DistLock
Distributed locks with Redis and Python

Why DistLock ?

This library implements the DistLock algorithm introduced by @antirez, Due to the origin fork from redlock has been inactive since 2015, Then this repo was born with additional implementations:

• Bug fix
• Python3 syntax improvements
• New tweak for adaptation

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 DistLock algorithm

To resolve this problem, the Distlock algorithm assume we have N Redis 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 DistLock algorithm can be found in the Redis documentation: Distributed locks with Redis.

APIs

The distlock.DistLock class shares a similar API with the threading.Lock class in the Python Standard Library.

Basic Usage

from distlock import DistLock
# By default, if no redis connection details are
# provided, DistLock uses redis://127.0.0.1:6379/0
lock =  DistLock("distributed_lock")
lock.acquire()
do_something()
lock.release()

With Statement / Context Manager

As with threading.Lock, distlock.DistLock objects are context managers thus support the With Statement. This way is more pythonic and recommended.

from distlock import DistLock
with DistLock("distributed_lock"):
    do_something()

Specify multiple Redis nodes

from distlock import DistLock
with DistLock("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_details parameter 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 DistLockFactory

Usually the connection details of the Redis nodes are fixed. DistLockFactory can help reuse them, create multiple DistLocks but only initialize the clients once.

from distlock import DistLockFactory
factory = DistLockFactory(
    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()