大话openstack之资源刷新

zhouyi

　　   在openstack上测试批量创建thin lun时，发现部分创建失败，分析代码发现，根本原因是主机的能力是周期性刷新（默认是1分钟），而不是即时刷新。现在来详细分析openstack的资源刷新机制，主要以cinder为例。      
　　   在cinder源代码中, 可以在很多函数上看到@periodic_task这样的修饰符, 我们根据_publish_service_capabilities这个函数知道这是cinder的定时任务,可以让这个函数周期性执行, 但是不了解这个修饰符产生作用的原理和用法, 这里将进行详细分析。
　　  关于元类：就像对象是类的实例一样，类是它的元类的实例。
　　 在manager.py中更新函数的定义如下：@periodic_task.periodic_task说明该函数被装饰器装饰。被此装饰器修饰后，函数新增了这个属性：f._periodic_task = True。
　　 

    @periodic_task.periodic_task
def _publish_service_capabilities(self, context):
"""Pass data back to the scheduler at a periodic interval."""
if self.last_capabilities:
LOG.debug('Notifying Schedulers of capabilities ...')
self.scheduler_rpcapi.update_service_capabilities(
context,
self.service_name,
self.host,
self.last_capabilities)
　　 

def periodic_task(*args, **kwargs):
"""Decorator to indicate that a method is a periodic task.
This decorator can be used in two ways:
1. Without arguments '@periodic_task', this will be run on the default
interval of 60 seconds.
2. With arguments:
@periodic_task(spacing=N [, run_immediately=[True|False]]
[, name=[None|"string"])
this will be run on approximately every N seconds. If this number is
negative the periodic task will be disabled. If the run_immediately
argument is provided and has a value of 'True', the first run of the
task will be shortly after task scheduler starts.  If
run_immediately is omitted or set to 'False', the first time the
task runs will be approximately N seconds after the task scheduler
starts. If name is not provided, __name__ of function is used.
"""
def decorator(f):
# Test for old style invocation
if 'ticks_between_runs' in kwargs:
raise InvalidPeriodicTaskArg(arg='ticks_between_runs')
# Control if run at all
f._periodic_task = True
f._periodic_external_ok = kwargs.pop('external_process_ok', False)
f._periodic_enabled = kwargs.pop('enabled', True)
f._periodic_name = kwargs.pop('name', f.__name__)
# Control frequency
f._periodic_spacing = kwargs.pop('spacing', 0)
f._periodic_immediate = kwargs.pop('run_immediately', False)
if f._periodic_immediate:
f._periodic_last_run = None
else:
f._periodic_last_run = now()
return f
# NOTE(sirp): The `if` is necessary to allow the decorator to be used with
# and without parenthesis.
#
# In the 'with-parenthesis' case (with kwargs present), this function needs
# to return a decorator function since the interpreter will invoke it like:
#
#   periodic_task(*args, **kwargs)(f)
#
# In the 'without-parenthesis' case, the original function will be passed
# in as the first argument, like:
#
#   periodic_task(f)
if kwargs:
return decorator
else:
return decorator(args[0])
　　  periodic_task.py中的类_PeriodicTasksMeta是PeriodicTasks类的元类，在PeriodicTasks初始化时会调用_PeriodicTasksMeta的init方法，这三个语句将有_periodic_task这个属性的函数对象加入到Manager._periodic_tasks中。
　　

        for value in cls.__dict__.values():
if getattr(value, '_periodic_task', False):
cls._add_periodic_task(value)
　　 
　　
　　  

class _PeriodicTasksMeta(type):
def _add_periodic_task(cls, task):
"""Add a periodic task to the list of periodic tasks.
The task should already be decorated by @periodic_task.
:return: whether task was actually enabled
"""
name = task._periodic_name
if task._periodic_spacing < 0:
LOG.info(_LI('Skipping periodic task %(task)s because '
'its interval is negative'),
{'task': name})
return False
if not task._periodic_enabled:
LOG.info(_LI('Skipping periodic task %(task)s because '
'it is disabled'),
{'task': name})
return False
# A periodic spacing of zero indicates that this task should
# be run on the default interval to avoid running too
# frequently.
if task._periodic_spacing == 0:
task._periodic_spacing = DEFAULT_INTERVAL
cls._periodic_tasks.append((name, task))
cls._periodic_spacing[name] = task._periodic_spacing
return True
def __init__(cls, names, bases, dict_):
"""Metaclass that allows us to collect decorated periodic tasks."""
super(_PeriodicTasksMeta, cls).__init__(names, bases, dict_)
# NOTE(sirp): if the attribute is not present then we must be the base
# class, so, go ahead an initialize it. If the attribute is present,
# then we're a subclass so make a copy of it so we don't step on our
# parent's toes.
try:
cls._periodic_tasks = cls._periodic_tasks[:]
except AttributeError:
cls._periodic_tasks = []
try:
cls._periodic_spacing = cls._periodic_spacing.copy()
except AttributeError:
cls._periodic_spacing = {}
for value in cls.__dict__.values():
if getattr(value, '_periodic_task', False):
cls._add_periodic_task(value)
　　 Manager的这个类有一个函数periodic_tasks，阅读函数代码可以发现这个函数实际上的作用就是把所有在self._periodic_tasks中的函数对象(也就是所有用@periodic_task修饰符修饰过的函数)全部遍历并调用一遍. 如果能定期调用这个函数的话, 就能实现类似linux中crontab的定时任务功能.
　　

class Manager(base.Base, PeriodicTasks):
# Set RPC API version to 1.0 by default.
RPC_API_VERSION = '1.0'
target = messaging.Target(version=RPC_API_VERSION)
def __init__(self, host=None, db_driver=None):
if not host:
host = CONF.host
self.host = host
self.additional_endpoints = []
super(Manager, self).__init__(db_driver)
def periodic_tasks(self, context, raise_on_error=False):
"""Tasks to be run at a periodic interval."""
return self.run_periodic_tasks(context, raise_on_error=raise_on_error)
　　 
　　

@six.add_metaclass(_PeriodicTasksMeta)
class PeriodicTasks(object):
def __init__(self, conf):
super(PeriodicTasks, self).__init__()
self.conf = conf
self.conf.register_opts(_options.periodic_opts)
self._periodic_last_run = {}
for name, task in self._periodic_tasks:
self._periodic_last_run[name] = task._periodic_last_run
def add_periodic_task(self, task):
"""Add a periodic task to the list of periodic tasks.
The task should already be decorated by @periodic_task.
"""
if self.__class__._add_periodic_task(task):
self._periodic_last_run[task._periodic_name] = (
task._periodic_last_run)
def run_periodic_tasks(self, context, raise_on_error=False):
"""Tasks to be run at a periodic interval."""
idle_for = DEFAULT_INTERVAL
for task_name, task in self._periodic_tasks:
if (task._periodic_external_ok and not
self.conf.run_external_periodic_tasks):
continue
full_task_name = '.'.join([self.__class__.__name__, task_name])
spacing = self._periodic_spacing[task_name]
last_run = self._periodic_last_run[task_name]
# Check if due, if not skip
idle_for = min(idle_for, spacing)
if last_run is not None:
delta = last_run + spacing - now()
if delta > 0:
idle_for = min(idle_for, delta)
continue
LOG.debug("Running periodic task %(full_task_name)s",
{"full_task_name": full_task_name})
self._periodic_last_run[task_name] = _nearest_boundary(
last_run, spacing)
try:
task(self, context)
except Exception:
if raise_on_error:
raise
LOG.exception(_LE("Error during %(full_task_name)s"),
{"full_task_name": full_task_name})
time.sleep(0)
return idle_for
　　 service.py 中server类RPC server服务启动时，调用start函数，该函数中会周期性的调用Manager的periodic_tasks函数。即这样实现了host的资源刷新。
　　

        if self.periodic_interval:
if self.periodic_fuzzy_delay:
initial_delay = random.randint(0, self.periodic_fuzzy_delay)
else:
initial_delay = None
periodic = loopingcall.FixedIntervalLoopingCall(
self.periodic_tasks)
periodic.start(interval=self.periodic_interval,
initial_delay=initial_delay)
self.timers.append(periodic)

　　 

    def periodic_tasks(self, raise_on_error=False):
"""Tasks to be run at a periodic interval."""
ctxt = context.get_admin_context()
self.manager.periodic_tasks(ctxt, raise_on_error=raise_on_error)