OpenStack L版 neutron server的启动流程三

上一篇提到，函数_run_wsgi中加载配置操作load_paste_app，看代码直观一些，如下所示

def _run_wsgi(app_name):
    app = config.load_paste_app(app_name)
    if not app:
        LOG.error(_LE('No known API applications configured.'))
        return
    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

继续跟踪load_paste_app函数，在neutron.common.config中定义了该函数有（注意，习惯用模块导入的格式写代码的路径，比如neutron.common.config实际上是neutron/common/config.py，下面的所有都是这种情况）

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
    :raises ConfigFilesNotFoundError when config file cannot be located
    :raises RuntimeError when application cannot be loaded from config file
    """

    config_path = cfg.CONF.find_file(cfg.CONF.api_paste_config)
    if not config_path:
        raise cfg.ConfigFilesNotFoundError(
            config_files=[cfg.CONF.api_paste_config])
    config_path = os.path.abspath(config_path)
    LOG.info(_LI("Config paste file: %s"), config_path)

    try:
        app = deploy.loadapp("config:%s" % config_path, name=app_name)
    except (LookupError, ImportError):
        msg = (_("Unable to load %(app_name)s from "
                 "configuration file %(config_path)s.") %
               {'app_name': app_name,
                'config_path': config_path})
        LOG.exception(msg)
        raise RuntimeError(msg)
    return app

该函数调用了deploy.loadapp函数，继续跟踪这个函数，此函数是在paste.deploy.loadwsgi.py中实现

def loadapp(uri, name=None, **kw):
    return loadobj(APP, uri, name=name, **kw)

loadobj函数详情如下

def loadobj(object_type, uri, name=None, relative_to=None,
            global_conf=None):
    context = loadcontext(
        object_type, uri, name=name, relative_to=relative_to,
        global_conf=global_conf)
    return context.create()

接着看loadcontext函数

def loadcontext(object_type, uri, name=None, relative_to=None,
                global_conf=None):
    if '#' in uri:
        if name is None:
            uri, name = uri.split('#', 1)
        else:
            # @@: Ignore fragment or error?
            uri = uri.split('#', 1)[0]
    if name is None:
        name = 'main'
    if ':' not in uri:
        raise LookupError("URI has no scheme: %r" % uri)
    scheme, path = uri.split(':', 1)
    scheme = scheme.lower()
    if scheme not in _loaders:
        raise LookupError(
            "URI scheme not known: %r (from %s)"
            % (scheme, ', '.join(_loaders.keys())))
    return _loaders[scheme](
        object_type,
        uri, path, name=name, relative_to=relative_to,
        global_conf=global_conf)

往后就暂不追究了，在这里最重要的东西是下面的代码

      app = deploy.loadapp("config:%s" % config_path, name=app_name)

这个app到底是什么呢？