Celery is an asynchronous task queue/job queue that is based on distributed message passing. It is used for executing tasks concurrently across multiple worker processes while managing task distribution, execution status, and retries. In many scenarios, it becomes crucial to retrieve the current state of tasks in a queue for monitoring and debugging purposes. This article dives into how we can retrieve a list of tasks within a Celery queue using various approaches.

Technical Overview

Celery operates with a broker — commonly RabbitMQ or Redis — which holds the tasks until a worker processes them. Workers, defined in separate processes or machines, execute these tasks. Each task in Celery is an instance of the Task class which can hold states and results in a backend of your choice — from RPC to database storage solutions.

Task States

Celery tasks can have several states:

• `PENDING`: Task is waiting to be executed.
• `STARTED`: Task has been started.
• `SUCCESS`: Task has been successfully executed.
• `FAILURE`: Task execution failed.
• `RETRY`: Task is being retried after encountering an error.
• Other states such as `REVOKED` and `RECEIVED` depending on configuration.

When tasks are put into a queue, they usually start with a `PENDING` state, transition to `STARTED`, and finally `SUCCESS` or `FAILURE`. These states are essential for monitoring the progress or bottlenecks within processing pipelines.

Connection to Broker

To retrieve tasks queued in Celery, you generally connect directly to the message broker used by Celery. This connection can either be programmatically handled or manually if required for one-off debugging purposes.

Retrieving the Task List

Imposing a Solution Using Celery Events

Celery provides a powerful tool known as Celery Events, which is part of the `celery.events` module. Celery Events work by listening to the messages produced by the task’s state changes and worker’s life-cycle events. The `celery events` command, part of celery's command-line tool, can provide real-time insights and task tracking.

Example using the command: