Skip to content



Prefect integrations with the Ray execution framework, a flexible distributed computing framework for Python.

Provides a RayTaskRunner that enables flows to run tasks requiring parallel execution using Ray.

Getting Started

Python setup

Requires an installation of Python 3.7+.

We recommend using a Python virtual environment manager such as pipenv, conda, or virtualenv.

These tasks are designed to work with Prefect 2.0. For more information about how to use Prefect, please refer to the Prefect documentation.


Install prefect-ray with pip:

pip install prefect-ray

Users running Apple Silicon (such as M1 macs) will need to additionally run:

pip uninstall grpcio
conda install grpcio
Click here for more details.

Running tasks on Ray

The RayTaskRunner is a Prefect task runner that submits tasks to Ray for parallel execution.

By default, a temporary Ray instance is created for the duration of the flow run.

For example, this flow counts to 3 in parallel.

import time

from prefect import flow, task
from prefect_ray import RayTaskRunner

def shout(number):

def count_to(highest_number):
    for number in range(highest_number):

if __name__ == "__main__":

# outputs

If you already have a Ray instance running, you can provide the connection URL via an address argument.

To configure your flow to use the RayTaskRunner:

  1. Make sure the prefect-ray collection is installed as described earlier: pip install prefect-ray.
  2. In your flow code, import RayTaskRunner from prefect_ray.task_runners.
  3. Assign it as the task runner when the flow is defined using the task_runner=RayTaskRunner argument.

For example, this flow uses the RayTaskRunner with a local, temporary Ray instance created by Prefect at flow run time.

from prefect import flow
from prefect_ray.task_runners import RayTaskRunner

def my_flow():

This flow uses the RayTaskRunner configured to access an existing Ray instance at ray://

from prefect import flow
from prefect_ray.task_runners import RayTaskRunner

def my_flow():

RayTaskRunner accepts the following optional parameters:

Parameter Description
address Address of a currently running Ray instance, starting with the ray:// URI.
init_kwargs Additional kwargs to use when calling ray.init.

Note that Ray Client uses the ray:// URI to indicate the address of a Ray instance. If you don't provide the address of a Ray instance, Prefect creates a temporary instance automatically.

Ray environment limitations

While we're excited about adding support for parallel task execution via Ray to Prefect, there are some inherent limitations with Ray you should be aware of:

Ray currently does not support Python 3.10.

Ray support for non-x86/64 architectures such as ARM/M1 processors with installation from pip alone and will be skipped during installation of Prefect. It is possible to manually install the blocking component with conda. See the Ray documentation for instructions.

See the Ray installation documentation for further compatibility information.


If you encounter and bugs while using prefect-ray, feel free to open an issue in the prefect-ray repository.

If you have any questions or issues while using prefect-ray, you can find help in either the Prefect Discourse forum or the Prefect Slack community.


If you'd like to install a version of prefect-ray for development, clone the repository and perform an editable install with pip:

git clone

cd prefect-ray/

pip install -e ".[dev]"

# Install linting pre-commit hooks
pre-commit install