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"Database cache backend."
import base64
import time
from datetime import datetime
try:
import cPickle as pickle
except ImportError:
import pickle
from django.conf import settings
from django.core.cache.backends.base import BaseCache
from django.db import connections, router, transaction, DatabaseError
from django.utils import timezone
class Options(object):
"""A class that will quack like a Django model _meta class.
This allows cache operations to be controlled by the router
"""
def __init__(self, table):
self.db_table = table
self.app_label = 'django_cache'
self.module_name = 'cacheentry'
self.verbose_name = 'cache entry'
self.verbose_name_plural = 'cache entries'
self.object_name = 'CacheEntry'
self.abstract = False
self.managed = True
self.proxy = False
class BaseDatabaseCache(BaseCache):
def __init__(self, table, params):
BaseCache.__init__(self, params)
self._table = table
class CacheEntry(object):
_meta = Options(table)
self.cache_model_class = CacheEntry
class DatabaseCache(BaseDatabaseCache):
# This class uses cursors provided by the database connection. This means
# it reads expiration values as aware or naive datetimes depending on the
# value of USE_TZ. They must be compared to aware or naive representations
# of "now" respectively.
# But it bypasses the ORM for write operations. As a consequence, aware
# datetimes aren't made naive for databases that don't support time zones.
# We work around this problem by always using naive datetimes when writing
# expiration values, in UTC when USE_TZ = True and in local time otherwise.
def get(self, key, default=None, version=None):
key = self.make_key(key, version=version)
self.validate_key(key)
db = router.db_for_read(self.cache_model_class)
table = connections[db].ops.quote_name(self._table)
cursor = connections[db].cursor()
cursor.execute("SELECT cache_key, value, expires FROM %s "
"WHERE cache_key = %%s" % table, [key])
row = cursor.fetchone()
if row is None:
return default
now = timezone.now()
if row[2] < now:
db = router.db_for_write(self.cache_model_class)
cursor = connections[db].cursor()
cursor.execute("DELETE FROM %s "
"WHERE cache_key = %%s" % table, [key])
transaction.commit_unless_managed(using=db)
return default
value = connections[db].ops.process_clob(row[1])
return pickle.loads(base64.decodestring(value))
def set(self, key, value, timeout=None, version=None):
key = self.make_key(key, version=version)
self.validate_key(key)
self._base_set('set', key, value, timeout)
def add(self, key, value, timeout=None, version=None):
key = self.make_key(key, version=version)
self.validate_key(key)
return self._base_set('add', key, value, timeout)
def _base_set(self, mode, key, value, timeout=None):
if timeout is None:
timeout = self.default_timeout
db = router.db_for_write(self.cache_model_class)
table = connections[db].ops.quote_name(self._table)
cursor = connections[db].cursor()
cursor.execute("SELECT COUNT(*) FROM %s" % table)
num = cursor.fetchone()[0]
now = timezone.now()
now = now.replace(microsecond=0)
if settings.USE_TZ:
exp = datetime.utcfromtimestamp(time.time() + timeout)
else:
exp = datetime.fromtimestamp(time.time() + timeout)
exp = exp.replace(microsecond=0)
if num > self._max_entries:
self._cull(db, cursor, now)
pickled = pickle.dumps(value, pickle.HIGHEST_PROTOCOL)
encoded = base64.encodestring(pickled).strip()
cursor.execute("SELECT cache_key, expires FROM %s "
"WHERE cache_key = %%s" % table, [key])
try:
result = cursor.fetchone()
if result and (mode == 'set' or
(mode == 'add' and result[1] < now)):
cursor.execute("UPDATE %s SET value = %%s, expires = %%s "
"WHERE cache_key = %%s" % table,
[encoded, connections[db].ops.value_to_db_datetime(exp), key])
else:
cursor.execute("INSERT INTO %s (cache_key, value, expires) "
"VALUES (%%s, %%s, %%s)" % table,
[key, encoded, connections[db].ops.value_to_db_datetime(exp)])
except DatabaseError:
# To be threadsafe, updates/inserts are allowed to fail silently
transaction.rollback_unless_managed(using=db)
return False
else:
transaction.commit_unless_managed(using=db)
return True
def delete(self, key, version=None):
key = self.make_key(key, version=version)
self.validate_key(key)
db = router.db_for_write(self.cache_model_class)
table = connections[db].ops.quote_name(self._table)
cursor = connections[db].cursor()
cursor.execute("DELETE FROM %s WHERE cache_key = %%s" % table, [key])
transaction.commit_unless_managed(using=db)
def has_key(self, key, version=None):
key = self.make_key(key, version=version)
self.validate_key(key)
db = router.db_for_read(self.cache_model_class)
table = connections[db].ops.quote_name(self._table)
cursor = connections[db].cursor()
if settings.USE_TZ:
now = datetime.utcnow()
else:
now = datetime.now()
now = now.replace(microsecond=0)
cursor.execute("SELECT cache_key FROM %s "
"WHERE cache_key = %%s and expires > %%s" % table,
[key, connections[db].ops.value_to_db_datetime(now)])
return cursor.fetchone() is not None
def _cull(self, db, cursor, now):
if self._cull_frequency == 0:
self.clear()
else:
# When USE_TZ is True, 'now' will be an aware datetime in UTC.
now = now.replace(tzinfo=None)
table = connections[db].ops.quote_name(self._table)
cursor.execute("DELETE FROM %s WHERE expires < %%s" % table,
[connections[db].ops.value_to_db_datetime(now)])
cursor.execute("SELECT COUNT(*) FROM %s" % table)
num = cursor.fetchone()[0]
if num > self._max_entries:
cull_num = num / self._cull_frequency
if connections[db].vendor == 'oracle':
# Oracle doesn't support LIMIT + OFFSET
cursor.execute("""SELECT cache_key FROM
(SELECT ROW_NUMBER() OVER (ORDER BY cache_key) AS counter, cache_key FROM %s)
WHERE counter > %%s AND COUNTER <= %%s""" % table, [cull_num, cull_num + 1])
else:
# This isn't standard SQL, it's likely to break
# with some non officially supported databases
cursor.execute("SELECT cache_key FROM %s "
"ORDER BY cache_key "
"LIMIT 1 OFFSET %%s" % table, [cull_num])
cursor.execute("DELETE FROM %s "
"WHERE cache_key < %%s" % table,
[cursor.fetchone()[0]])
def clear(self):
db = router.db_for_write(self.cache_model_class)
table = connections[db].ops.quote_name(self._table)
cursor = connections[db].cursor()
cursor.execute('DELETE FROM %s' % table)
# For backwards compatibility
class CacheClass(DatabaseCache):
pass
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