This document describes the current stable version of Celery (5.6). For development docs, go here.

Concurrency

Release:

5.6

Date:

Jul 05, 2026

Concurrency in Celery enables the parallel execution of tasks. The default model, prefork, is well-suited for many scenarios and generally recommended for most users. In fact, switching to another mode will silently disable certain features like soft_timeout and max_tasks_per_child.

This page gives a quick overview of the available options which you can pick between using the –pool option when starting the worker.

Overview of Concurrency Options

• prefork: The default option, ideal for CPU-bound tasks and most use cases. It is robust and recommended unless there’s a specific need for another model.

• eventlet and gevent: Designed for IO-bound tasks, these models use greenlets for high concurrency. Note that certain features, like soft_timeout, are not available in these modes. These have detailed documentation pages linked below.

• solo: Executes tasks sequentially in the main thread.

• threads: Utilizes threading for concurrency, available if the concurrent.futures module is present.

• custom: Enables specifying a custom worker pool implementation through environment variables.

Note

On Windows, the default prefork pool uses spawn instead of fork to create worker processes. This means per-process worker optimizations set up in the main process are not inherited by workers. If you see RuntimeError: fast_trace_task: worker task registry is empty, switch to --pool=solo or --pool=threads.

• Concurrency with Eventlet
  • Introduction
  • Enabling Eventlet
  • Examples
• Concurrency with gevent
  • Introduction
  • Enabling gevent
  • Examples
  • Known issues

Note

While alternative models like eventlet and gevent are available, they may lack certain features compared to prefork. We recommend prefork as the starting point unless specific requirements dictate otherwise.