As we saw, Celery tasks may be restarted several times, especially if you have enabled late acknowledgments. This makes it important to control the side effects of a task. Hence, Celery recommends that all tasks should be idempotent. Idempotence is a mathematical property of a function that assures that it will return the same result if invoked with the same arguments, no matter how many times you call it.

You might have seen simple examples of idempotent functions in the Celery documentation itself, such as this:

@app.task 
def add(x, y): 
    return x + y 

No matter how many times we call this function, the result of add(2, 2) is always 4.

However, it is important to understand the difference between an idempotent function and a function having no side effects (a pure or nullipotent function). The side effect of an idempotent will be the same, regardless of whether it was called once or several times.

For example, a task that always places a fresh order when called is not idempotent, but a task that cancels an existing order is idempotent. Operations that only read the state of the world and do not have any side effects are nullipotent.

As Celery architecture relies on tasks being idempotent, it is important to try to study all the side effects of a non-idempotent task and convert it into an idempotent task. You can do this by either checking whether the tasks have been executed previously (if it was, then abort) or storing the result in a unique location based on the arguments. An example of the latter is given in the Avoid writing to shared or global state section.

Finally, call your task multiple times to test whether it leaves your system in the same state.