nova-api中ExtensionManager的构造

源码版本：H版

nova/api/openstack/__init__.py

APIRouter类：
def __init__(self, ext_mgr=None, init_only=None):
    if ext_mgr is None:
        if self.ExtensionManager:
            ext_mgr = self.ExtensionManager()
        else:
            raise Exception(_("Must specify an ExtensionManager class"))
    ...

其中的ExtensionManager由子类nova/api/openstack/compute/__init__.py中的APIRouter指定为nova/api/openstack/compute/extensions.py中的ExtensionManager类，下面看其构造函数：

class ExtensionManager(base_extensions.ExtensionManager):
    def __init__(self):
        LOG.audit(_('Initializing extension manager.'))
        self.cls_list = CONF.osapi_compute_extension
        self.extensions = {}
        self.sorted_ext_list = []
        self._load_extensions()

其中self.cls_list是扩展包的引导类列表，此处为[nova.api.openstack.compute.contrib.standard_extensions](注意只有一个元素)。接着看self._load_extensions()的内容，由于ExtensionManager的类继承关系，该函数为nova.api.openstack. extensions ExtensionManager的函数，内容如下：

def _load_extensions(self):
    extensions = list(self.cls_list)
    for ext_factory in extensions:
        try:
            self.load_extension(ext_factory)
        except Exception as exc:
            LOG.warn(_('Failed to load extension %(ext_factory)s: '
                       '%(exc)s'),
                     {'ext_factory': ext_factory, 'exc': exc})

其中self.load_extension函数内容如下：

def load_extension(self, ext_factory):
    LOG.debug(_("Loading extension %s"), ext_factory)
    if isinstance(ext_factory, basestring):
        factory = importutils.import_class(ext_factory)
    else:
        factory = ext_factory
    LOG.debug(_("Calling extension factory %s"), ext_factory)
    factory(self)

最终结果就是调用nova.api.openstack.compute.contrib.standard_extensions。接下来看standard_extensions函数，内容如下：

def standard_extensions(ext_mgr):
    extensions.load_standard_extensions(ext_mgr, LOG, __path__, __package__)

其中extensions为nova.api.openstack.extensions，看load_standard_extensions内容如下：

def load_standard_extensions(ext_mgr, logger, path, package, ext_list=None):
    our_dir = path[0]
    for dirpath, dirnames, filenames in os.walk(our_dir):
        relpath = os.path.relpath(dirpath, our_dir)
        if relpath == '.':
            relpkg = ''
        else:
            relpkg = '.%s' % '.'.join(relpath.split(os.sep))
        for fname in filenames:
            root, ext = os.path.splitext(fname)
            if ext != '.py' or root == '__init__':
                continue
            classname = "%s%s" % (root[0].upper(), root[1:])
            classpath = ("%s%s.%s.%s" %
                         (package, relpkg, root, classname))
            if ext_list is not None and classname not in ext_list:
                logger.debug("Skipping extension: %s" % classpath)
                continue
            try:
                ext_mgr.load_extension(classpath)
            except Exception as exc:
                logger.warn(_('Failed to load extension %(classpath)s: '
                              '%(exc)s'),
                            {'classpath': classpath, 'exc': exc})
        ...

实际上也就是调用之前ExtensionManager的 load_extension函数加载模块，并构造模块中与模块名对应的类，由于这些类都是nova.api.openstack.extensions.ExtensionDescriptor的子类，且看其__init__函数：

def __init__(self, ext_mgr):
    ext_mgr.register(self)
    self.ext_mgr = ext_mgr

　　可知构造这些扩展类会在ExtensionManager中进行注册，注册过程就是在ExtensionManag中将扩展类的实例存入ExtensionManager的extensions列表中，对应的key为扩展类的alias。

　　综上所述，ExtensionManager的构造就是对其中的self.cls_list进行加载调用，self.cls_list包含扩展包的引导类或引导函数，此处为nova.api.openstack.compute.contrib.standard_extensions，该引导函数负责加载扩展包中模块，并构造相应的类实例，这些类实例在构造时会向ExtensionManager进行注册。