Access Arguments to Parent Task from Subtask in Celery

Question

Is it possible to access the arguments with which a parent task A was called, from its child task Z? Put differently, when Task Z gets called in a chain, can it somehow access an argument V that was invoked when Task A was fired, but that was not passed through any intermediary nodes between tasks A and Z ? And if so, how?

Using Celery 3.0 with RabbitMQ for results backend.

Answer 1

I'm going to provisionally answer my own question with a "no" -- and the hopes that someone more knowledgeable than me will come back with a solution.

After looking more closely at the Celery documentation, I'm thinking that there simply isn't a way to access the arguments with which a parent task was called. According to the documentation :

During its lifetime a task will transition through several possible states, and each state may have arbitrary metadata attached to it. When a task moves into a new state the previous state is forgotten about..

A task can be in one of the following states: Pending, Started, Success, Failure, Retry, and Revoked. For my scenario, the key here would, it seems, be the Success state. What I want to do is, for a given succeessful task, locate the id of (one of its) parent task(s), and then view the arguments with which the parent was called. However, according to the documentation, a task in the SUCCESS state will not contain this sort of data (since the parent task itself will have been successful insofar as it has spawned a child task):

SUCCESS Task has been successfully executed.

metadata: result contains the return value of the task. propagates: Yes ready: Yes

In other words, it would seem that there's a hard limit here. Even if I can locate the ID of the parent task I'm interested in, because it has successfully executed, I will by definition not be able to access the arguments with which it was called because it only contains the return value (no longer its arguments).

Answer 2

I think you can, at least in chord . When you bind=True your task, you can access self.request . In self.request.chord You can find a detailed dict. In its kwargs or options['chord'] you will find what you're looking for, but it's not an elegant solution. Also, if the parent has been replaced, you will only be able to see the final state.