This app provides functionality for locking functions or blocks of code to prevent multiple simultaneous threads from executing them at the same time. On App Engine where simultaneous threads can be running on different instances, Python's normal threading locks cannot provide this functionality.
If you're using the
STRONG lock kind (which is the default), then you will want to add the
INSTALLED_APPS, and add the
cleanup-locks URL to your URL config and call it
periodically on a cron.
# settings.py INSTALLED_APPS = ( ... 'djangae.contrib.locking', ) # Your ROOT_URLCONF file from djangae.contrib.locking.urls import urlpatterns as locking_urls urlpatterns += locking_urls # Your cron.yaml file cron: - description: Cleanup locks url: /djangae-cleanup-locks/ schedule: every 1 days
The main utility is the
lock object, which can be used as a function decorator or context manager.
lock(identifier, wait=True, steal_after_ms=None, kind=LOCK_KINDS.STRONG)
identifier- a string which uniquely identifies the block of code that you want to lock.
wait- whether to wait if another thread has already got a lock with the same identifier, or to bail.
- In the function decorator case, bailing means that the function will not be run.
- In the context manager case, bailing means that
LockAcquisitionErrorwill be raised when entering
steal_after_ms- if passed, then any existing lock which is older than this value will be ignored.
kind- which kind of lock implementation to use.
- LOCK_KINDS.WEAK is not guaranteed to be robust, but can be used for situations where avoiding simultaneous code execution is preferable but not critical (uses memcache).
- LOCK_KINDS.STRONG is for where prevention of simultaneous code execution is required (uses the datastore).
Locking a function
from djangae.contrib.locking import lock, LOCK_KINDS, LockAcquisitionError @lock('my_lock') def refresh_user_oauth_token(user): """ This function refreshes a user's oauth token (thereby invalidating old one) and then saves the new token onto the user object. If multiple threads run this at the same time then we unecessarily hammer the external API and one thread potentially invalidates the token which was just fetched by another thread. @lock to the rescue! """ user.refresh_from_db() if token_is_expired(user): new_token = get_new_token_from_external_api(user) update_user(user, new_token)
Locking a block of code using the context manager
with lock('my_other_lock'): do_something_which_should_not_be_run_many_times_at_once()
Bailing if another thread is already holding the lock (decorator)
@lock('my_lock', wait=False): def my_function(): # This will not be called if another thread is already holding the lock
Bailing if another thread is already holding the lock (context manager)
try: with lock('my_lock', wait=False): # This will not be run if another thread is already holding the lock except LockAcquisitionError: pass
Lower Level Interface
If you want to be able to acquire and release the locks manually, then you can use the lower-level
Lock class directly.
Lock.acquire(identifier, wait=True, steal_after_ms=None, kind=LOCK_KINDS.STRONG)
- Class method which returns a
Lockobject or if
wait=Falseand another thread has the lock, returns
- Keyword arguments are the same as for
- Class method which returns a
- Instance method which releases the lock.
from djangae.contrib.locking import Lock lock = Lock.acquire('my_lock') do_something_which_should_not_be_run_many_times_at_once() lock.release()