Ordering the tasks

Note

Needs graph middleware, which is available as a contrib module

Occasionally you will find yourself in a situation where you want a particular task to get started when other finish. For instance, you have serious data processing and you want to send a report once all is finished. It is highly unlikely your code to be run with only with one worker and one queue. And you want to guarantee strictly defined sequence.

For such purpose redis-tasks offers a TaskGraph class which allows you to specify the exact order:

import tasks
from redis_tasks.contrib.graph import TaskGraph


graph = TaskGraph()  # Initialize the graph

after_node = graph.add_task(
    dict(func=tasks.send_report))
before_node = graph.add_task(
    dict(func=tasks.data_processing,
         args=['http://www.python.org']))

graph.add_dependency(before_node, after_node)
graph.enqueue()  # Push them into the queue

First you need to add the tasks as nodes and second to specify the dependency. In case you have more tasks to handle in specific manner you can use the chain helper:

import tasks
from redis_tasks.contrib.graph import chain


nodes = [
    dict(func=tasks.data_processing,
         args=['http://www.python.org']),
    dict(func=tasks.data_processing,
         args=['https://www.djangoproject.com']),
    dict(func=tasks.send_report),
]

graph = chain(nodes)
graph.enqueue()

In this case the last task on the list will be the last one to be executed.