首先，Neutron Server作为一种服务（neutron-server.service），可以到Neutron项目目录中的setup.cfg配置文件中找到对应的代码入口。

[entry_points]console_scripts =    neutron-db-manage = neutron.db.migration.cli:main    neutron-debug = neutron.debug.shell:main    neutron-dhcp-agent = neutron.cmd.eventlet.agents.dhcp:main    neutron-keepalived-state-change = neutron.cmd.keepalived_state_change:main    neutron-ipset-cleanup = neutron.cmd.ipset_cleanup:main    neutron-l3-agent = neutron.cmd.eventlet.agents.l3:main    neutron-linuxbridge-agent = neutron.cmd.eventlet.plugins.linuxbridge_neutron_agent:main    neutron-linuxbridge-cleanup = neutron.cmd.linuxbridge_cleanup:main    neutron-macvtap-agent = neutron.cmd.eventlet.plugins.macvtap_neutron_agent:main    neutron-metadata-agent = neutron.cmd.eventlet.agents.metadata:main    neutron-netns-cleanup = neutron.cmd.netns_cleanup:main    neutron-ns-metadata-proxy = neutron.cmd.eventlet.agents.metadata_proxy:main    neutron-openvswitch-agent = neutron.cmd.eventlet.plugins.ovs_neutron_agent:main    neutron-ovs-cleanup = neutron.cmd.ovs_cleanup:main    neutron-pd-notify = neutron.cmd.pd_notify:main    neutron-server = neutron.cmd.eventlet.server:main    neutron-rpc-server = neutron.cmd.eventlet.server:main_rpc_eventlet    neutron-rootwrap = oslo_rootwrap.cmd:main    neutron-rootwrap-daemon = oslo_rootwrap.cmd:daemon    neutron-usage-audit = neutron.cmd.eventlet.usage_audit:main    neutron-metering-agent = neutron.cmd.eventlet.services.metering_agent:main    neutron-sriov-nic-agent = neutron.cmd.eventlet.plugins.sriov_nic_neutron_agent:main    neutron-sanity-check = neutron.cmd.sanity_check:main

neutron-server代码入口：

def main():    server.boot_server(_main_neutron_server)

调用neutron.server.__init__中的boot_server函数：

def boot_server(server_func):    # the configuration will be read into the cfg.CONF global data structure    config.init(sys.argv[1:])    config.setup_logging()    config.set_config_defaults()    if not cfg.CONF.config_file:        sys.exit(_("ERROR: Unable to find configuration file via the default"                   " search paths (~/.neutron/, ~/, /etc/neutron/, /etc/) and"                   " the '--config-file' option!"))    try:        server_func()    except KeyboardInterrupt:        pass    except RuntimeError as e:        sys.exit(_("ERROR: %s") % e)

该函数在外加了一层初始化日志文件的壳，执行server_func（也就是这里的 _main_neutron_server），并进行异常的捕获。

def _main_neutron_server():    if cfg.CONF.web_framework == 'legacy':        wsgi_eventlet.eventlet_wsgi_server()    else:        wsgi_pecan.pecan_wsgi_server()

根据Neutron配置文件中的web_framework进行判断，默认为legacy。执行neutron.server.wsgi_eventlet.py中的eventlet_wsgi_server。

def eventlet_wsgi_server():    neutron_api = service.serve_wsgi(service.NeutronApiService)     # 初始化neutron_api    start_api_and_rpc_workers(neutron_api)                          # 启动neutron_api和rpc

先分析一下neutron_api的启动过程，rpc的启动过程下一节分析。调用neutron.service.py中的serve_wsgi函数，相关的类代码都在一块，一起贴出来分析。

class WsgiService(object):    """Base class for WSGI based services.    For each api you define, you must also define these flags:    :
     
      _listen: The address on which to listen    :
      
       _listen_port: The port on which to listen    """    def __init__(self, app_name):                    # 初始化类变量，wsgi_app变量在start函数中初始化        self.app_name = app_name        self.wsgi_app = None    def start(self):        self.wsgi_app = _run_wsgi(self.app_name)    def wait(self):        self.wsgi_app.wait()class NeutronApiService(WsgiService):    """Class for neutron-api service."""    def __init__(self, app_name):        profiler.setup('neutron-server', cfg.CONF.host)        super(NeutronApiService, self).__init__(app_name)     # super调用父类的构造函数    @classmethod    def create(cls, app_name='neutron'):        # Setup logging early        config.setup_logging()        service = cls(app_name)          # 调用构造函数，app_name为neutron        return service                   # 返回一个NeutronApiService的实例def serve_wsgi(cls):                    # 传入的cls参数为service.NeutronApiService    try:        service = cls.create()          # 调用NeutronApiService.create        service.start()                 # 初始化service.wsgi_app    except Exception:        with excutils.save_and_reraise_exception():            LOG.exception(_LE('Unrecoverable error: please check log '                              'for details.'))    registry.notify(resources.PROCESS, events.BEFORE_SPAWN, service)    return service
      
     

下面分析_run_wsgi函数，传入参数app_name为'neutron'。

def _run_wsgi(app_name):    app = config.load_paste_app(app_name)          # 加载wsgi_app    if not app:        LOG.error(_LE('No known API applications configured.'))        return    return run_wsgi_app(app)                       # 启动wsgi_server

def load_paste_app(app_name):    """Builds and returns a WSGI app from a paste config file.    :param app_name: Name of the application to load    """    loader = wsgi.Loader(cfg.CONF)    app = loader.load_app(app_name)    return app

load_paste_app函数首先实例化oslo_service.wsgi.py中的Loader类，返回一个loader对象。然后再调用loader对象的load_app函数来加载wsgi_app。

class Loader(object):    """Used to load WSGI applications from paste configurations."""    def __init__(self, conf):        """Initialize the loader, and attempt to find the config.        :param conf: Application config        :returns: None        """        conf.register_opts(_options.wsgi_opts)        self.config_path = None        config_path = conf.api_paste_config                    # 获取配置文件中api_paste_config指定的api-paste.ini文件路径        if not os.path.isabs(config_path):            self.config_path = conf.find_file(config_path)        elif os.path.exists(config_path):            self.config_path = config_path                     # 赋值给loader.config_path，供load_app中调用        if not self.config_path:            raise ConfigNotFound(path=config_path)    def load_app(self, name):        """Return the paste URLMap wrapped WSGI application.        :param name: Name of the application to load.        :returns: Paste URLMap object wrapping the requested application.        :raises: PasteAppNotFound        """        try:            LOG.debug("Loading app %(name)s from %(path)s",                      {
    'name': name, 'path': self.config_path})            return deploy.loadapp("config:%s" % self.config_path, name=name)    # 调用第三方库paste.deploy的loadapp函数来加载wsgi_app并返回，后续章节分析其配置文件          except LookupError:            LOG.exception("Couldn't lookup app: %s", name)            raise PasteAppNotFound(name=name, path=self.config_path)

完成加载的wsgi_app在_run_wsgi函数中作为参数传入run_wsgi_app函数中，进行wsgi_server的创建。

def run_wsgi_app(app):    server = wsgi.Server("Neutron")    server.start(app, cfg.CONF.bind_port, cfg.CONF.bind_host,                 workers=_get_api_workers())    LOG.info(_LI("Neutron service started, listening on %(host)s:%(port)s"),             {
    'host': cfg.CONF.bind_host, 'port': cfg.CONF.bind_port})    return server

run_wsgi_app函数首先实例化Server，然后调用start函数来启动wsgi_server。这里的workers指启动workers个进程来运行Neutron API。先分析_get_api_workers函数再分析Server类中的相关内容。

def _get_api_workers():    workers = cfg.CONF.api_workers    if workers is None:        workers = processutils.get_worker_count()    return workers

_get_api_workers先获取配置文件中的api_workers。如果没有配置，则workers默认为CPU个数。

class Server(object):    def start(self, application, port, host='0.0.0.0', workers=0):        """Run a WSGI server with the given application."""        self._host = host        self._port = port        backlog = CONF.backlog        self._socket = self._get_socket(self._host,                                        self._port,                                        backlog=backlog)        self._launch(application, workers)    def _launch(self, application, workers=0):        service = WorkerService(self, application, self.disable_ssl, workers)        if workers < 1:            # The API service should run in the current process.            self._server = service            # Dump the initial option values            cfg.CONF.log_opt_values(LOG, logging.DEBUG)            service.start()            systemd.notify_once()        else:            # dispose the whole pool before os.fork, otherwise there will            # be shared DB connections in child processes which may cause            # DB errors.            api.context_manager.dispose_pool()            # The API service runs in a number of child processes.            # Minimize the cost of checking for child exit by extending the            # wait interval past the default of 0.01s.            self._server = common_service.ProcessLauncher(cfg.CONF,                                                          wait_interval=1.0)            self._server.launch_service(service,                                        workers=service.worker_process_count)

首先，_launch函数会把传入的wsgi_app封装成一个WorkerService，然后对workers进行判断。当workers的值大于或等于1时（默认情况下，workers的值为1，因此会创建一个子进程来运行Neutron API），与接下来分析的rpc服务启动部分和其他服务启动部分一样，先实例化oslo_service.service.py中的ProcessLauncher类，再调用其中的launch_service来启动WorkerService。

class ProcessLauncher(object):    def launch_service(self, service, workers=1):        """Launch a service with a given number of workers.       :param service: a service to launch, must be an instance of              :class:`oslo_service.service.ServiceBase`       :param workers: a number of processes in which a service              will be running        """        _check_service_base(service)                        # 检查传入参数service的基类是否为oslo_service.service.ServiceBase        wrap = ServiceWrapper(service, workers)             # 将servcie相关参数记录到wrap变量中，包括service和workers        LOG.info('Starting %d workers', wrap.workers)        while self.running and len(wrap.children) < wrap.workers:   # 依次启动子进程，数量为wrap.workers            self._start_child(wrap)

class WorkerService(neutron_worker.NeutronWorker):    """Wraps a worker to be handled by ProcessLauncher"""    def __init__(self, service, application, disable_ssl=False,                 worker_process_count=0):        super(WorkerService, self).__init__(worker_process_count)        self._service = service               # service为neutron.wsgi.py中Server的实例server（run_wsgi_app函数中的server）        self._application = application       # wsgi_app        self._disable_ssl = disable_ssl        self._server = None    def start(self):        super(WorkerService, self).start()        # When api worker is stopped it kills the eventlet wsgi server which        # internally closes the wsgi server socket object. This server socket        # object becomes not usable which leads to "Bad file descriptor"        # errors on service restart.        # Duplicate a socket object to keep a file descriptor usable.        dup_sock = self._service._socket.dup()        if CONF.use_ssl and not self._disable_ssl:            dup_sock = sslutils.wrap(CONF, dup_sock)        self._server = self._service.pool.spawn(self._service._run,                                                     self._application,                                                dup_sock)          # 这里调用self._service也就是上面提到的server对象的pool（eventlet的GreenPool）                                                                   # 的spawn函数来运行Server的_run函数

def _run(self, application, socket):    """Start a WSGI server in a new green thread."""    eventlet.wsgi.server(socket, application,                        max_size=self.num_threads,                        log=LOG,                        keepalive=CONF.wsgi_keep_alive,                        socket_timeout=self.client_socket_timeout)

 

Neutron API部分启动过程总结：

Neutron API的启动过程是Neutron Server启动过程中的一大重要步骤，另一个为RPC部分。主要分为两个部分：wsgi app的加载和wsgi server的启动。其中，wsgi app的加载最终是调用paste.deploy库中的loadapp函数来实现的，默认的配置文件api-paste.ini在/etc/neutron目录下。wsgi server是通过进程+Eventlet的GreenPool的方式来启动服务，后面要分析的RPC服务也是通过这种方式启动。最终实例化oslo_service.service::ProcessLaucher类并调用其launch_service函数来实现，传入的service参数（这里是WorkerService ）必须是oslo_service.service::ServiceBase的派生类并需要实现start函数。