Using the Ray Cluster Launcher

Ray comes with a built-in cluster launcher that makes deploying a Ray cluster simple.

# First, run `pip install boto3` and `aws configure`
# Create or update the cluster. When the command finishes, it will print
# out the command that can be used to SSH into the cluster head node.
$ ray up ray/python/ray/autoscaler/aws/example-full.yaml

This will provision resources from a node provider (like AWS EC2 or Kubernetes) to instantiate the specified cluster, and start a Ray cluster on the provisioned resources. Once the Ray cluster is running, you can manually SSH into it or use provided commands like ray attach, ray rsync-up, and ray exec to access it and run Ray programs.

You can configure the Ray Cluster Launcher to use with a cloud provider, an existing Kubernetes cluster, or a private cluster of machines.

To launch a cluster, you must first create a cluster configuration file, which specifies some important details about the cluster. Here is an example cluster configuration file for AWS:

# A unique identifier for this cluster.
cluster_name: basic-ray

# The maximum number of workers nodes to launch in addition to the head
# node.
max_workers: 0 # this means zero workers

# Cloud-provider specific configuration.
   type: aws
   region: us-west-2
   availability_zone: us-west-2a

# How Ray will authenticate with newly launched nodes.
   ssh_user: ubuntu

  - pip install ray[all]
  # The following line demonstrate that you can specify arbitrary
  # startup scripts on the cluster.
  - touch /tmp/some_file.txt

In another example, the AWS example configuration file cluster config file will create a small cluster with an m5.large head node (on-demand) configured to autoscale up to two m5.large spot workers.

See the Cluster Configuration docs on how to customize the configuration file.

Here are the commands covered in this document:

Launching a cluster (ray up)

This will start up the machines in the cloud, install your dependencies and run any setup commands that you have, configure the Ray cluster automatically, and prepare you to scale your distributed system. See the documentation for ray up.


The worker nodes will start only after the head node has finished starting. To monitor the progress of the cluster setup, you can run ray monitor <cluster yaml>.

# Replace '<your_backend>' with one of: 'aws', 'gcp', 'kubernetes', or 'local'.
$ BACKEND=<your_backend>

# Create or update the cluster.
$ ray up ray/python/ray/autoscaler/$BACKEND/example-full.yaml

# Tear down the cluster.
$ ray down ray/python/ray/autoscaler/$BACKEND/example-full.yaml

Updating an existing cluster (ray up)

If you want to update your cluster configuration (add more files, change dependencies), run ray up again on the existing cluster.

This command checks if the local configuration differs from the applied configuration of the cluster. This includes any changes to synced files specified in the file_mounts section of the config. If so, the new files and config will be uploaded to the cluster. Following that, Ray services/processes will be restarted.


Don’t do this for the cloud provider specifications (e.g., change from AWS to GCP on a running cluster) or change the cluster name (as this will just start a new cluster and orphan the original one).

You can also run ray up to restart a cluster if it seems to be in a bad state (this will restart all Ray services even if there are no config changes).

If you don’t want the update to restart services (e.g., because the changes don’t require a restart), pass --no-restart to the update call.

If you want to force re-generation of the config to pick up possible changes in the cloud environment, pass --no-config-cache to the update call.

See the documentation for ray up.

# Reconfigure autoscaling behavior without interrupting running jobs.
$ ray up ray/python/ray/autoscaler/$BACKEND/example-full.yaml \
    --max-workers=N --no-restart

Running shell commands on the cluster (ray exec)

You can use ray exec to conveniently run commands on clusters. Note that python scripts that you want to scale should connect to Ray via ray.init(address="auto"). See the documentation for ray exec.

# Run a command on the cluster
$ ray exec cluster.yaml 'echo "hello world"'

# Run a command on the cluster, starting it if needed
$ ray exec cluster.yaml 'echo "hello world"' --start

# Run a command on the cluster, stopping the cluster after it finishes
$ ray exec cluster.yaml 'echo "hello world"' --stop

# Run a command on a new cluster called 'experiment-1', stopping it after
$ ray exec cluster.yaml 'echo "hello world"' \
    --start --stop --cluster-name experiment-1

# Run a command in a detached tmux session
$ ray exec cluster.yaml 'echo "hello world"' --tmux

# Run a command in a screen (experimental)
$ ray exec cluster.yaml 'echo "hello world"' --screen

If you want to run applications on the cluster that are accessible from a web browser (e.g., Jupyter notebook), you can use the --port-forward. The local port opened is the same as the remote port.

$ ray exec cluster.yaml --port-forward=8899 'source ~/anaconda3/bin/activate tensorflow_p36 && jupyter notebook --port=8899'


For Kubernetes clusters, the port-forward option cannot be used while executing a command. To port forward and run a command you need to call ray exec twice separately.

Running Ray scripts on the cluster (ray submit)

You can also use ray submit to execute Python scripts on clusters. This will rsync the designated file onto the cluster and execute it with the given arguments. See the documentation for ray submit.

# Run a Python script in a detached tmux session
$ ray submit cluster.yaml --tmux --start --stop

Attaching to a running cluster (ray attach)

You can use ray attach to attach to an interactive screen session on the cluster. See the documentation for ray attach or run ray attach --help.

# Open a screen on the cluster
$ ray attach cluster.yaml

# Open a screen on a new cluster called 'session-1'
$ ray attach cluster.yaml --start --cluster-name=session-1

# Attach to tmux session on cluster (creates a new one if none available)
$ ray attach cluster.yaml --tmux

Synchronizing files from the cluster (ray rsync-up/down)

To download or upload files to the cluster head node, use ray rsync_down or ray rsync_up:

$ ray rsync_down cluster.yaml '/path/on/cluster' '/local/path'
$ ray rsync_up cluster.yaml '/local/path' '/path/on/cluster'

Monitoring cluster status (ray dashboard/monitor)

The ray also comes with an online dashboard. The dashboard is accessible via HTTP on the head node (by default it listens on localhost:8265). You can also use the built-in ray dashboard to do this automatically.

$ ray dashboard cluster.yaml

You can monitor cluster usage and auto-scaling status by tailing the autoscaling logs in /tmp/ray/session_*/logs/monitor*.

$ ray monitor cluster.yaml

The Ray autoscaler also reports per-node status in the form of instance tags. In your cloud provider console, you can click on a Node, go the the “Tags” pane, and add the ray-node-status tag as a column. This lets you see per-node statuses at a glance:


Common Workflow: Syncing git branches

A common use case is syncing a particular local git branch to all workers of the cluster. However, if you just put a git checkout <branch> in the setup commands, the autoscaler won’t know when to rerun the command to pull in updates. There is a nice workaround for this by including the git SHA in the input (the hash of the file will change if the branch is updated):

file_mounts: {
    "/tmp/current_branch_sha": "/path/to/local/repo/.git/refs/heads/<YOUR_BRANCH_NAME>",

    - test -e <REPO_NAME> || git clone<REPO_ORG>/<REPO_NAME>.git
    - cd <REPO_NAME> && git fetch && git checkout `cat /tmp/current_branch_sha`

This tells ray up to sync the current git branch SHA from your personal computer to a temporary file on the cluster (assuming you’ve pushed the branch head already). Then, the setup commands read that file to figure out which SHA they should checkout on the nodes. Note that each command runs in its own session. The final workflow to update the cluster then becomes just this:

  1. Make local changes to a git branch

  2. Commit the changes with git commit and git push

  3. Update files on your Ray cluster with ray up


The Ray Cluster Launcher will automatically enable a load-based autoscaler. When cluster resource usage exceeds a configurable threshold (80% by default), new nodes will be launched up the specified max_workers limit (in the cluster config). When nodes are idle for more than a timeout, they will be removed, down to the min_workers limit. The head node is never removed.

The default idle timeout is 5 minutes, which can be set in the cluster config. This is to prevent excessive node churn which could impact performance and increase costs (in AWS / GCP there is a minimum billing charge of 1 minute per instance, after which usage is billed by the second).

Questions or Issues?

You can post questions or issues or feedback through the following channels:

  1. For discussions about development or any general questions and feedback.

  2. StackOverflow: For questions about how to use Ray.

  3. GitHub Issues: For bug reports and feature requests.