Ray Client

What is the Ray Client?

The Ray Client is an API that connects a python script to a remote Ray cluster. Effectively, it allows you to leverage a remote Ray cluster just like you would with Ray running on your local machine.

By changing ray.init() to ray.init("ray://<head_node_host>:<port>"), you can connect from your laptop (or anywhere) directly to a remote cluster and scale-out your Ray code, while maintaining the ability to develop interactively in a python shell. This will only work with Ray 1.5+. If you’re using an older version of ray, see the 1.4.1 docs

# You can run this code outside of the Ray cluster!
import ray

# Starting the Ray client. This connects to a remote Ray cluster.
# If you're using a version of Ray prior to 1.5, use the 1.4.1 example
# instead: https://docs.ray.io/en/releases-1.4.1/cluster/ray-client.html
ray.init("ray://<head_node_host>:10001")

# Normal Ray code follows
@ray.remote
def do_work(x):
    return x ** x

do_work.remote(2)
#....

Client arguments

Ray client is used when the address passed into ray.init is prefixed with ray://. Client mode currently accepts two arguments:

  • namespace: Sets the namespace for the session

  • runtime_env: Sets the runtime environment for the session

# Connects to an existing cluster at 1.2.3.4 listening on port 10001, using
# the namespace "my_namespace"
ray.init("ray://1.2.3.4:10001", namespace="my_namespace")
#....

When to use Ray client

Ray client should be used when you want to connect a script or an interactive shell session to a remote cluster.

  • Use ray.init("ray://<head_node_host>:10001") (Ray client) if you’ve set up a remote cluster at <head_node_host>. This will connect your local script or shell to the cluster. See the section on using ray client for more details on setting up your cluster.

  • Use ray.init("localhost:<port>") (non-client connection, local address) if you’re developing locally or on the head node of your cluster and you have already started the cluster (i.e. ray start --head has already been run)

  • Use ray.init() (non-client connection, no address specified) if you’re developing locally and want to automatically create a local cluster and attach directly to it.

How do you use the Ray client?

Step 1: set up your Ray cluster

First, you’ll want to create a remote Ray cluster. Follow the directions in Ray Cluster Quick Start to do this.

If using the Ray cluster launcher, the remote cluster will be listening on port 10001 of the head node. If necessary, you can modify this port by setting --ray-client-server-port to the ray start command.

If not using the Ray cluster launcher, you can start the “Ray Client Server” manually on the head node of your remote cluster by running the following:

python -m ray.util.client.server [--host host_ip] [--port port] [--redis-address address] [--redis-password password]

Step 2: Check ports

Ensure that the Ray Client port on the head node is reachable from your local machine. This means opening that port up by configuring security groups or other access controls (on EC2) or proxying from your local machine to the cluster (on K8s).

Step 3: Run Ray code

Now, connect to the Ray Cluster with the following and then use Ray like you normally would:

import ray

# replace with the appropriate host and port
ray.init("ray://<head_node_host>:10001")

# Normal Ray code follows
@ray.remote
def do_work(x):
    return x ** x

do_work.remote(2)

#....

Things to know

Client disconnections

When the client disconnects, any object or actor references held by the server on behalf of the client are dropped, as if directly disconnecting from the cluster.

Versioning requirements

Generally, the client Ray version must match the server Ray version. An error will be raised if an incompatible version is used.

Similarly, the minor Python (e.g., 3.6 vs 3.7) must match between the client and server. An error will be raised if this is not the case.

Starting a connection on older Ray versions

If you encounter socket.gaierror: [Errno -2] Name or service not known when using ray.init("ray://...") then you may be on a version of Ray prior to 1.5 that does not support starting client connections through ray.init. If this is the case, see the 1.4.1 docs for Ray client.

Connection through the Ingress

If you encounter the following error message when connecting to the Ray Cluster using an Ingress, it may be caused by the Ingress’s configuration.

grpc._channel._MultiThreadedRendezvous: <_MultiThreadedRendezvous of RPC that terminated with:
    status = StatusCode.INVALID_ARGUMENT
    details = ""
    debug_error_string = "{"created":"@1628668820.164591000","description":"Error received from peer ipv4:10.233.120.107:443","file":"src/core/lib/surface/call.cc","file_line":1062,"grpc_message":"","grpc_status":3}"
>
Got Error from logger channel -- shutting down: <_MultiThreadedRendezvous of RPC that terminated with:
    status = StatusCode.INVALID_ARGUMENT
    details = ""
    debug_error_string = "{"created":"@1628668820.164713000","description":"Error received from peer ipv4:10.233.120.107:443","file":"src/core/lib/surface/call.cc","file_line":1062,"grpc_message":"","grpc_status":3}"
>

If you are using the nginx-ingress-controller, you may be able to resolve the issue by adding the following Ingress configuration.

metadata:
  annotations:
     nginx.ingress.kubernetes.io/server-snippet: |
       underscores_in_headers on;
       ignore_invalid_headers on;