class CacheGenerator(object):
"""This class is a thread-safe way to retrieve data that was cached
using group_cache. With CacheGenerator, when data is not present
in the cache, only one thread is delegated to update it. The
rest of the threads wait on file locks, thereby reducing the
server load.
.. note::
:class:`CacheGenerator` should be used to retrieve data
from group_cache. That being said, it is safe to invalidate
group_cache without thread synchronisation.
.. note::
:class:`CacheGenerator` is a context manager, so it should
be used in a ``with`` statement.
"""
def __init__(self, cache_key, cache_group):
self.cache_key = cache_key
self.cache_group = cache_group
self.name = self.get_instance_name(cache_key, cache_group)
self.lock = None
self._in_context = 0
def __enter__(self):
self._in_context += 1
if self._in_context == 1:
self.lock = FileLock(self.name)
return self
def __exit__(self, exc_type, exc_value, traceback):
self._in_context -= 1
if self._in_context == 0:
self.lock.unlock()
@staticmethod
def get_instance_name(cache_key, cache_group):
return group_cache.generate_cache_key(cache_key, cache_group)
def _get_obj_from_cache(self):
return group_cache.get(self.cache_key, self.cache_group)
def _cache_obj(self, obj, timeout):
group_cache.set(self.cache_key, self.cache_group, obj, timeout)
def get_cached_obj(self, generate_obj, timeout):
""":param generate_obj: A function taking no arguments and
returning a current version of the
object to cache.
:param timeout: Timeout in seconds for the cached object.
"""
self.lock.lock_shared()
cached_obj = self._get_obj_from_cache()
if cached_obj is not None:
# Object is in the cache, so we return it and *maintain*
# the lock for the lifetime of this object.
return cached_obj
self.lock.unlock()
self.lock.lock_exclusive()
# After acquiring an exclusive lock we check if the object is
# still missing from the cache.
cached_obj = self._get_obj_from_cache()
if cached_obj is not None:
self.lock.lock_shared()
return cached_obj
try:
cached_obj = generate_obj()
self._cache_obj(cached_obj, timeout)
except:
self.lock.unlock()
raise
self.lock.lock_shared()
return cached_obj
def __enter__(self):
self._in_context += 1
if self._in_context == 1:
self.lock = FileLock(self.name)
return self
