Examples

How to Use Django DB Mutex

The Django DB Mutex app provides a context manager and function decorator for locking a critical section of code. The context manager is used in the following way:

from db_mutex import DBMutexError, DBMutexTimeoutError
from db_mutex.db_mutex import db_mutex

# Lock a critical section of code
try:
    with db_mutex('lock_id'):
        # Run critical code here
        pass
except DBMutexError:
    print('Could not obtain lock')
except DBMutexTimeoutError:
    print('Task completed but the lock timed out')

You’ll notice that two errors were caught from this context manager. The first one, DBMutexError, is thrown if the lock cannot be acquired. The second one, DBMutexTimeoutError, is thrown if the critical code completes but the lock timed out. More about lock timeout in the next section.

The db_mutex decorator can also be used in a similar manner for locking a function:

from db_mutex import DBMutexError, DBMutexTimeoutError
from db_mutex.db_mutex import db_mutex

@db_mutex('lock_id')
def critical_function():
    pass

try:
    critical_function()
except DBMutexError:
    print('Could not obtain lock')
except DBMutexTimeoutError:
    print('Task completed but the lock timed out')

Lock Timeout

Django DB Mutex comes with lock timeout baked in. This ensures that a lock cannot be held forever. This is especially important when working with segments of code that may run out of memory or produce errors that do not raise exceptions.

In the default setup of this app, a lock is only valid for 30 minutes. As shown earlier in the example code, if the lock times out during the execution of a critical piece of code, a DBMutexTimeoutError will be thrown. This error basically says that a critical section of your code could have overlapped (but it doesn’t necessarily say if a section of code overlapped or didn’t).

In order to change the duration of a lock, set the DB_MUTEX_TTL_SECONDS variable in your settings.py file to a number of seconds. If you want your locks to never expire (beware!), set the setting to None.

Usage with Celery

Django DB Mutex can be used with celery’s tasks in the following manner:

from db_mutex import DBMutexError, DBMutexTimeoutError
from db_mutex.db_mutex import db_mutex
from abc import ABCMeta
from celery import Task

class NonOverlappingTask(Task):
    __metaclass__ = ABCMeta

    def run_worker(self, *args, **kwargs):
        """
        Run worker code here.
        """
        raise NotImplementedError()

    def run(self, *args, **kwargs):
        try:
            with db_mutex(self.__class__.__name__):
                self.run_worker(*args, **kwargs):
        except DBMutexError:
            # Ignore this task since the same one is already running
            pass
        except DBMutexTimeoutError:
            # A task ran for a long time and another one may have overlapped with it. Report the error
            pass