Question

I have distributed hardware sensor nodes that will be interrogated by celery tasks. Each sensor node has a object associated holding recent readings, and config data.

I never want more than one celery task interrogating a single sensornode. But requests might come to interrogate the node while it is still being worked on from a previous request.

I didn’t see any example of this sort of task tracking in any of the celery docs. But I assume its a fairly common requirement.

My first thought was to just mark the model object at the beginning and end of the task with a task_in_progress like flag.

Is there anything in the task instantiation that I can use to better realize my task tracking?

Answer 1

What you want is to lock a task on a given resource, there is a very nice example on the Celery.

To summarize the example suggests to use a cache key to hold the lock, a task will check the lock key (you can generate a instance specific cache key like “sensor-%(id)s”) before starting and execute only if the cache key is not set.

example.

def check_sensor(sensor_id):
    if check_lock_from_cache(sensor_id):
        ... handle the lock ...
    else:
        lock(sensor_id)
        ... use the sensor ...
        unlock(sensor_id)

you probably want to be really sure to do the unlock properly (try except finally)

here’s the celery example http://ask.github.com/celery/cookbook/tasks.html#ensuring-a-task-is-only-executed-one-at-a-time