title: Executing Celery tasks with dependency graph
body: <p>I would like to have Celery tasks that depend on the result of 2 or more other tasks. I have looked into <a href="https://stackoverflow.com/questions/3901101/pythoncelery-chaining-jobs">Python+Celery: Chaining jobs?</a> and <a href="http://pypi.python.org/pypi/celery-tasktree" rel="noreferrer">http://pypi.python.org/pypi/celery-tasktree</a> , but those are good only if tasks have just one dependent task.</p>
  
  <p>I know about TaskSet, but there does not seem to be a way to instantly execute a callback when TaskSetResult.ready() becomes True. What I have in mind right now is to have a periodic task that polls TaskSetResult.ready() every few [milli]seconds or so and fire the callback as it returns True, but that sounds rather inelegant to me.</p>
  
  <p>Any suggestions?</p>