Serving RLlib Models#

In this guide, we will train and deploy a simple Ray RLlib model. In particular, we show:

  • How to train and store an RLlib model.

  • How to load this model from a checkpoint.

  • How to parse the JSON request and evaluate the payload in RLlib.

We will train and checkpoint a simple PPO model with the CartPole-v1 environment from gymnasium. In this tutorial we simply write to local disk, but in production you might want to consider using a cloud storage solution like S3 or a shared file system.

Let’s get started by defining a PPO instance, training it for one iteration and then creating a checkpoint:

import ray
import ray.rllib.algorithms.ppo as ppo
from ray import serve

def train_ppo_model():
    # Configure our PPO algorithm.
    config = (
    # Create a `PPO` instance from the config.
    algo =
    # Train for one iteration.
    # Save state of the trained Algorithm in a checkpoint.
    checkpoint_dir ="/tmp/rllib_checkpoint")
    return checkpoint_dir

checkpoint_path = train_ppo_model()

You create deployments with Ray Serve by using the @serve.deployment on a class that implements two methods:

  • The __init__ call creates the deployment instance and loads your data once. In the below example we restore our PPO Algorithm from the checkpoint we just created.

  • The __call__ method will be invoked every request. For each incoming request, this method has access to a request object, which is a Starlette Request.

We can load the request body as a JSON object and, assuming there is a key called observation, in your deployment you can use request.json()["observation"] to retrieve observations (obs) and pass them into the restored Algorithm using the compute_single_action method.

from starlette.requests import Request

class ServePPOModel:
    def __init__(self, checkpoint_path) -> None:
        # Re-create the originally used config.
        config = ppo.PPOConfig()\
        # Build the Algorithm instance using the config.
        self.algorithm ="CartPole-v1")
        # Restore the algo's state from the checkpoint.

    async def __call__(self, request: Request):
        json_input = await request.json()
        obs = json_input["observation"]

        action = self.algorithm.compute_single_action(obs)
        return {"action": int(action)}


Although we used a single input and Algorithm.compute_single_action(...) here, you can process a batch of input using Ray Serve’s batching feature and use Algorithm.compute_actions(...) to process a batch of inputs.

Now that we’ve defined our ServePPOModel service, let’s deploy it to Ray Serve.

ppo_model = ServePPOModel.bind(checkpoint_path)

Note that the checkpoint_path that we passed to the bind() method will be passed to the __init__ method of the ServePPOModel class that we defined above.

Now that the model is deployed, let’s query it!

# Note: `gymnasium` (not `gym`) will be **the** API supported by RLlib from Ray 2.3 on.
    import gymnasium as gym
    gymnasium = True
except Exception:
    import gym
    gymnasium = False

import requests

env = gym.make("CartPole-v1")

for _ in range(5):
    if gymnasium:
        obs, infos = env.reset()
        obs = env.reset()

    print(f"-> Sending observation {obs}")
    resp = requests.get(
        "http://localhost:8000/", json={"observation": obs.tolist()}
    print(f"<- Received response {resp.json()}")

You should see output like this (observation values will differ):

<- Received response {'action': 1}
-> Sending observation [0.04228249 0.02289503 0.00690076 0.03095441]
<- Received response {'action': 0}
-> Sending observation [ 0.04819471 -0.04702759 -0.00477937 -0.00735569]
<- Received response {'action': 0}


In this example the client used the requests library to send a request to the server. We defined a json object with an observation key and a Python list of observations (obs.tolist()). Since obs = env.reset() is a numpy.ndarray, we used tolist() for conversion. On the server side, we used obs = json_input["observation"] to retrieve the observations again, which has list type. In the simple case of an RLlib algorithm with a simple observation space, it’s possible to pass this obs list to the Algorithm.compute_single_action(...) method. We could also have created a numpy array from it first and then passed it into the Algorithm.

In more complex cases with tuple or dict observation spaces, you will have to do some preprocessing of your json_input before passing it to your Algorithm instance. The exact way to process your input depends on how you serialize your observations on the client.