Starting Ray

This page covers how to start Ray on your single machine or cluster of machines.

Tip

Be sure to have installed Ray before following the instructions on this page.

What is the Ray runtime?

Ray programs are able to parallelize and distribute by leveraging an underlying Ray runtime. The Ray runtime consists of multiple services/processes started in the background for communication, data transfer, scheduling, and more. The Ray runtime can be started on a laptop, a single server, or multiple servers.

There are three ways of starting the Ray runtime:

In all cases, ray.init() will try to automatically find a Ray instance to connect to. It checks, in order: 1. The RAY_ADDRESS OS environment variable. 2. The concrete address passed to ray.init(address=<address>). 3. If no address is provided, the latest Ray instance that was started on the same machine using ray start.

Starting Ray on a single machine

Calling ray.init() starts a local Ray instance on your laptop/machine. This laptop/machine becomes the “head node”.

Note

In recent versions of Ray (>=1.5), ray.init() will automatically be called on the first use of a Ray remote API.

import ray
# Other Ray APIs will not work until `ray.init()` is called.
ray.init()
import io.ray.api.Ray;

public class MyRayApp {

  public static void main(String[] args) {
    // Other Ray APIs will not work until `Ray.init()` is called.
    Ray.init();
    ...
  }
}
#include <ray/api.h>
// Other Ray APIs will not work until `ray::Init()` is called.
ray::Init()

When the process calling ray.init() terminates, the Ray runtime will also terminate. To explicitly stop or restart Ray, use the shutdown API.

import ray
ray.init()
... # ray program
ray.shutdown()
import io.ray.api.Ray;

public class MyRayApp {

  public static void main(String[] args) {
    Ray.init();
    ... // ray program
    Ray.shutdown();
  }
}
#include <ray/api.h>
ray::Init()
... // ray program
ray::Shutdown()

To check if Ray is initialized, use the is_initialized API.

import ray
ray.init()
assert ray.is_initialized()

ray.shutdown()
assert not ray.is_initialized()
import io.ray.api.Ray;

public class MyRayApp {

public static void main(String[] args) {
        Ray.init();
        Assert.assertTrue(Ray.isInitialized());
        Ray.shutdown();
        Assert.assertFalse(Ray.isInitialized());
    }
}
#include <ray/api.h>

int main(int argc, char **argv) {
    ray::Init();
    assert(ray::IsInitialized());

    ray::Shutdown();
    assert(!ray::IsInitialized());
}

See the Configuration documentation for the various ways to configure Ray.

Starting Ray via the CLI (ray start)

Use ray start from the CLI to start a 1 node ray runtime on a machine. This machine becomes the “head node”.

$ ray start --head --port=6379

Local node IP: 192.123.1.123
2020-09-20 10:38:54,193 INFO services.py:1166 -- View the Ray dashboard at http://localhost:8265

--------------------
Ray runtime started.
--------------------

...

You can connect to this Ray instance by starting a driver process on the same node as where you ran ray start. ray.init() will now automatically connect to the latest Ray instance.

import ray
ray.init()
import io.ray.api.Ray;

public class MyRayApp {

  public static void main(String[] args) {
    Ray.init();
    ...
  }
}
java -classpath <classpath> \
  -Dray.address=<address> \
  <classname> <args>
#include <ray/api.h>

int main(int argc, char **argv) {
  ray::Init();
  ...
}
RAY_ADDRESS=<address> ./<binary> <args>

You can connect other nodes to the head node, creating a Ray cluster by also calling ray start on those nodes. See Launching an On-Premise Cluster for more details. Calling ray.init() on any of the cluster machines will connect to the same Ray cluster.

Launching a Ray cluster (ray up)

Ray clusters can be launched with the Cluster Launcher. The ray up command uses the Ray cluster launcher to start a cluster on the cloud, creating a designated “head node” and worker nodes. Underneath the hood, it automatically calls ray start to create a Ray cluster.

Your code only needs to execute on one machine in the cluster (usually the head node). Read more about running programs on a Ray cluster.

To connect to the Ray cluster, call ray.init from one of the machines in the cluster. This will connect to the latest Ray cluster:

ray.init()

Note that the machine calling ray up will not be considered as part of the Ray cluster, and therefore calling ray.init on that same machine will not attach to the cluster.

What’s next?

Check out our Deployment section for more information on deploying Ray in different settings, including Kubernetes, YARN, and SLURM.