間為/etc/nova/nova.conf配置檔案中的vif_plugging_timeout引數值，其預設值為300s。

如此，VM便開始呼叫底層libvirt的介面建立VM，同時nova側去等待neutron側傳送network-vif-pluggend事件。

下面我們轉到neutron側，分析neutron-linuxbridge-agent服務如何檢測tap裝置以及neutron-server如何傳送event事件給nova-api。

注意：閱讀下面內容之前，先大致瀏覽一下我的《neutron-server的啟動流程》的文章。

首先，我們需要找到neutron-linuxbridge-agent的入口。即檢視setup.cfg檔案。

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py
def main():
    common_config.init(sys.argv[1:])

    common_config.setup_logging()
    try:
        interface_mappings = q_utils.parse_mappings(
            cfg.CONF.LINUX_BRIDGE.physical_interface_mappings)
    except ValueError as e:
        LOG.error(_LE("Parsing physical_interface_mappings failed: %s. "
                      "Agent terminated!"), e)
        sys.exit(1)
    LOG.info(_LI("Interface mappings: %s"), interface_mappings)

    polling_interval = cfg.CONF.AGENT.polling_interval
    agent = LinuxBridgeNeutronAgentRPC(interface_mappings,
                                       polling_interval)
    LOG.info(_LI("Agent initialized successfully, now running... "))
    agent.daemon_loop()
    sys.exit(0)


if __name__ == "__main__":
    main()

這裡首先解析/etc/neutron/plugins/linuxbridge/linuxbridge_conf.ini配置檔案中的physical_interface_mappings引數。我的OpenStack環境設定的該引數如下。

這裡physnet1用作資料網路，physnet2用作外部網路。

#/neutron/common/utils.py
def parse_mappings(mapping_list, unique_values=True):
    """Parse a list of mapping strings into a dictionary.

    :param mapping_list: a list of strings of the form '<key>:<value>'
    :param unique_values: values must be unique if True
    :returns: a dict mapping keys to values
    """
    mappings = {}
    for mapping in mapping_list:
        mapping = mapping.strip()
        if not mapping:
            continue
        split_result = mapping.split(':')
        if len(split_result) != 2:
            raise ValueError(_("Invalid mapping: '%s'") % mapping)
        key = split_result[0].strip()
        if not key:
            raise ValueError(_("Missing key in mapping: '%s'") % mapping)
        value = split_result[1].strip()
        if not value:
            raise ValueError(_("Missing value in mapping: '%s'") % mapping)
        if key in mappings:
            raise ValueError(_("Key %(key)s in mapping: '%(mapping)s' not "
                               "unique") % {'key': key, 'mapping': mapping})
        if unique_values and value in mappings.itervalues():
            raise ValueError(_("Value %(value)s in mapping: '%(mapping)s' "
                               "not unique") % {'value': value,
                                                'mapping': mapping})
        mappings[key] = value
    return mappings

最終返回interface_mappings是一個字典，其中key為physnet1和physnet2，相對應的value為eth1和eth2。

繼續回到neutron-linuxbridge-agent的main函式。然後讀取/etc/neutron/plugins/linuxbridge/linuxbridge_conf.ini配置檔案中的polling_interval引數，用於建立LinuxBridgeNeutronAgentRPC物件。

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py:LinuxBridgeNeutronAgentRPC
class LinuxBridgeNeutronAgentRPC(object):

    def __init__(self, interface_mappings, polling_interval):
        self.polling_interval = polling_interval
        self.setup_linux_bridge(interface_mappings)
        configurations = {'interface_mappings': interface_mappings}
        if self.br_mgr.vxlan_mode != lconst.VXLAN_NONE:
            configurations['tunneling_ip'] = self.br_mgr.local_ip
            configurations['tunnel_types'] = [p_const.TYPE_VXLAN]
            configurations['l2_population'] = cfg.CONF.VXLAN.l2_population
        self.agent_state = {
            'binary': 'neutron-linuxbridge-agent',
            'host': cfg.CONF.host,
            'topic': constants.L2_AGENT_TOPIC,
            'configurations': configurations,
            'agent_type': constants.AGENT_TYPE_LINUXBRIDGE,
            'start_flag': True}

        # stores received port_updates for processing by the main loop
        self.updated_devices = set()
        self.context = context.get_admin_context_without_session()
        self.plugin_rpc = agent_rpc.PluginApi(topics.PLUGIN)
        self.sg_plugin_rpc = sg_rpc.SecurityGroupServerRpcApi(topics.PLUGIN)
        self.sg_agent = sg_rpc.SecurityGroupAgentRpc(self.context,
                self.sg_plugin_rpc)
        self.setup_rpc(interface_mappings.values())

self.setup_linux_bridge(interface_mappings)語句時建立一個LinuxBridgeManager物件。

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py:LinuxBridgeNeutronAgentRPC
    def setup_linux_bridge(self, interface_mappings):
        self.br_mgr = LinuxBridgeManager(interface_mappings)

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py:LinuxBridgeManager
class LinuxBridgeManager(object):
    def __init__(self, interface_mappings):
        self.interface_mappings = interface_mappings
        self.ip = ip_lib.IPWrapper()
        # VXLAN related parameters:
        self.local_ip = cfg.CONF.VXLAN.local_ip
        self.vxlan_mode = lconst.VXLAN_NONE
        if cfg.CONF.VXLAN.enable_vxlan:
            self.local_int = self.get_interface_by_ip(self.local_ip)
            if self.local_int:
                self.check_vxlan_support()
            else:
                LOG.warning(_LW('VXLAN is enabled, a valid local_ip '
                                'must be provided'))
        # Store network mapping to segments
        self.network_map = {}

這裡/etc/neutron/plugins/linuxbridge/linuxbridge_conf.ini配置檔案中的enable_vxlan引數值為False。且LinuxBridgeManager物件的vxlan_mode = lconst.VXLAN_NONE(即'not_supported')。

繼續回到LinuxBridgeNeutronAgentRPC類的初始化函式，後續的操作便是建立RPC-client和RPC-server，對於RPC-server(client)所對應的另一端RPC-client(server)請檢視《neutron-server的啟動流程(二)》，這裡簡要分析self.setup_rpc(interface_mappings.values())語句執行流程。

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py:LinuxBridgeNeutronAgentRPC
    def setup_rpc(self, physical_interfaces):
        if physical_interfaces:
            mac = utils.get_interface_mac(physical_interfaces[0])
        else:
            devices = ip_lib.IPWrapper().get_devices(True)
            if devices:
                mac = utils.get_interface_mac(devices[0].name)
            else:
                LOG.error(_LE("Unable to obtain MAC address for unique ID. "
                              "Agent terminated!"))
                exit(1)
        self.agent_id = '%s%s' % ('lb', (mac.replace(":", "")))
        LOG.info(_LI("RPC agent_id: %s"), self.agent_id)

        self.topic = topics.AGENT
        self.state_rpc = agent_rpc.PluginReportStateAPI(topics.PLUGIN)
        # RPC network init
        # Handle updates from service
        self.endpoints = [LinuxBridgeRpcCallbacks(self.context, self,
                                                  self.sg_agent)]
        # Define the listening consumers for the agent
        consumers = [[topics.PORT, topics.UPDATE],
                     [topics.NETWORK, topics.DELETE],
                     [topics.SECURITY_GROUP, topics.UPDATE]]
        if cfg.CONF.VXLAN.l2_population:
            consumers.append([topics.L2POPULATION,
                              topics.UPDATE, cfg.CONF.host])
        self.connection = agent_rpc.create_consumers(self.endpoints,
                                                     self.topic,
                                                     consumers)
        report_interval = cfg.CONF.AGENT.report_interval
        if report_interval:
            heartbeat = loopingcall.FixedIntervalLoopingCall(
                self._report_state)
            heartbeat.start(interval=report_interval)

這裡涉及到neutron-linuxbridge-agent服務的一個RPC-server的建立和一個上報neutron-linuxbridge-agent服務狀態(_report_state)的RPC-client的建立。

其中neutron-linuxbridge-agent服務的一個RPC-server的建立如下。

#/neutron/agent/rpc.py
def create_consumers(endpoints, prefix, topic_details, start_listening=True):
    """Create agent RPC consumers.

    :param endpoints: The list of endpoints to process the incoming messages.
    :param prefix: Common prefix for the plugin/agent message queues.
    :param topic_details: A list of topics. Each topic has a name, an
                          operation, and an optional host param keying the
                          subscription to topic.host for plugin calls.
    :param start_listening: if True, it starts the processing loop

    :returns: A common Connection.
    """

    connection = n_rpc.create_connection(new=True)
    for details in topic_details:
        topic, operation, node_name = itertools.islice(
            itertools.chain(details, [None]), 3)

        topic_name = topics.get_topic_name(prefix, topic, operation)
        connection.create_consumer(topic_name, endpoints, fanout=True)
        if node_name:
            node_topic_name = '%s.%s' % (topic_name, node_name)
            connection.create_consumer(node_topic_name,
                                       endpoints,
                                       fanout=False)
    if start_listening:
        connection.consume_in_threads()
    return connection

#/neutron/common/rpc.py:Connection
class Connection(object):

    def __init__(self):
        super(Connection, self).__init__()
        self.servers = []

    def create_consumer(self, topic, endpoints, fanout=False):
        target = oslo_messaging.Target(
            topic=topic, server=cfg.CONF.host, fanout=fanout)
        server = get_server(target, endpoints)
        self.servers.append(server)

    def consume_in_threads(self):
        for server in self.servers:
            server.start()
        return self.servers

    def close(self):
        for server in self.servers:
            server.stop()
        for server in self.servers:
            server.wait()

這裡注意一個python語法。

我們知道itertools.islice返回的是一個迭代器型別，需執行next方法或隱式執行next方法(如for)才能獲取所需的資訊。而這裡直接就獲取到我們所需的資訊了，是否有錯？

這裡沒有錯誤，在python語法中，實際上不僅僅只是元祖和列表，只要物件是可迭代的，那麼就可以執行分解操作(這裡就是分解操作)，這包括字串、檔案、迭代器以及生成器。

這裡/neutron/agent/rpc.py的create_consumers函式建立/neutron/common/rpc.py:Connection物件，呼叫Connection類的create_consumer函式建立RPC-server，通過consume_in_threads函式開啟RPC-server等到RPC-client的請求。

上報neutron-linuxbridge-agent服務狀態(_report_state)的RPC-client的建立如下。

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py:LinuxBridgeNeutronAgentRPC
    def _report_state(self):
        try:
            devices = len(self.br_mgr.get_tap_devices())
            self.agent_state.get('configurations')['devices'] = devices
            self.state_rpc.report_state(self.context,
                                        self.agent_state)
            self.agent_state.pop('start_flag', None)
        except Exception:
            LOG.exception(_LE("Failed reporting state!"))

其中上報狀態的時間間隔為30s(/etc/neutron/neutron.conf配置檔案的report_interval引數值)。對於迴圈週期執行_report_state函式的函式為loopingcall.FixedIntervalLoopingCall，具體如何執行請參考《nova-computePeriodic tasks 機制》。

目前LinuxBridgeNeutronAgentRPC物件的建立便分析完成。下面便是執行LinuxBridgeNeutronAgentRPC類的daemon_loop函式去迴圈(時間間隔polling_interval為2s)檢測tap的增加或刪除。

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py:LinuxBridgeNeutronAgentRPC
    def daemon_loop(self):
        LOG.info(_LI("LinuxBridge Agent RPC Daemon Started!"))
        device_info = None
        sync = True

        while True:
            start = time.time()

            device_info = self.scan_devices(previous=device_info, sync=sync)

            if sync:
                LOG.info(_LI("Agent out of sync with plugin!"))
                sync = False

            if self._device_info_has_changes(device_info):
                LOG.debug("Agent loop found changes! %s", device_info)
                try:
                    sync = self.process_network_devices(device_info)
                except Exception:
                    LOG.exception(_LE("Error in agent loop. Devices info: %s"),
                                  device_info)
                    sync = True

            # sleep till end of polling interval
            elapsed = (time.time() - start)
            if (elapsed < self.polling_interval):
                time.sleep(self.polling_interval - elapsed)
            else:
                LOG.debug("Loop iteration exceeded interval "
                          "(%(polling_interval)s vs. %(elapsed)s)!",
                          {'polling_interval': self.polling_interval,
                           'elapsed': elapsed})

首先是呼叫scan_devices函式來掃描host上的tap裝置。

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py:LinuxBridgeNeutronAgentRPC
    def scan_devices(self, previous, sync):
        device_info = {}

        # Save and reinitialise the set variable that the port_update RPC uses.
        # This should be thread-safe as the greenthread should not yield
        # between these two statements.
        updated_devices = self.updated_devices
        self.updated_devices = set()

        current_devices = self.br_mgr.get_tap_devices()
        device_info['current'] = current_devices

        if previous is None:
            # This is the first iteration of daemon_loop().
            previous = {'added': set(),
                        'current': set(),
                        'updated': set(),
                        'removed': set()}

        if sync:
            # This is the first iteration, or the previous one had a problem.
            # Re-add all existing devices.
            device_info['added'] = current_devices

            # Retry cleaning devices that may not have been cleaned properly.
            # And clean any that disappeared since the previous iteration.
            device_info['removed'] = (previous['removed'] | previous['current']
                                      - current_devices)

            # Retry updating devices that may not have been updated properly.
            # And any that were updated since the previous iteration.
            # Only update devices that currently exist.
            device_info['updated'] = (previous['updated'] | updated_devices
                                      & current_devices)
        else:
            device_info['added'] = current_devices - previous['current']
            device_info['removed'] = previous['current'] - current_devices
            device_info['updated'] = updated_devices & current_devices

        return device_info

將掃描的tap裝置資訊與上一次掃描的結果做比對，然記錄下對比結果返回。其中它是怎麼去掃描的呢？

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py:LinuxBridgeManager
    def get_tap_devices(self):
        devices = set()
        for device in os.listdir(BRIDGE_FS):
            if device.startswith(constants.TAP_DEVICE_PREFIX):
                devices.add(device)
        return devices

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py
BRIDGE_FS = "/sys/class/net/"

#/neutron/common/constants.py
# Device names start with "tap"
TAP_DEVICE_PREFIX = 'tap'

這裡是在nova-compute服務所在host的/sys/class/net/目錄下去查詢以‘tap’開頭的裝置。如

此時呼叫get_tap_devices函式將獲取最後3個以tap開頭的裝置。

這裡我們關注的是建立VM時，tap裝置增加的情況，所以這裡neutron-linuxbridge-agent服務將檢測到device發生變化，所以處理device裝置。

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py:LinuxBridgeNeutronAgentRPC
    def process_network_devices(self, device_info):
        resync_a = False
        resync_b = False

        self.sg_agent.prepare_devices_filter(device_info.get('added'))

        if device_info.get('updated'):
            self.sg_agent.refresh_firewall()

        # Updated devices are processed the same as new ones, as their
        # admin_state_up may have changed. The set union prevents duplicating
        # work when a device is new and updated in the same polling iteration.
        devices_added_updated = (set(device_info.get('added'))
                                 | set(device_info.get('updated')))
        if devices_added_updated:
            resync_a = self.treat_devices_added_updated(devices_added_updated)

        if device_info.get('removed'):
            resync_b = self.treat_devices_removed(device_info['removed'])
        # If one of the above operations fails => resync with plugin
        return (resync_a | resync_b)

在process_network_devices函式中，首先為新增的device增加security group rule，對於rule的制定的分析，我們在後續文章進行分析。因為本文分析的情況是device有增加，所以執行下面的函式。

#/neutron/plugins/linuxbridge/agent/linuxbridge_neutron_agent.py:LinuxBridgeNeutronAgentRPC
    def treat_devices_added_updated(self, devices):
        try:
            devices_details_list = self.plugin_rpc.get_devices_details_list(
                self.context, devices, self.agent_id)
        except Exception as e:
            LOG.debug("Unable to get port details for "
                      "%(devices)s: %(e)s",
                      {'devices': devices, 'e': e})
            # resync is needed
            return True

        for device_details in devices_details_list:
            device = device_details['device']
            LOG.debug("Port %s added", device)

            if 'port_id' in device_details:
                LOG.info(_LI("Port %(device)s updated. Details: %(details)s"),
                         {'device': device, 'details': device_details})
                if device_details['admin_state_up']:
                    # create the networking for the port
                    network_type = device_details.get('network_type')
                    if network_type:
                        segmentation_id = device_details.get('segmentation_id')
                    else:
                        # compatibility with pre-Havana RPC vlan_id encoding
                        vlan_id = device_details.get('vlan_id')
                        (network_type,
                         segmentation_id) = lconst.interpret_vlan_id(vlan_id)
                    if self.br_mgr.add_interface(
                        device_details['network_id'],
                        network_type,
                        device_details['physical_network'],
                        segmentation_id,
                        device_details['port_id']):

                        # update plugin about port status
                        self.plugin_rpc.update_device_up(self.context,
                                                         device,
                                                         self.agent_id,
                                                         cfg.CONF.host)
                    else:
                        self.plugin_rpc.update_device_down(self.context,
                                                           device,
                                                           self.agent_id,
                                                           cfg.CONF.host)
                else:
                    self.remove_port_binding(device_details['network_id'],
                                             device_details['port_id'])
            else:
                LOG.info(_LI("Device %s not defined on plugin"), device)
        return False

經測試發現，建立VM時，neutron資料庫中ports表的status欄位將由DOWN->BUILD->ACTIVE。其中nova與neutron互動建立ports資訊時，其狀態為DOWN，當在treat_devices_added_updated函式中呼叫get_devices_details_list函式將由DOWN更新到BUILD狀態，最後在treat_devices_added_updated函式中呼叫update_device_up函式由BUILD更新到ACTIVE狀態。

對於更新neutron資料庫中的ports表的status狀態都是呼叫update_port_status函式。

#/neutron/plugins/ml2/plugin.py:Ml2Plugin
    def update_port_status(self, context, port_id, status, host=None):
        """
        Returns port_id (non-truncated uuid) if the port exists.
        Otherwise returns None.
        """
        updated = False
        session = context.session
        # REVISIT: Serialize this operation with a semaphore to
        # prevent deadlock waiting to acquire a DB lock held by
        # another thread in the same process, leading to 'lock wait
        # timeout' errors.
        with contextlib.nested(lockutils.lock('db-access'),
                               session.begin(subtransactions=True)):
            port = db.get_port(session, port_id)
            if not port:
                LOG.warning(_LW("Port %(port)s updated up by agent not found"),
                            {'port': port_id})
                return None
            if (port.status != status and
                port['device_owner'] != const.DEVICE_OWNER_DVR_INTERFACE):
                original_port = self._make_port_dict(port)
                port.status = status
                updated_port = self._make_port_dict(port)
                network = self.get_network(context,
                                           original_port['network_id'])
                levels = db.get_binding_levels(session, port.id,
                                               port.port_binding.host)
                mech_context = driver_context.PortContext(
                    self, context, updated_port, network, port.port_binding,
                    levels, original_port=original_port)
                self.mechanism_manager.update_port_precommit(mech_context)
                updated = True
            elif port['device_owner'] == const.DEVICE_OWNER_DVR_INTERFACE:
                binding = db.get_dvr_port_binding_by_host(
                    session, port['id'], host)
                if not binding:
                    return
                binding['status'] = status
                binding.update(binding)
                updated = True

        if (updated and
            port['device_owner'] == const.DEVICE_OWNER_DVR_INTERFACE):
            with contextlib.nested(lockutils.lock('db-access'),
                                   session.begin(subtransactions=True)):
                port = db.get_port(session, port_id)
                if not port:
                    LOG.warning(_LW("Port %s not found during update"),
                                port_id)
                    return
                original_port = self._make_port_dict(port)
                network = self.get_network(context,
                                           original_port['network_id'])
                port.status = db.generate_dvr_port_status(session, port['id'])
                updated_port = self._make_port_dict(port)
                levels = db.get_binding_levels(session, port_id, host)
                mech_context = (driver_context.PortContext(
                    self, context, updated_port, network,
                    binding, levels, original_port=original_port))
                self.mechanism_manager.update_port_precommit(mech_context)

        if updated:
            self.mechanism_manager.update_port_postcommit(mech_context)

        if port['device_owner'] == const.DEVICE_OWNER_DVR_INTERFACE:
            db.delete_dvr_port_binding_if_stale(session, binding)

        return port['id']

這裡我建立VM時，增加的tap裝置資訊如下。

不過我查詢很久，也未發現更新neutron資料庫的ports的status的函式介面，當時本以為update_port_precommit會做相應的處理，但發現linuxbridge並未重新改函式，而是繼承父類未做任何操作的update_port_precommit函式。不過測試發現的確是在update_port_status函式去更新neutron資料庫的ports的status的，所以希望瞭解的大神告知，謝謝。

下面我們看看建立Ml2Plugin物件(這裡neutron-linuxbridge-agent通過rpc呼叫，在執行neutron-server上的函式)時，在哪裡建立的監聽neutron資料庫的ports表的呢？

#/neutron/db/db_base_plugin_v2.py:NeutronDbPluginV2
class NeutronDbPluginV2(neutron_plugin_base_v2.NeutronPluginBaseV2,
                        common_db_mixin.CommonDbMixin):
    """V2 Neutron plugin interface implementation using SQLAlchemy models.

    Whenever a non-read call happens the plugin will call an event handler
    class method (e.g., network_created()).  The result is that this class
    can be sub-classed by other classes that add custom behaviors on certain
    events.
    """

    # This attribute specifies whether the plugin supports or not
    # bulk/pagination/sorting operations. Name mangling is used in
    # order to ensure it is qualified by class
    __native_bulk_support = True
    __native_pagination_support = True
    __native_sorting_support = True

    def __init__(self):
        if cfg.CONF.notify_nova_on_port_status_changes:
            from neutron.notifiers import nova
            # NOTE(arosen) These event listeners are here to hook into when
            # port status changes and notify nova about their change.
            self.nova_notifier = nova.Notifier()
            event.listen(models_v2.Port, 'after_insert',
                         self.nova_notifier.send_port_status)
            event.listen(models_v2.Port, 'after_update',
                         self.nova_notifier.send_port_status)
            event.listen(models_v2.Port.status, 'set',
                         self.nova_notifier.record_port_status_changed)

NeutronDbPluginV2類是Ml2Plugin類的父類，所以在建立Ml2Plugin物件時，將初始化NeutronDbPluginV2類。而NeutronDbPluginV2類則在初始化時根據/etc/neutron/neutron.conf配置檔案中的notify_nova_on_port_status_changes引數值去決定是否通知nova。

首先，當neutron資料庫的ports表的status被設定時，如BUILD->ACTIVE，將觸發呼叫self.nova_notifier.record_port_status_changed函式。

#/neutron/notifiers/nova.py:Notifier
    def record_port_status_changed(self, port, current_port_status,
                                   previous_port_status, initiator):
        """Determine if nova needs to be notified due to port status change.
        """
        # clear out previous _notify_event
        port._notify_event = None
        # If there is no device_id set there is nothing we can do here.
        if not port.device_id:
            LOG.debug("device_id is not set on port yet.")
            return

        if not port.id:
            LOG.warning(_LW("Port ID not set! Nova will not be notified of "
                            "port status change."))
            return

        # We only want to notify about nova ports.
        if not self._is_compute_port(port):
            return

        # We notify nova when a vif is unplugged which only occurs when
        # the status goes from ACTIVE to DOWN.
        if (previous_port_status == constants.PORT_STATUS_ACTIVE and
                current_port_status == constants.PORT_STATUS_DOWN):
            event_name = VIF_UNPLUGGED

        # We only notify nova when a vif is plugged which only occurs
        # when the status goes from:
        # NO_VALUE/DOWN/BUILD -> ACTIVE/ERROR.
        elif (previous_port_status in [sql_attr.NO_VALUE,
                                       constants.PORT_STATUS_DOWN,
                                       constants.PORT_STATUS_BUILD]
              and current_port_status in [constants.PORT_STATUS_ACTIVE,
                                          constants.PORT_STATUS_ERROR]):
            event_name = VIF_PLUGGED
        # All the remaining state transitions are of no interest to nova
        else:
            LOG.debug("Ignoring state change previous_port_status: "
                      "%(pre_status)s current_port_status: %(cur_status)s"
                      " port_id %(id)s",
                      {'pre_status': previous_port_status,
                       'cur_status': current_port_status,
                       'id': port.id})
            return

        port._notify_event = (
            {'server_uuid': port.device_id,
             'name': event_name,
             'status': NEUTRON_NOVA_EVENT_STATUS_MAP.get(current_port_status),
             'tag': port.id})

record_port_status_changed函式將記錄傳送給nova的event事件，這裡只是記錄event事件，並未真正傳送該event事件給nova。

不過record_port_status_changed函式只會將兩類event事件通知給nova：

1. ports表的status由ACTIVE->DOWN.

2. ports表的status由NO_VALUE/DOWN/BUILD -> ACTIVE/ERROR

所以在建立VM時，neutron-linuxbridge-agent會將neutron資料庫的ports表的status由DOWN->BUILD->ACTIVE。但最終通知nova的event事件是由BUILD->ACTIVE觸發的。而該event事件為：

在neutron資料庫的ports表更新完成後(after_update)，則真正傳送event事件給nova是執行self.nova_notifier.send_port_status函式完成的。

#/neutron/notifiers/nova.py:Notifier
    def send_port_status(self, mapper, connection, port):
        event = getattr(port, "_notify_event", None)
        self.batch_notifier.queue_event(event)
        port._notify_event = None

send_port_status函式中event = getattr(port, "_notify_event",None)為:

        port._notify_event = (
            {'server_uuid': port.device_id,
             'name': event_name,
             'status': NEUTRON_NOVA_EVENT_STATUS_MAP.get(current_port_status),
             'tag': port.id})
NEUTRON_NOVA_EVENT_STATUS_MAP = {constants.PORT_STATUS_ACTIVE: 'completed',
                                 constants.PORT_STATUS_ERROR: 'failed',
                                 constants.PORT_STATUS_DOWN: 'completed'}

由於ports表的status由BUILD->ACTIVE，所以port._notify_event中的status為completed。

同時send_port_status函式中的self.batch_notifier是在/neutron/notifiers/nova.py的Notifier類初始化的時候建立的一個BatchNotifier物件，如下。

#/neutron/notifiers/nova.py:Notifier
class Notifier(object):

    def __init__(self):
        # FIXME(jamielennox): A notifier is being created for each Controller
        # and each Notifier is handling it's own auth. That means that we are
        # authenticating the exact same thing len(controllers) times. This
        # should be an easy thing to optimize.
        auth = ks_auth.load_from_conf_options(cfg.CONF, 'nova')
        endpoint_override = None

        if not auth:
            LOG.warning(_LW('Authenticating to nova using nova_admin_* options'
                            ' is deprecated. This should be done using'
                            ' an auth plugin, like password'))

            if cfg.CONF.nova_admin_tenant_id:
                endpoint_override = "%s/%s" % (cfg.CONF.nova_url,
                                               cfg.CONF.nova_admin_tenant_id)

            auth = DefaultAuthPlugin(
                auth_url=cfg.CONF.nova_admin_auth_url,
                username=cfg.CONF.nova_admin_username,
                password=cfg.CONF.nova_admin_password,
                tenant_id=cfg.CONF.nova_admin_tenant_id,
                tenant_name=cfg.CONF.nova_admin_tenant_name,
                endpoint_override=endpoint_override)

        session = ks_session.Session.load_from_conf_options(cfg.CONF,
                                                            'nova',
                                                            auth=auth)

        # NOTE(andreykurilin): novaclient.v1_1 was renamed to v2 and there is
        # no way to import the contrib module directly without referencing v2,
        # which would only work for novaclient >= 2.21.0.
        novaclient_cls = nova_client.get_client_class(NOVA_API_VERSION)
        server_external_events = importutils.import_module(
            novaclient_cls.__module__.replace(
                ".client", ".contrib.server_external_events"))

        self.nclient = novaclient_cls(
            session=session,
            region_name=cfg.CONF.nova.region_name,
            extensions=[server_external_events])
        self.batch_notifier = batch_notifier.BatchNotifier(
            cfg.CONF.send_events_interval, self.send_events)

其中send_events_interval為/etc/neutron/neutron.conf配置檔案中的send_events_interval引數值。如下

#/neutron/notifiers/batch_notifier.py:BatchNotifier
class BatchNotifier(object):
    def __init__(self, batch_interval, callback):
        self.pending_events = []
        self._waiting_to_send = False
        self.callback = callback
        self.batch_interval = batch_interval

    def queue_event(self, event):
        """Called to queue sending an event with the next batch of events.

        Sending events individually, as they occur, has been problematic as it
        can result in a flood of sends.  Previously, there was a loopingcall
        thread that would send batched events on a periodic interval.  However,
        maintaining a persistent thread in the loopingcall was also
        problematic.

        This replaces the loopingcall with a mechanism that creates a
        short-lived thread on demand when the first event is queued.  That
        thread will sleep once for the same batch_duration to allow other
        events to queue up in pending_events and then will send them when it
        wakes.

        If a thread is already alive and waiting, this call will simply queue
        the event and return leaving it up to the thread to send it.

        :param event: the event that occurred.
        """
        if not event:
            return

        self.pending_events.append(event)

        if self._waiting_to_send:
            return

        self._waiting_to_send = True

        def last_out_sends():
            eventlet.sleep(self.batch_interval)
            self._waiting_to_send = False
            self._notify()

        eventlet.spawn_n(last_out_sends)

    def _notify(self):
        if not self.pending_events:
            return

        batched_events = self.pending_events
        self.pending_events = []
        self.callback(batched_events)

所以當send_port_status函式執行self.batch_notifier.queue_event(event)將呼叫BatchNotifier類的queue_event函式，queue_event函式將開啟一個新的執行緒去執行last_out_sends函式，最終呼叫BatchNotifier類初始化時指定的callback函式。這裡的callback函式為/neutron/notifiers/nova.py的Notifier類的send_events函式。

#/neutron/notifiers/nova.py:Notifier
    def send_events(self, batched_events):
        LOG.debug("Sending events: %s", batched_events)
        try:
            response = self.nclient.server_external_events.create(
                batched_events)
        except nova_exceptions.NotFound:
            LOG.warning(_LW("Nova returned NotFound for event: %s"),
                        batched_events)
        except Exception:
            LOG.exception(_LE("Failed to notify nova on events: %s"),
                          batched_events)
        else:
            if not isinstance(response, list):
                LOG.error(_LE("Error response returned from nova: %s"),
                          response)
                return
            response_error = False
            for event in response:
                try:
                    code = event['code']
                except KeyError:
                    response_error = True
                    continue
                if code != 200:
                    LOG.warning(_LW("Nova event: %s returned with failed "
                                    "status"), event)
                else:
                    LOG.info(_LI("Nova event response: %s"), event)
            if response_error:
                LOG.error(_LE("Error response returned from nova: %s"),
                          response)

send_events函式將通過HTTP請求的方式將event事件傳送給nova。

下面我們看看nova側如何處理neutron傳送過來的event事件。

#/nova/api/openstack/compute/contrib/server_external_events.py:ServerExternalEventsController
class ServerExternalEventsController(wsgi.Controller):

    def __init__(self):
        self.compute_api = compute.API()
        super(ServerExternalEventsController, self).__init__()

    def create(self, req, body):
        """Creates a new instance event."""
        context = req.environ['nova.context']
        authorize(context, action='create')

        response_events = []
        accepted_events = []
        accepted_instances = set()
        instances = {}
        result = 200

        body_events = body.get('events', [])
        if not isinstance(body_events, list) or not len(body_events):
            raise webob.exc.HTTPBadRequest()

        for _event in body_events:
            client_event = dict(_event)
            event = objects.InstanceExternalEvent(context)

            try:
                event.instance_uuid = client_event.pop('server_uuid')
                event.name = client_event.pop('name')
                event.status = client_event.pop('status', 'completed')
                event.tag = client_event.pop('tag', None)
            except KeyError as missing_key:
                msg = _('event entity requires key %(key)s') % missing_key
                raise webob.exc.HTTPBadRequest(explanation=msg)

            if client_event:
                msg = (_('event entity contains unsupported items: %s') %
                       ', '.join(client_event.keys()))
                raise webob.exc.HTTPBadRequest(explanation=msg)

            if event.status not in external_event_obj.EVENT_STATUSES:
                raise webob.exc.HTTPBadRequest(
                    _('Invalid event status `%s\'') % event.status)

            instance = instances.get(event.instance_uuid)
            if not instance:
                try:
                    instance = objects.Instance.get_by_uuid(
                        context, event.instance_uuid)
                    instances[event.instance_uuid] = instance
                except exception.InstanceNotFound:
                    LOG.debug('Dropping event %(name)s:%(tag)s for unknown '
                              'instance %(instance_uuid)s',
                              dict(event.iteritems()))
                    _event['status'] = 'failed'
                    _event['code'] = 404
                    result = 207

            # NOTE: before accepting the event, make sure the instance
            # for which the event is sent is assigned to a host; otherwise
            # it will not be possible to dispatch the event
            if instance:
                if instance.host:
                    accepted_events.append(event)
                    accep