With Celery Canvas, what is the difference between chord() and chain(group(), task)

Question

We're implementing a workflow with Celery. First we need to run some tasks in parallel, and when they are all finished we need to run a single task.

It seems we can use chord, or group and chain:

chord(tasks, task)

vs

group(tasks) | task

What is the exact difference between those two? They seem to do the same thing.

score 7 · Accepted Answer · edited Jul 10 '18 at 13:14

7

The Canvas docs say:

Chaining a group together with another task will automatically upgrade it to be a chord:
 >>> c3 = (group(add.s(i, i) for i in xrange(10)) | xsum.s())
>>> res = c3()
>>> res.get()
90

edited Jul 10 '18 at 13:14

fedorqui

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answered Jul 02 '15 at 10:29

scytale

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Not sure why this is upvoted or accepted. It doesn't answer the question. Still not clear what the difference is here. – Taylor Halliday Nov 13 '20 at 19:41

score 3 · Answer 2 · answered Jun 21 '21 at 18:12

From Canvas documentation:

group

The group primitive is a signature that takes a list of tasks that should be applied in parallel.

chain

The chain primitive lets us link together signatures so that one is called after the other, essentially forming a chain of callbacks.

chord

A chord is just like a group but with a callback. A chord consists of a header group and a body, where the body is a task that should execute after all of the tasks in the header are complete.

With Celery Canvas, what is the difference between chord() and chain(group(), task)

2 Answers2