Openstack中的服务是分布式部署，因此，各个服务的启停状态，决定了此系统的可用性。我们可以
通过openstack提供的接口来查看服务的运行状态，比如：
nova service的相关配置
Nova相关的配置项都nova/conf这个目录下，如果我们相查看service相关的配置项，可以看nova/conf/service.py。其中，注意一下这两个配置项：
service_down_time
report_interval
具体作用可以查看注释。大概意思就是说，超过service_down_time的时间没有收到的心跳 的话，就认为服务是挂 了的，report_interval是定义了心跳的周期，即多长时间
上报一次自己的心跳。
nova服务的启动
nova-compute的服务的入口在nova/cmd/compute.py:main()
其中最重要的一行代码就是高亮的部分。这里首先会初始一个service.Service的类：
def main(): config.parse_args(sys.argv) logging.setup(CONF, 'nova') priv_context.init(root_helper=shlex.split(utils.get_root_helper())) utils.monkey_patch() objects.register_all() # Ensure os-vif objects are registered and plugins loaded os_vif.initialize() gmr.TextGuruMeditation.setup_autorun(version) cmd_common.block_db_access('nova-compute') objects_base.NovaObject.indirection_api = conductor_rpcapi.ConductorAPI() server = service.Service.create(binary='nova-compute', topic=CONF.compute_topic) service.serve(server) service.wait()
我们详细看一下service.Service()这个类：
class Service(service.Service): """Service object for binaries running on hosts. A service takes a manager and enables rpc by listening to queues based on topic. It also periodically runs tasks on the manager and reports its state to the database services table. """ def __init__(self, host, binary, topic, manager, report_interval=None, periodic_enable=None, periodic_fuzzy_delay=None, periodic_interval_max=None, *args, **kwargs): super(Service, self).__init__() self.host = host self.binary = binary self.topic = topic self.manager_class_name = manager self.servicegroup_api = servicegroup.API() manager_class = importutils.import_class(self.manager_class_name) self.manager = manager_class(host=self.host, *args, **kwargs) self.rpcserver = None self.report_interval = report_interval self.periodic_enable = periodic_enable self.periodic_fuzzy_delay = periodic_fuzzy_delay self.periodic_interval_max = periodic_interval_max self.saved_args, self.saved_kwargs = args, kwargs self.backdoor_port = None if objects_base.NovaObject.indirection_api: conductor_api = conductor.API() conductor_api.wait_until_ready(context.get_admin_context()) setup_profiler(binary, self.host)这里重点看一下高亮的部分，看到一个report_interval这个变量。还有一个servicegroup.API()。
接下来我们看一下serivce.Service的start()方法：
# Add service to the ServiceGroup membership group. self.servicegroup_api.join(self.host, self.topic, self)会看上面一行代码，这里调用了servicegroup_api中的join方法，具体路径在nova/servicegroup/api.py:join()
def join(self, member, group, service=None): """Add a new member to a service group. :param member: the joined member ID/name :param group: the group ID/name, of the joined member :param service: a `nova.service.Service` object """ return self._driver.join(member, group, service)
从servicegroup.API()的初始化函数我们看到
class API(object): def __init__(self, *args, **kwargs): '''Create an instance of the servicegroup API. args and kwargs are passed down to the servicegroup driver when it gets created. ''' # Make sure report interval is less than service down time report_interval = CONF.report_interval if CONF.service_down_time <= report_interval: new_service_down_time = int(report_interval * 2.5) LOG.warning(_LW("Report interval must be less than service down " "time. Current config: <service_down_time: " "%(service_down_time)s, report_interval: " "%(report_interval)s>. Setting service_down_time " "to: %(new_service_down_time)s"), {'service_down_time': CONF.service_down_time, 'report_interval': report_interval, 'new_service_down_time': new_service_down_time}) CONF.set_override('service_down_time', new_service_down_time) driver_class = _driver_name_class_mapping[CONF.servicegroup_driver] self._driver = importutils.import_object(driver_class, *args, **kwargs)回到nova/conf/servicegrouup.py中我们可以看到，CONF.servicegroup_driver目前只支持两个类型：
SERVICEGROUP_OPTS = [ cfg.StrOpt('servicegroup_driver', default='db', choices=['db', 'mc'], help=""" This option specifies the driver to be used for the servicegroup service. ServiceGroup API in nova enables checking status of a compute node. When a compute worker running the nova-compute daemon starts, it calls the join API to join the compute group. Services like nova scheduler can query the ServiceGroup API to check if a node is alive. Internally, the ServiceGroup client driver automatically updates the compute worker status. There are multiple backend implementations for this service: Database ServiceGroup driver and Memcache ServiceGroup driver. Possible Values: * db : Database ServiceGroup driver * mc : Memcache ServiceGroup driver Related Options: * service_down_time (maximum time since last check-in for up service) """), ]我们假定用的是db这种方式，因此，我们要找的join方法应该在nova/securitygroup/drivers/db.py中：
def join(self, member, group, service=None): """Add a new member to a service group. :param member: the joined member ID/name :param group: the group ID/name, of the joined member :param service: a `nova.service.Service` object """ LOG.debug('DB_Driver: join new ServiceGroup member %(member)s to ' 'the %(group)s group, service = %(service)s', {'member': member, 'group': group, 'service': service}) if service is None: raise RuntimeError(_('service is a mandatory argument for DB based' ' ServiceGroup driver')) report_interval = service.report_interval if report_interval: service.tg.add_timer(report_interval, self._report_state, api.INITIAL_REPORTING_DELAY, service)在这里我们很容易发现了他的回调函数：self._report_state()
def _report_state(self, service): """Update the state of this service in the datastore.""" try: service.service_ref.report_count += 1 service.service_ref.save() # TODO(termie): make this pattern be more elegant. if getattr(service, 'model_disconnected', False): service.model_disconnected = False LOG.info( _LI('Recovered from being unable to report status.'))
其中service是从数据库中取得的最新service数据，该函数只是仅仅把report_count加一，然后调用save方法保存到数据库中。这里需要注意的是，save方法每次都会记录更新的时间,在数据库的字段为updated_at。
由此，nova服务的心跳机制，本质就是每隔一段时间往数据库更新report_count值，并记录最后更新时间作为接收到的最新心跳时间戳。
这个值可以从数据库中查到，如下：
服务状态监控机制
我们从nova-api的分析入手。其中与service相关的代码位于： nova/api/openstack/compute/services.py。当我们调用nova service-list的命令时，实际是调用：
1. nova/api/openstack/compute/serivces.py: index()
@extensions.expected_errors(()) def index(self, req): """Return a list of all running services. Filter by host & service name """ if api_version_request.is_supported(req, min_version='2.11'): _services = self._get_services_list(req, ['forced_down']) else: _services = self._get_services_list(req) return {'services': _services}2. nova/api/openstack/compute/serivces.py: _get_services_list():
def _get_services_list(self, req, additional_fields=()): _services = self._get_services(req) return [self._get_service_detail(svc, additional_fields) for svc in _services]3. nova/api/openstack/compute/serivces.py: _get_services_detail():
def _get_service_detail(self, svc, additional_fields): alive = self.servicegroup_api.service_is_up(svc) state = (alive and "up") or "down" active = 'enabled' if svc['disabled']: active = 'disabled' service_detail = {'binary': svc['binary'], 'host': svc['host'], 'id': svc['id'], 'zone': svc['availability_zone'], 'status': active, 'state': state, 'updated_at': svc['updated_at'], 'disabled_reason': svc['disabled_reason']} for field in additional_fields: service_detail[field] = svc[field] return service_detail
nova/servicegroup/api.py
def service_is_up(self, member): """Check if the given member is up.""" # NOTE(johngarbutt) no logging in this method, # so this doesn't slow down the scheduler if member.get('forced_down'): return False return self._driver.is_up(member)nova/serivcegroup/drivers/db.py
def is_up(self, service_ref): """Moved from nova.utils Check whether a service is up based on last heartbeat. """ last_heartbeat = (service_ref.get('last_seen_up') or service_ref['created_at']) if isinstance(last_heartbeat, six.string_types): # NOTE(russellb) If this service_ref came in over rpc via # conductor, then the timestamp will be a string and needs to be # converted back to a datetime. last_heartbeat = timeutils.parse_strtime(last_heartbeat) else: # Objects have proper UTC timezones, but the timeutils comparison # below does not (and will fail) last_heartbeat = last_heartbeat.replace(tzinfo=None) # Timestamps in DB are UTC. elapsed = timeutils.delta_seconds(last_heartbeat, timeutils.utcnow()) is_up = abs(elapsed) <= self.service_down_time if not is_up: LOG.debug('Seems service %(binary)s on host %(host)s is down. ' 'Last heartbeat was %(lhb)s. Elapsed time is %(el)s', {'binary': service_ref.get('binary'), 'host': service_ref.get('host'), 'lhb': str(last_heartbeat), 'el': str(elapsed)}) return is_up
看到高亮的部分，我们就很容易明白，nova是如何判断一个服务是up还是down的了：
首先获取service实例的最后更新时间戳，即最后心跳时间，然后计算最后心跳时间距离现在时间的间隔，如果小于等于service_down_time的值，则认为服务是up的，否则是down。
比如假设我们设置的report_interval时间为10秒，正常的话检查最后心跳到当前时间一定小于10秒，不幸的是可能中间丢了2个心跳，那检查的最后心跳距离当前时间可能为20多秒，由于小于我们的service_down_time（假设为60秒)，因此还是认为服务是up的。如果连续丢掉超过6个心跳包，则服务就会返回down了。
问题分析
当Openstack不正常工作时，首先查看下服务状态，比如执行nova service-list命令查看Nova相关的服务状态。
如果服务状态为down，根据Openstack服务的心跳机制和状态监控原理，可能有以下几种故障情形：
1. 数据库访问错误导致心跳更新失败，这种情况看日志就能发现错误日志。
2. Rabbitmq连接失败，nova-compute不能直接访问数据库，更新时是通过RPC调用nova-conductor完成的，如果rabbitmq连接失败，RPC将无法执行，导致心跳发送失败。
3. nova-conductor故障，原因同上，不过这种情况概率很低，除非人为关闭了该服务。
4. 时间不同步。这种情况排查非常困难，因为你在日志中是发现不了任何错误信息的，我们知道数据库操作由nova-conductor组件完成的，而计算心跳间隔是在nova-api服务完成的，假如这两个服务所在的主机时间不同步，将可能导致服务误判为down。对于多API节点部署时尤其容易出现这种情况，所有节点务必保证时间同步，NTP服务必须能够正常工作，否则将影响Openstack服务的心跳机制和状态监控。
nova的这种上报方式其实这种方法效率是非常低的，并且当服务众多时，数据库的压力将会非常大，因此有人提出引入Zookeeper服务发现机制维护Openstack服务状态，参考Services Heartbeat with ZooKeeper。