Ceilometer RESTful框架

ceilometer-api使用pecan和flask来构建restful api,这里简单介绍一下pecan和flask的使用。

ceilomter-api服务启动流程

/usr/bin/ceilometer-api

...
from ceilometer.cli import api

if __name__ == "__main__":
    sys.exit(api())
...

/usr/lib/python2.6/site-packages/ceilometer/cli.py

...
def api():
    service.prepare_service()
    srv = app.build_server()
    srv.serve_forever()
...

/usr/lib/python2.6/site-packages/ceilometer/api/app.py

...
def build_server():
    # Build the WSGI app
    root = VersionSelectorApplication()

    # Create the WSGI server and start it
    host, port = cfg.CONF.api.host, cfg.CONF.api.port
    server_cls = get_server_cls(host)
    srv = simple_server.make_server(host, port, root, server_cls)
...

...
class VersionSelectorApplication(object):
    def __init__(self):
        pc = get_pecan_config()
        pc.app.debug = CONF.debug
        pc.app.enable_acl = (CONF.auth_strategy == 'keystone')
        if cfg.CONF.enable_v1_api:
            from ceilometer.api.v1 import app as v1app
            self.v1 = v1app.make_app(cfg.CONF, enable_acl=pc.app.enable_acl)
        else:
            def not_found(environ, start_response):
                start_response('404 Not Found', [])
                return []
            self.v1 = not_found
        self.v2 = setup_app(pecan_config=pc)

    def __call__(self, environ, start_response):
        if environ['PATH_INFO'].startswith('/v1/'):
            return self.v1(environ, start_response)
        return self.v2(environ, start_response)
...

如上图所示，ceilometer-api有两种版本即V1和V2，分别使用Flask和Pecan来构建RESTful api，其中V1在Havana已deprecated。

Pecan 是轻量的web框架，主页http://pecan.readthedocs.org/ ，其中URL Mapping如下图：

：

ceilometer使用pecan代码如下：

/usr/lib/python2.6/site-packages/ceilometer/api/app.py

...
def setup_app(pecan_config=None, extra_hooks=None):
    # FIXME: Replace DBHook with a hooks.TransactionHook
    app_hooks = [hooks.ConfigHook(),
                 hooks.DBHook(
                     storage.get_connection(cfg.CONF, 'metric'),
                     storage.get_connection(cfg.CONF, 'event'),
                 ),
                 hooks.PipelineHook(),
                 hooks.TranslationHook()]
    if extra_hooks:
        app_hooks.extend(extra_hooks)

    if not pecan_config:
        pecan_config = get_pecan_config()

    pecan.configuration.set_config(dict(pecan_config), overwrite=True)

    app = pecan.make_app(
        pecan_config.app.root,
        static_root=pecan_config.app.static_root,
        template_path=pecan_config.app.template_path,
        debug=CONF.debug,
        force_canonical=getattr(pecan_config.app, 'force_canonical', True),
        hooks=app_hooks,
        wrap_app=middleware.ParsableErrorMiddleware,
        guess_content_type_from_ext=False
    )

    if getattr(pecan_config.app, 'enable_acl', True):
        return acl.install(app, cfg.CONF)

    return app
...

ceilometer中pecan的配置 /usr/lib/python2.6/site-packages/ceilometer/api/config.py

...
app = {
    'root': 'ceilometer.api.controllers.root.RootController',
    'modules': ['ceilometer.api'],
    'static_root': '%(confdir)s/public',
    'template_path': '%(confdir)s/ceilometer/api/templates',
}
...

RootController /usr/lib/python2.6/site-packages/ceilometer/api/controllers/root.py

...
class RootController(object):

    v2 = v2.V2Controller()

    @pecan.expose(generic=True, template='index.html')
    def index(self):
        # FIXME: Return version information
        return dict()
...

V2Controller包含多个Resource的Controller, /usr/lib/python2.6/site-packages/ceilometer/api/controllers/v2.py

class V2Controller(object):
    """Version 2 API controller root."""

    resources = ResourcesController()
    meters = MetersController()
    samples = SamplesController()
    alarms = AlarmsController()
    event_types = EventTypesController()
    events = EventsController()
    status = StatusController()
    query = QueryController()
    capabilities = CapabilitiesController()

下面以MetersController进行分析：/usr/lib/python2.6/site-packages/ceilometer/api/controllers/v2.py

class MetersController(rest.RestController):
    """Works on meters."""

    @pecan.expose()
    def _lookup(self, meter_name, *remainder):
        return MeterController(meter_name), remainder

    #wsme_pecan是来自wsmeext.pecan
    @wsme_pecan.wsexpose([Meter], [Query])
    def get_all(self, q=[]):
        """Return all known meters, based on the data recorded so far.

        :param q: Filter rules for the meters to be returned.
        """
        #Timestamp field is not supported for Meter queries
        kwargs = _query_to_kwargs(q, pecan.request.metric_conn.get_meters,
                                  allow_timestamps=False)
        return [Meter.from_db_model(m)
                for m in pecan.request.metric_conn.get_meters(**kwargs)]

   这里用到了wsme和wsmeext.pecan。wsme即Web Services Made Easy，简化了REST Web服务的编写。可以运行在另一个框架的顶层，如（Cornice、Flask、Pecan）等，这里就是运行在了pecan之上来更加简单的构建restapi，用法:

wsmeext.pecan.wsexpose(return_type, *arg_types, **options)

Flask与Pecan类似，是基于Python的开源微RESTful框架，ceilometer使用Flask代码如下： /usr/lib/python2.6/site-packages/ceilometer/api/v1/app.py

...
def make_app(conf, enable_acl=True, attach_storage=True,
             sources_file='sources.json'):
    app = flask.Flask('ceilometer.api')
    app.register_blueprint(v1_blueprint.blueprint, url_prefix='/v1')

    app.json_encoder = JSONEncoder

    try:
        with open(sources_file, "r") as f:
            sources = jsonutils.load(f)
    except IOError:
        sources = {}

    @app.before_request
    def attach_config():
        flask.request.cfg = conf
        flask.request.sources = sources

    if attach_storage:
        @app.before_request
        def attach_storage():
            flask.request.storage_conn = 
                storage.get_connection(conf)

    # Install the middleware wrapper
    if enable_acl:
        app.wsgi_app = acl.install(app.wsgi_app, conf)

    return app
...

app.register_blueprint 注册RESTful方法，如下图所示

...
blueprint = flask.Blueprint('v1', __name__,
                            template_folder='templates',
                            static_folder='static')

@blueprint.route('/meters')
def list_meters_all():
    """Return a list of meters.
    :param metadata.<key>: match on the metadata within the resource.
                           (optional)
    """
    rq = flask.request
    meters = rq.storage_conn.get_meters(
        project=acl.get_limited_to_project(rq.headers),
        metaquery=_get_metaquery(rq.args))
    return flask.jsonify(meters=[m.as_dict() for m in meters])
...