BACKEND_GUIDE.rst

Contributing a new backend.

Contributing a new locking backend is greatly appreciated! Each new backend must implement the following methods…

class Backend(object):
    """
    Each new backend must implement the following methods,
    - __init__
    - raise_or_lock
    - clear_lock
    """
    def __init__(self, settings):
        pass

    def raise_or_lock(self, key, timeout):
        pass

    def clear_lock(self, key):
        pass

def raise_or_lock(self, key, timeout)

Checks if the task is locked based on the key argument (str). If already locked should raise an AlreadyQueued exception. If not, locks the task by the key. A timeout argument (int) can also be passed in. The key should be cleared after the timeout (in seconds) has passed.

def clear_lock(self, key)

Removes the lock based on the key argument (str). This is called after a task completes (either successfully, or fails beyond celery’s retry limit).

def __init__(self, settings)

The settings argument (dict) is based on the celery once configuration. This can be used to setup the connection/client to the backend. Any imports for backend specific modules should happen inside here.

The redis backend is a good example of all of this in practice. If you’d like to contribute a new backend and still feel unsure how to do so, feel free to open an issue with any questions.