启动一个进程，如要想要这个进程的某个方法定时得进行执行的话，在openstack有两种方式: 一种是通过继承 periodic_task.PeriodicTasks，另一种是使用loopingcall.py，针对两种方式分别说一下实现原理。
（1） 继承periodic_task.PeriodicTasks
这种方式比较复杂，用到了python中的一些比较高级的特性，装饰器和元类；首先看一下periodic_task.py，在nova/openstack/common中，其他组件也有。
看一下PeriodicTasks 这个类。
144 @six.add_metaclass(_PeriodicTasksMeta)
145 class PeriodicTasks(object):
146 def __init__(self):
147 super(PeriodicTasks, self).__init__()
148 self._periodic_last_run = {}
149 for name, task in self._periodic_tasks:
150 self._periodic_last_run[name] = task._periodic_last_run
151
152 def run_periodic_tasks(self, context, raise_on_error=False):
153 """Tasks to be run at a periodic interval."""
154 idle_for = DEFAULT_INTERVAL
155 for task_name, task in self._periodic_tasks:
156 full_task_name = '.'.join([self.__class__.__name__, task_name])
157
158 spacing = self._periodic_spacing[task_name]
159 last_run = self._periodic_last_run[task_name]
160
161 # If a periodic task is _nearly_ due, then we'll run it early
162 if spacing is not None:
163 idle_for = min(idle_for, spacing)
164 if last_run is not None:
165 delta = last_run + spacing - time.time()
166 if delta > 0.2:
167 idle_for = min(idle_for, delta)
168 continue
169
170 LOG.debug("Running periodic task %(full_task_name)s",
171 {"full_task_name": full_task_name})
172 self._periodic_last_run[task_name] = time.time()
173
174 try:
175 task(self, context)
176 except Exception as e:
177 if raise_on_error:
178 raise
179 LOG.exception(_LE("Error during %(full_task_name)s: %(e)s"),
180 {"full_task_name": full_task_name, "e": e})
181 time.sleep(0)
182
183 return idle_for
run_periodic_tasks 函数是用户启动各个定时任务的，其中里面有几个数据结构比较重要，self._periodic_tasks：记录来每个task和每个task的函数句柄；self._periodic_spacing: 记录每一个task的运行间隔时间。在__init__函数中，还有构造一个self._periodic_last_run 结构用来记录每一个task上一次运行的时间；具体运行的时候会根据上次运行时间和间隔时间来确定是否运行，函数第162～168行；那具体的self._periodic_tasks和self._periodic_spacing是怎么得来的，是通过元类的方式来实现的；
元类可以干预一个类的实现形式，比方说在为一个类添加一个方法，或者为了一个类统一添加某种行为；上文的@six.add_metaclass(_PeriodicTasksMeta)语法就是添加了一个元类，我们看一下_PeriodicTasksMeta的实现；
100 class _PeriodicTasksMeta(type):
101 def __init__(cls, names, bases, dict_):
102 """Metaclass that allows us to collect decorated periodic tasks."""
103 super(_PeriodicTasksMeta, cls).__init__(names, bases, dict_)
104
105 # NOTE(sirp): if the attribute is not present then we must be the base
106 # class, so, go ahead an initialize it. If the attribute is present,
107 # then we're a subclass so make a copy of it so we don't step on our
108 # parent's toes.
109 try:
110 cls._periodic_tasks = cls._periodic_tasks[:]
111 except AttributeError:
112 cls._periodic_tasks = []
113
114 try:
115 cls._periodic_spacing = cls._periodic_spacing.copy()
116 except AttributeError:
117 cls._periodic_spacing = {}
118
119 for value in cls.__dict__.values():
120 if getattr(value, '_periodic_task', False):
121 task = value
122 name = task.__name__
123
124 if task._periodic_spacing < 0:
125 LOG.info(_LI('Skipping periodic task %(task)s because '
126 'its interval is negative'),
127 {'task': name})
128 continue
129 if not task._periodic_enabled:
130 LOG.info(_LI('Skipping periodic task %(task)s because '
131 'it is disabled'),
132 {'task': name})
133 continue
134
135 # A periodic spacing of zero indicates that this task should
136 # be run every pass
137 if task._periodic_spacing == 0:
138 task._periodic_spacing = None
139
140 cls._periodic_tasks.append((name, task))
141 cls._periodic_spacing[name] = task._periodic_spacing
其中109～117为类添加_periodic_tasks与_periodic_spacing两个类变量， for value in cls.__dict__.values() 语句访问类的各个成员，主要是函数成员；如果发现成员中有_periodic_task属性，并且等于True，则构造_periodic_tasks与_periodic_spacing两个数据结构；那么剩下就要弄清楚task的结构了，task就是类中的一个函数，它为什么具有_periodic_task属性和_periodic_spacing的呢？这个活就是装饰器做的事情了。
当你给一个函数设置定时任务的装饰器时，一般会这样写：
@periodic_task.periodic_task(spacing=…, run_immediately=...)
def f(args,kwargs):
…….
奥妙就在这个装饰器里面了。
42 def periodic_task(*args, **kwargs):
43 """Decorator to indicate that a method is a periodic task.
44
45 This decorator can be used in two ways:
46
47 1. Without arguments '@periodic_task', this will be run on every cycle
48 of the periodic scheduler.
49
50 2. With arguments:
51 @periodic_task(spacing=N [, run_immediately=[True|False]])
52 this will be run on approximately every N seconds. If this number is
53 negative the periodic task will be disabled. If the run_immediately
54 argument is provided and has a value of 'True', the first run of the
55 task will be shortly after task scheduler starts. If
56 run_immediately is omitted or set to 'False', the first time the
57 task runs will be approximately N seconds after the task scheduler
58 starts.
59 """
60 def decorator(f):
61 # Test for old style invocation
62 if 'ticks_between_runs' in kwargs:
63 raise InvalidPeriodicTaskArg(arg='ticks_between_runs')
64
65 # Control if run at all
66 f._periodic_task = True
67 f._periodic_external_ok = kwargs.pop('external_process_ok', False)
68 if f._periodic_external_ok and not CONF.run_external_periodic_tasks:
69 f._periodic_enabled = False
70 else:
71 f._periodic_enabled = kwargs.pop('enabled', True)
72
73 # Control frequency
74 f._periodic_spacing = kwargs.pop('spacing', 0)
75 f._periodic_immediate = kwargs.pop('run_immediately', False)
76 if f._periodic_immediate:
77 f._periodic_last_run = None
78 else:
79 f._periodic_last_run = time.time()
80 return f
81
82 # NOTE(sirp): The `if` is necessary to allow the decorator to be used with
83 # and without parents.
84 #
85 # In the 'with-parents' case (with kwargs present), this function needs to
86 # return a decorator function since the interpreter will invoke it like:
87 #
88 # periodic_task(*args, **kwargs)(f)
89 #
90 # In the 'without-parents' case, the original function will be passed
91 # in as the first argument, like:
92 #
93 # periodic_task(f)
94 if kwargs:
95 return decorator
96 else:
97 return decorator(args[0])
在装饰器中，66～80行就是为一个函数设置_periodic_task属性，并从装饰器的kwargs中取得spacing参数，如果有run_immediately（启动之后先立刻执行一次，然后再定时执行）会设置_periodic_last_run属性。这样定时函数运行需要的信息就都齐全了；
（2）另一个种方法是使用loopingcall.py ，也在nova/openstack/common中。
使用方法是：obj = loopingcall.FixedIntervalLoopingCall(f, args，kwargs)，
obj.start(interval,initial_delay)
代码：
62 class FixedIntervalLoopingCall(LoopingCallBase):
63 """A fixed interval looping call."""
64
65 def start(self, interval, initial_delay=None):
66 self._running = True
67 done = event.Event()
68
69 def _inner():
70 if initial_delay:
71 greenthread.sleep(initial_delay)
72
73 try:
74 while self._running:
75 start = timeutils.utcnow()
76 self.f(*self.args, **self.kw)
77 end = timeutils.utcnow()
78 if not self._running:
79 break
80 delay = interval - timeutils.delta_seconds(start, end)
81 if delay <= 0:
82 LOG.warn(_LW('task run outlasted interval by %s sec') %
83 -delay)
84 greenthread.sleep(delay if delay > 0 else 0)
85 except LoopingCallDone as e:
86 self.stop()
87 done.send(e.retvalue)
88 except Exception:
89 LOG.exception(_LE('in fixed duration looping call'))
90 done.send_exception(*sys.exc_info())
91 return
92 else:
93 done.send(True)
94
95 self.done = done
96
97 greenthread.spawn_n(_inner)
98 return self.done
start()方法运行定时任务，initial_delay表示是否有延时，75~84每次运行任务要记录开始时间和结束时间，如果开始时间减去结束时间比interval还大的话，那么就不等待了，立刻运行：greenthread.sleep(delay if delay > 0 else 0)