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Python Mongodb based Distributed Lock

Project description

Usage
_____

from mongolock import MongoLock

lock = MongoLock()

# you can use it as context:
# (note: if lock is already taken by another worker, MongoLockLocked will raised)
with lock('key', 'my_worker_name', expires=60, timeout=10):
# some work here

# or simply calling methods:
if lock.lock('key', 'my_worker_name'):
try:
# some usefull work
finally:
lock.release('key', 'my_worker_name')

# you can also renew lock by touching it:
with lock('key', 'my_worker_name', expires=60, timeout=10):
# some loong loong work here
lock.touch('key', 'my_worker_name')


Parameters in `lock` method:

* `key` - name of task to lock
* `owner` - name of worker, which takes a lock
* `expire` (optional) - duration in seconds after which lock can be stealed
* `timeout` (optional) - how long we can wait for a lock (in seconds)

Configuration nuances
_____________________

You can configure connection either by specifying connection string,

::
lock = MongoLock('localhost:27017')


or by passing configured instance of MongoClient/MongoReplicaSetClient in MongoLock constructor:

::
client = MongoClient('localhost:27017')
lock = MongoLock(client=client)

The second is preferred, as in such a way you can perform more fine grained configuration:

* use MongoReplicaSetClient
* specify write concern
* specify tags
* etc

Important things to think about
_______________________________

Lock stealing and releasing
~~~~~~~~~~~~~~~~~~~~~~~~~~~

You should use unique names for all your workers, if you don't - some strange things may happens.
Consider following sequence:

* worker1 and worker2 has the same names: worker
* worker1 achieve lock with expire +30s
* worker2 try to get lock after 30s (worker1 still working)
* as lock expires, worker2 steal it
* now worker1 ends his work and releases lock, as they has same names - lock released
* worker3 takes a lock, while worker2 still working ...

Lock simply not works
~~~~~~~~~~~~~~~~~~~~~

If you use cron (or some analogue) to launch tasks on different machines, take in account time drift between machines as well
as duration of critical section of your tasks.

* machine1 and machine2 has a time drift 5s
* worker on machine1 takes a lock on TaskA
* worker on machine1 done a TaskA in 1s
* now worker on machine2 comes in play
* he don't know, that worker on machine1 already complete TaskA
* he takes a lock on TaskA (without problem as lock was released 4s before) and do it

To prevent such a weird situation, you can simply add some part of time (for example YmdHM) to a key with task name.

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