Adds argument-based unique constraints to Celery tasks

Configuring unique constraints on Celery requires the following:

• A modified Task base class for Celery (provided by this package)
• A redis database connection
• A simple (lambda) function that generates the "unique key" identifier for a task, based on the same arguments which are part of the task's signature.

• Step 1: Configure Celery

  # my_application/__init__.py
  #
  # Create and configure Celery app object, as usual
  from celery import Celery
  
  celery_app = Celery()
  
  # Add celery-unique capabilities to the original Celery Task class
  from celery_unique import unique_task_factory
  
  task_base_cls = celery_app.Task
  new_task_cls = unique_task_factory(task_base_cls)
  celery_app.Task = new_task_cls

• Step 2: Configure your tasks to be unique

  # my_application/celery_tasks.py
  from . import celery_app
  from redis import Redis
  
  my_redis_client = Redis()
  
  # Configure a unique task by providing a key-generator and Redis
  # connection as `unique_key` and `redis_client` keyword arguments,
  # respectively.
  
  @celery_app.task(
      unique_key=lambda a, b, c=0: '{} with {}'.format(a, c), 
      redis_client=my_redis_client
  )
  def add_first_and_last(a, b, c=0):
      return a + c

• Step 3: Run the tasks with a delay and see what happens

  import time
  from my_application.celery_tasks import add_first_and_last
  
  # Unique-handling will only take effect when the above functions are called
  # via `apply_async()` with an ETA or countdown...
  async_result_1 = add_first_and_last.apply_async(args=(1, 2, 3), countdown=100)
  async_result_2 = add_first_and_last.apply_async(args=(3, 2, 1), countdown=100)
  async_result_3 = add_first_and_last.apply_async(
      args=(1, 2), 
      kwargs={'c': 3}, 
      countdown=50
  )
  
  # Wait 100 seconds for all tasks to complete
  time.sleep(100)
  
  # Check and see the status of each task
  assert async_result_1.status == 'REVOKED'
  assert async_result_2.status == 'SUCCESS'
  assert async_result_3.status == 'SUCCESS'

In the above example, three Celery tasks were created. However, in processing our third call, the handling provided by celery-unique found that there was already a pending result for add_first_and_last() with a unique key of "1 with 3" (generated by the lambda). So what happened? The first task was automatically revoked, and the most recent task was then sent along to be handled by Celery.

This is especially useful for creating time-based tasks like emails. If we create a task to re-engage users via email 30 days after the last time we saw them, we could create a task with an ETA of 30 days from the current time each time they visited. If we didn't have celery-unique in this scenario and a user made visits on Monday, Tuesday, and Wednesday, then they would get an email 30 days after Monday, 30 days after Tuesday, and 30 days after Wednesday. With celery-unique (and a proper task configuration, of course), the only email sent would be 30 days after Wednesday. Huzzah!

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Copyright 2015-2017, Shiftgig Inc

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.