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"""Redis lock and store wrapper.
The lock implementation was mostly lifted from
http://chris-lamb.co.uk/2010/06/07/distributing-locking-python-and-redis/
"""
import shelve
from .time import time, sleep
class Lock(object):
def __init__(self, db, key, expires=60):
"""
Distributed locking using Redis SETNX and GETSET.
Usage::
with Lock('my_lock'):
print "Critical section"
:param expires We consider any existing lock older than
``expires`` seconds to be invalid in order to
detect crashed clients. This value must be higher
than it takes the critical section to execute.
:param timeout If another client has already obtained the lock,
sleep for a maximum of ``timeout`` seconds before
giving up. A value of 0 means we never wait.
"""
self.db = db
self.key = key
self.expires = expires
def acquire(self, timeout):
delay = 0.1
while timeout >= 0:
expires = time() + self.expires + 1
if self.db.setnx(self.key, expires):
# We gained the lock; enter critical section
return True
current_value = self.db.get(self.key)
# We found an expired lock and nobody raced us to replacing it
if current_value and float(current_value) < time() and \
self.db.getset(self.key, expires) == current_value:
return True
sleep(min(timeout, delay))
timeout -= delay
delay *= 2
return False
def release(self):
self.db.delete(self.key)
def wrap(redis, lock_class=Lock):
def lock(key):
return lock_class(redis, key + '.lock')
db = shelve.Shelf(redis)
db.lock = lock
return db
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