glasslion/ redlock
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Description
Distributed locks with Redis and Python
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RedLock - Distributed locks with Redis and Python
This library implements the RedLock algorithm introduced by @antirez
Yet another ...
There are already a few redis based lock implementations in the Python world, e.g. retools, redis-lock.
However, these libraries can only work with single-master redis server. When the Redis master goes down, your application has to face a single point of failure. We can't rely on the master-slave replication, because Redis replication is asynchronous.
This is an obvious race condition with the master-slave replication model : 1. Client A acquires the lock into the master. 2. The master crashes before the write to the key is transmitted to the slave. 3. The slave gets promoted to master. 4. Client B acquires the lock to the same resource A already holds a lock for. SAFETY VIOLATION!
A quick introduction to the RedLock algorithm
To resolve this problem, the Redlock algorithm assume we have
NRedis masters. These nodes are totally independent (no replications). In order to acquire the lock, the client will try to acquire the lock in all the N instances sequentially. If and only if the client was able to acquire the lock in the majority (
(N+1)/2)of the instances, the lock is considered to be acquired.
The detailed description of the RedLock algorithm can be found in the Redis documentation: Distributed locks with Redis.
APIs
The
redlock.RedLockclass shares a similar API with the
threading.Lockclass in the Python Standard Library.
Basic Usage
from redlock import RedLock
# By default, if no redis connection details are
# provided, RedLock uses redis://127.0.0.1:6379/0
lock = RedLock("distributed_lock")
lock.acquire()
do_something()
lock.release()
With Statement / Context Manager
As with
threading.Lock,
redlock.RedLockobjects are context managers thus support the With Statement. This way is more pythonic and recommended.
from redlock import RedLock
with RedLock("distributed_lock"):
do_something()
Specify multiple Redis nodes
from redlock import RedLock
with RedLock("distributed_lock",
connection_details=[
{'host': 'xxx.xxx.xxx.xxx', 'port': 6379, 'db': 0},
{'host': 'xxx.xxx.xxx.xxx', 'port': 6379, 'db': 0},
{'host': 'xxx.xxx.xxx.xxx', 'port': 6379, 'db': 0},
{'host': 'xxx.xxx.xxx.xxx', 'port': 6379, 'db': 0},
]
):
do_something()
The
connection_detailsparameter expects a list of keyword arguments for initializing Redis clients. Other acceptable Redis client arguments can be found on the redis-py doc.
Reuse Redis clients with the RedLockFactory
Usually the connection details of the Redis nodes are fixed.
RedLockFactorycan help reuse them, create multiple RedLocks but only initialize the clients once.
from redlock import RedLockFactory
factory = RedLockFactory(
connection_details=[
{'host': 'xxx.xxx.xxx.xxx'},
{'host': 'xxx.xxx.xxx.xxx'},
{'host': 'xxx.xxx.xxx.xxx'},
{'host': 'xxx.xxx.xxx.xxx'},
])
with factory.create_lock("distributed_lock"):
do_something()
with factory.create_lock("another_lock"):
do_something()