关于python:Openstack服务的启动

从明天开始将陆陆续续发表一些 openstack 相干的文章。

openstack 服务的启动
根本所有的 openstack 服务都依赖 evenlet 实现各种并发工作，它的过程可分为两类：
1、WSGIService: 接管和解决 http 申请，依赖 eventlet.wsgi 的 wsgi server 解决 http 申请，比方 nova-api
2、Service: 接管和解决 rpc 申请，如 nova-operation 等
无论是 WSGIService 还是 Service 类型的过程，每当接管到一个申请(http 或 rpc)，都会在线程池中调配一个协程解决该申请

一、WSGIService 的启动
上面以 nova 服务为例。
nova-api 由 nova/cmd/api.py 启动，它初始化一个 WSGIService(由 service.py 定义) 对象。

def main():

objects.register_all()
CONF(sys.argv[1:], project='nova',
     version=version.version_string())
logging.setup(CONF, "nova")

rpc.init(CONF)
launcher = service.get_launcher()
server = service.WSGIService('osapi_nova')
launcher.launch_service(server, workers=server.workers)
launcher.wait()

api 中从 service 层获取一个启动器对象，最初将 server 对象传入启动器对象的 launch_service 办法中，launch_service（server, workers=server.workers）办法定义如下：

class Launcher(object):

def __init__(self):
    super(Launcher, self).__init__()
    self.launch_service = serve
    self.wait = wait

该办法被援用到 serve 办法，serve 办法定义如下：

def serve(server, workers=None):

global _launcher
if _launcher:
    raise RuntimeError(_('serve() can only be called once'))

_launcher = service.launch(CONF, server, workers=workers)

最终调用了 oslo_service/service.py 下的 launch 办法，launch 办法定义如下：

def launch(conf, service, workers=1, restart_method=’reload’):

…

if workers is not None and workers <= 0:
    raise ValueError(_("Number of workers should be positive!"))

if workers is None or workers == 1:
    launcher = ServiceLauncher(conf, restart_method=restart_method)
else:
    launcher = ProcessLauncher(conf, restart_method=restart_method)
launcher.launch_service(service, workers=workers)

能够看到这里应用到了两种启动器，在进一步解说启动的过程中先介绍下 openstack 中的启动器

二、Openstack 中的 Launcher
Openstack 中有一个叫 Launcher 的概念，即专门用来启动服务的，这个类被放在了 oslo_service 这个包外面，Launcher 分为两种:
一种是 ServiceLauncher；
另一种为 ProcessLauncher。
ServiceLauncher 用来启动单过程的服务；
而 ProcessLauncher 用来启动有多个 worker 子过程的服务，如各类 api 服务 (nova-api、cinder-api) 等

oslo_service/service.py

1、ServiceLauncher
ServiceLauncher 继承自 Launcher，启动服务的一个重要成员就是 launcher_service，ServiceLauncher 的该成员就是继承于 Launcher

def launch_service(self, service, workers=1):

 …
if workers is not None and workers != 1:
    raise ValueError(_("Launcher asked to start multiple workers"))
_check_service_base(service)
service.backdoor_port = self.backdoor_port
self.services.add(service)

aucher_service 就是将服务增加到 self.services 成员外面，services 成员的类型是 class Services，看看它的 add 办法

class Services(object):

def __init__(self):
    self.services = []
    self.tg = threadgroup.ThreadGroup()
    self.done = event.Event()

def add(self, service):
    """Add a service to a list and create a thread to run it.

    :param service: service to run
    """
    self.services.append(service)
    self.tg.add_thread(self.run_service, service, self.done)

Services 这个类的初始化很简略，即创立一个 ThreadGroup，ThreadGroup 其实是 eventlet 的 GreenPool，Openstack 利用 eventlet 实现并发，add 办法，将 self.run_service 这个办法放入 pool 中，而 service 就是它的参数。run_service 办法很简略，就是调用 service 的 start 办法，这样就实现了服务的启动

2、ProcessLauncher
ProcessLauncher 间接继承于 Object，同样也有 launch_service 办法

def launch_service(self, service, workers=1):

 …
_check_service_base(service)
wrap = ServiceWrapper(service, workers)

LOG.info('Starting %d workers', wrap.workers)
while self.running and len(wrap.children) < wrap.workers:
    self._start_child(wrap)

lauch_service 除了承受 service 以外，还须要承受一个 workers 参数，即子过程的个数，而后调用_start_child 启动多个子过程

def _start_child(self, wrap):

if len(wrap.forktimes) > wrap.workers:
    # Limit ourselves to one process a second (over the period of
    # number of workers * 1 second). This will allow workers to
    # start up quickly but ensure we don't fork off children that
    # die instantly too quickly.
    if time.time() - wrap.forktimes[0] < wrap.workers:
        LOG.info('Forking too fast, sleeping')
        time.sleep(1)

    wrap.forktimes.pop(0)

wrap.forktimes.append(time.time())

pid = os.fork()
if pid == 0:
    self.launcher = self._child_process(wrap.service)
    while True:
        self._child_process_handle_signal()
        status, signo = self._child_wait_for_exit_or_signal(self.launcher)
        if not _is_sighup_and_daemon(signo):
            self.launcher.wait()
            break
        self.launcher.restart()

    os._exit(status)

LOG.debug('Started child %d', pid)

wrap.children.add(pid)
self.children[pid] = wrap

看见相熟的 fork 没有，只是简略的调用了一个 os.fork()，而后子过程开始运行，子过程调用_child_process

def _child_process(self, service):

self._child_process_handle_signal()

# Reopen the eventlet hub to make sure we don't share an epoll
# fd with parent and/or siblings, which would be bad
eventlet.hubs.use_hub()

# Close write to ensure only parent has it open
os.close(self.writepipe)
# Create greenthread to watch for parent to close pipe
eventlet.spawn_n(self._pipe_watcher)

# Reseed random number generator
random.seed()

launcher = Launcher(self.conf, restart_method=self.restart_method)
launcher.launch_service(service)
return launcher

_child_process 其实很简略，创立一个 Launcher，调用 Laucher.launch_service 办法，后面介绍过，其实 ServiceLauncher 继承自 Launcher，也是调用的 launcher_service 办法，将服务启动，因而接下来的步骤能够参考后面，最终都将调用 service.start 办法启动服务

三、WSGIService 的启动—续
回到后面的启动局部，从 launcher 节的阐明，咱们晓得服务的启动最终调用了 service 的 start 办法，而这里的 service 就是咱们最开始在 api.py 中创立的 service，而后一层层传进前面的启动器中的，咱们持续回到 WSGIService 类中的 start(self)办法

def start(self):

…
if self.manager:
    self.manager.init_host()
self.server.start()
self.port = self.server.port

这里调用了 oslo_service/wsgi.py 中的 start(self)办法

def start(self):

…
self.dup_socket = self.socket.dup()

if self._use_ssl:
    self.dup_socket = sslutils.wrap(self.conf, self.dup_socket)

wsgi_kwargs = {
    'func': eventlet.wsgi.server,
    'sock': self.dup_socket,
    'site': self.app,
    'protocol': self._protocol,
    'custom_pool': self._pool,
    'log': self._logger,
    'log_format': self.conf.wsgi_log_format,
    'debug': False,
    'keepalive': self.conf.wsgi_keep_alive,
    'socket_timeout': self.client_socket_timeout
    }

if self._max_url_len:
    wsgi_kwargs['url_length_limit'] = self._max_url_len

self._server = eventlet.spawn(**wsgi_kwargs)

留神 wsgi_kwargs 中的参数 func，它的值为 eventlet.wsgi.server，在 eventlet/wsgi.py 的定义如下：

def server(sock, site,

…
 try:
    serv.log.info("(%s) wsgi starting up on %s" % (serv.pid, socket_repr(sock)))
    while is_accepting:
        try:
            client_socket = sock.accept()
            client_socket[0].settimeout(serv.socket_timeout)
            serv.log.debug("(%s) accepted %r" % (serv.pid, client_socket[1]))
            try:
                pool.spawn_n(serv.process_request, client_socket)
            except AttributeError:
                warnings.warn("wsgi's pool should be an instance of ""eventlet.greenpool.GreenPool, is %s. Please convert your"
                              "call site to use GreenPool instead" % type(pool),
                              DeprecationWarning, stacklevel=2)
                pool.execute_async(serv.process_request, client_socket)
        except ACCEPT_EXCEPTIONS as e:
            if support.get_errno(e) not in ACCEPT_ERRNO:
                raise
        except (KeyboardInterrupt, SystemExit):
            serv.log.info("wsgi exiting")
            break
finally:
    pool.waitall()
    …

看，是不是看到相熟的一幕了！sock.accept() 监听申请，每当接管到一个新申请，调用 pool.spawn_n() 启动一个协程解决该申请

四、Service 的启动
Service 类型的过程同样由 nova/cmd/* 目录下某些文件创建：

nova-schedule: nova/cmd/schedule.py

……
作为消息中间件的消费者，它们监听各自的 queue，每当有 rpc 申请来长期，它们创立一个新的协程解决 rpc 申请。以 nova-schedule 为例，启动时初始化一个 Server(由 service.py 定义) 对象。
整个 Launcher 过程跟 WSGIServer 一样，只是 service 的 start()有些区别而已

def start(self):

…
target = messaging.Target(topic=self.topic, server=self.host)
endpoints = [self.manager]
endpoints.extend(self.manager.additional_endpoints)
serializer = objects_base.KarborObjectSerializer()
self.rpcserver = rpc.get_server(target, endpoints, serializer)
self.rpcserver.start()

通过层层调用，最终生成了这样一个 RPCServer 对象

class RPCServer(msg_server.MessageHandlingServer):

def __init__(self, transport, target, dispatcher, executor='blocking'):
    super(RPCServer, self).__init__(transport, dispatcher, executor)
    self._target = target

该类继承自 MessageHandlingServer；
注：nova 的各个组件都依赖 oslo.messaging 拜访音讯服务器，通过 oslo/messaging/server.py 初始化一个 MessageHandlingServer 的对象，监听音讯队列。
最终调用了该 service 的 start 办法

def start(self, override_pool_size=None):

…
if self._started:
    LOG.warning(_LW('Restarting a MessageHandlingServer is inherently'
                    'racy. It is deprecated, and will become a noop'
                    'in a future release of oslo.messaging. If you'
                    'need to restart MessageHandlingServer you should'
                    'instantiate a new object.'))
self._started = True

executor_opts = {}

if self.executor_type in ("threading", "eventlet"):
    executor_opts["max_workers"] = (override_pool_size or self.conf.executor_thread_pool_size)
self._work_executor = self._executor_cls(**executor_opts)

try:
    self.listener = self._create_listener()
except driver_base.TransportDriverError as ex:
    raise ServerListenError(self.target, ex)

# HACK(sileht): We temporary pass the executor to the rabbit
# listener to fix a race with the deprecated blocking executor.
# We do this hack because this is need only for 'synchronous'
# executor like blocking. And this one is deprecated. Making
# driver working in an sync and an async way is complicated
# and blocking have 0% tests coverage.
if hasattr(self.listener, '_poll_style_listener'):
    l = self.listener._poll_style_listener
    if hasattr(l, "_message_operations_handler"):
        l._message_operations_handler._executor = (self.executor_type)

self.listener.start(self._on_incoming)

上述的对象又初始化一个 EventletExecutor(由 oslo/messaging/_executors/impl_eventlet.py) 类型的 excuete 对象，它调用 self.listener.poll() 监听 rpc 申请，每当接管到一个申请，创立一个协程解决该申请。

class EventletExecutor(base.ExecutorBase):

......

def start(self):
    if self._thread is not None:
        return

    @excutils.forever_retry_uncaught_exceptions
    def _executor_thread():
        try:
            while True:
                incoming = self.listener.poll()
                spawn_with(ctxt=self.dispatcher(incoming),
                           pool=self._greenpool)
        except greenlet.GreenletExit:
            return

    self._thread = eventlet.spawn(_executor_thread)

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