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"""
Caching framework.
This module defines set of cache backends that all conform to a simple API.
In a nutshell, a cache is a set of values -- which can be any object that
may be pickled -- identified by string keys. For the complete API, see
the abstract Cache object, below.
Client code should not access a cache backend directly; instead
it should use the get_cache() function. This function will look at
settings.CACHE_BACKEND and use that to create and load a cache object.
The CACHE_BACKEND setting is a quasi-URI; examples are:
memcached://127.0.0.1:11211/ A memcached backend; the server is running
on localhost port 11211.
pgsql://tablename/ A pgsql backend (the pgsql backend uses
the same database/username as the rest of
the CMS, so only a table name is needed.)
file:///var/tmp/django.cache/ A file-based cache at /var/tmp/django.cache
simple:/// A simple single-process memory cache; you
probably don't want to use this except for
testing. Note that this cache backend is
NOT threadsafe!
All caches may take arguments; these are given in query-string style. Valid
arguments are:
timeout
Default timeout, in seconds, to use for the cache. Defaults
to 5 minutes (300 seconds).
max_entries
For the simple, file, and database backends, the maximum number of
entries allowed in the cache before it is cleaned. Defaults to
300.
cull_percentage
The percentage of entries that are culled when max_entries is reached.
The actual percentage is 1/cull_percentage, so set cull_percentage=3 to
cull 1/3 of the entries when max_entries is reached.
A value of 0 for cull_percentage means that the entire cache will be
dumped when max_entries is reached. This makes culling *much* faster
at the expense of more cache misses.
For example:
memcached://127.0.0.1:11211/?timeout=60
pgsql://tablename/?timeout=120&max_entries=500&cull_percentage=4
Invalid arguments are silently ignored, as are invalid values of known
arguments.
So far, only the memcached and simple backend have been implemented; backends
using postgres, and file-system storage are planned.
"""
##############
# Exceptions #
##############
class InvalidCacheBackendError(Exception):
pass
################################
# Abstract base implementation #
################################
class _Cache:
def __init__(self, params):
timeout = params.get('timeout', 300)
try:
timeout = int(timeout)
except (ValueError, TypeError):
timeout = 300
self.default_timeout = timeout
def get(self, key, default=None):
'''
Fetch a given key from the cache. If the key does not exist, return
default, which itself defaults to None.
'''
raise NotImplementedError
def set(self, key, value, timeout=None):
'''
Set a value in the cache. If timeout is given, that timeout will be
used for the key; otherwise the default cache timeout will be used.
'''
raise NotImplementedError
def delete(self, key):
'''
Delete a key from the cache, failing silently.
'''
raise NotImplementedError
def get_many(self, keys):
'''
Fetch a bunch of keys from the cache. For certain backends (memcached,
pgsql) this can be *much* faster when fetching multiple values.
Returns a dict mapping each key in keys to its value. If the given
key is missing, it will be missing from the response dict.
'''
d = {}
for k in keys:
val = self.get(k)
if val is not None:
d[k] = val
return d
def has_key(self, key):
'''
Returns True if the key is in the cache and has not expired.
'''
return self.get(key) is not None
###########################
# memcached cache backend #
###########################
try:
import memcache
except ImportError:
_MemcachedCache = None
else:
class _MemcachedCache(_Cache):
"""Memcached cache backend."""
def __init__(self, server, params):
_Cache.__init__(self, params)
self._cache = memcache.Client([server])
def get(self, key, default=None):
val = self._cache.get(key)
if val is None:
return default
else:
return val
def set(self, key, value, timeout=0):
self._cache.set(key, value, timeout)
def delete(self, key):
self._cache.delete(key)
def get_many(self, keys):
return self._cache.get_multi(keys)
##################################
# Single-process in-memory cache #
##################################
import time
class _SimpleCache(_Cache):
"""Simple single-process in-memory cache"""
def __init__(self, host, params):
_Cache.__init__(self, params)
self._cache = {}
self._expire_info = {}
max_entries = params.get('max_entries', 300)
try:
self._max_entries = int(max_entries)
except (ValueError, TypeError):
self._max_entries = 300
cull_frequency = params.get('cull_frequency', 3)
try:
self._cull_frequency = int(cull_frequency)
except (ValueError, TypeError):
self._cull_frequency = 3
def get(self, key, default=None):
now = time.time()
exp = self._expire_info.get(key, now)
if exp is not None and exp < now:
del self._cache[key]
del self._expire_info[key]
return default
else:
return self._cache.get(key, default)
def set(self, key, value, timeout=None):
if len(self._cache) >= self._max_entries:
self._cull()
if timeout is None:
timeout = self.default_timeout
self._cache[key] = value
self._expire_info[key] = time.time() + timeout
def delete(self, key):
try:
del self._cache[key]
except KeyError:
pass
try:
del self._expire_info[key]
except KeyError:
pass
def has_key(self, key):
return self._cache.has_key(key)
def _cull(self):
if self._cull_frequency == 0:
self._cache.clear()
self._expire_info.clear()
else:
doomed = [k for (i, k) in enumerate(self._cache) if i % self._cull_frequency == 0]
for k in doomed:
self.delete(k)
##########################################
# Read settings and load a cache backend #
##########################################
from cgi import parse_qsl
_BACKENDS = {
'memcached' : _MemcachedCache,
'simple' : _SimpleCache,
}
def get_cache(backend_uri):
if backend_uri.find(':') == -1:
raise InvalidCacheBackendError("Backend URI must start with scheme://")
scheme, rest = backend_uri.split(':', 1)
if not rest.startswith('//'):
raise InvalidCacheBackendError("Backend URI must start with scheme://")
if scheme not in _BACKENDS.keys():
raise InvalidCacheBackendError("%r is not a valid cache backend" % scheme)
host = rest[2:]
qpos = rest.find('?')
if qpos != -1:
params = dict(parse_qsl(rest[qpos+1:]))
host = rest[:qpos]
else:
params = {}
if host.endswith('/'):
host = host[:-1]
return _BACKENDS[scheme](host, params)
from django.conf.settings import CACHE_BACKEND
cache = get_cache(CACHE_BACKEND)
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