[Solved]-Celery: clean way of revoking the entire chain from within a task


I think I found the answer to this issue: this seems the right way to proceed, indeed. I wonder why such common scenario is not documented anywhere, though.

For completeness I post the basic code snapshot:

@app.task(bind=True)  # Note that we need bind=True for self to work
def task1(self, other_args):
    if end_chain:
        self.request.callbacks[:] = []


I implemented a more elegant way to cope with the issue and I want to share it with you. I am using a decorator called revoke_chain_authority, so that it can revoke automatically the chain without rewriting the code I previously described.

from functools import wraps

class RevokeChainRequested(Exception):
    def __init__(self, return_value):
        Exception.__init__(self, "")

        # Now for your custom code...
        self.return_value = return_value

def revoke_chain_authority(a_shared_task):
    @see: https://gist.github.com/bloudermilk/2173940
    @param a_shared_task: a @shared_task(bind=True) celery function.
    def inner(self, *args, **kwargs):
            return a_shared_task(self, *args, **kwargs)
        except RevokeChainRequested, e:
            # Drop subsequent tasks in chain (if not EAGER mode)
            if self.request.callbacks:
                self.request.callbacks[:] = []
            return e.return_value

    return inner

This decorator can be used on a shared task as follows:

def apply_fetching_decision(self, latitude, longitude):

    if condition:
        raise RevokeChainRequested(False)

Please note the use of @wraps. It is necessary to preserve the signature of the original function, otherwise this latter will be lost and celery will make a mess at calling the right wrapped task (e.g. it will call always the first registered function instead of the right one)


As of Celery 4.0, what I found to be working is to remove the remaining tasks from the current task instance’s request using the statement:

self.request.chain = None

Let’s say you have a chain of tasks a.s() | b.s() | c.s(). You can only access the self variable inside a task if you bind the task by passing bind=True as argument to the tasks’ decorator.

@app.task(name='main.a', bind=True):
def a(self):
  if something_happened:
    self.request.chain = None

If something_happened is truthy, b and c wouldn’t be executed.

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