RLlib Models and Preprocessors

The following diagram provides a conceptual overview of data flow between different components in RLlib. We start with an Environment, which given an action produces an observation. The observation is preprocessed by a Preprocessor and Filter (e.g. for running mean normalization) before being sent to a neural network Model. The model output is in turn interpreted by an ActionDistribution to determine the next action.


The components highlighted in green can be replaced with custom user-defined implementations, as described in the next sections. The purple components are RLlib internal, which means they can only be modified by changing the algorithm source code.

Built-in Models and Preprocessors

RLlib picks default models based on a simple heuristic: a vision network for image observations, and a fully connected network for everything else. These models can be configured via the model config key, documented in the model catalog. Note that you’ll probably have to configure conv_filters if your environment observations have custom sizes, e.g., "model": {"dim": 42, "conv_filters": [[16, [4, 4], 2], [32, [4, 4], 2], [512, [11, 11], 1]]} for 42x42 observations.

In addition, if you set "model": {"use_lstm": true}, then the model output will be further processed by a LSTM cell. More generally, RLlib supports the use of recurrent models for its algorithms (A3C, PG out of the box), and RNN support is built into its policy evaluation utilities.

For preprocessors, RLlib tries to pick one of its built-in preprocessor based on the environment’s observation space. Discrete observations are one-hot encoded, Atari observations downscaled, and Tuple observations flattened (there isn’t native tuple support yet, but you can reshape the flattened observation in a custom model). Note that for Atari, DQN defaults to using the DeepMind preprocessors, which are also used by the OpenAI baselines library.

Custom Models

Custom models should subclass the common RLlib model class and override the _build_layers method. This method takes in a tensor input (observation), and returns a feature layer and float vector of the specified output size. The model can then be registered and used in place of a built-in model:

import ray
import ray.rllib.agents.ppo as ppo
from ray.rllib.models import ModelCatalog, Model

class MyModelClass(Model):
    def _build_layers(self, inputs, num_outputs, options):
        layer1 = slim.fully_connected(inputs, 64, ...)
        layer2 = slim.fully_connected(inputs, 64, ...)
        return layerN, layerN_minus_1

ModelCatalog.register_custom_model("my_model", MyModelClass)

agent = ppo.PPOAgent(env="CartPole-v0", config={
    "model": {
        "custom_model": "my_model",
        "custom_options": {},  # extra options to pass to your model

For a full example of a custom model in code, see the Carla RLlib model and associated training scripts. The CarlaModel class defined there operates over a composite (Tuple) observation space including both images and scalar measurements.

Custom Preprocessors

Similarly, custom preprocessors should subclass the RLlib preprocessor class and be registered in the model catalog:

import ray
import ray.rllib.agents.ppo as ppo
from ray.rllib.models.preprocessors import Preprocessor

class MyPreprocessorClass(Preprocessor):
    def _init(self):
        self.shape = ...  # perhaps varies depending on self._options

    def transform(self, observation):
        return ...  # return the preprocessed observation

ModelCatalog.register_custom_preprocessor("my_prep", MyPreprocessorClass)

agent = ppo.PPOAgent(env="CartPole-v0", config={
    "model": {
        "custom_preprocessor": "my_prep",
        "custom_options": {},  # extra options to pass to your preprocessor

Customizing Policy Graphs

For deeper customization of algorithms, you can modify the policy graphs of the agent classes. Here’s an example of extending the DDPG policy graph to specify custom sub-network modules:

from ray.rllib.models import ModelCatalog
from ray.rllib.agents.ddpg.ddpg_policy_graph import DDPGPolicyGraph as BaseDDPGPolicyGraph

class CustomPNetwork(object):
    def __init__(self, dim_actions, hiddens, activation):
        action_out = ...
        # Use sigmoid layer to bound values within (0, 1)
        # shape of action_scores is [batch_size, dim_actions]
        self.action_scores = layers.fully_connected(
            action_out, num_outputs=dim_actions, activation_fn=tf.nn.sigmoid)

class CustomQNetwork(object):
    def __init__(self, action_inputs, hiddens, activation):
        q_out = ...
        self.value = layers.fully_connected(
            q_out, num_outputs=1, activation_fn=None)

class CustomDDPGPolicyGraph(BaseDDPGPolicyGraph):
    def _build_p_network(self, obs):
        return CustomPNetwork(

    def _build_q_network(self, obs, actions):
        return CustomQNetwork(

Then, you can create an agent with your custom policy graph by:

from ray.rllib.agents.ddpg.ddpg import DDPGAgent
from custom_policy_graph import CustomDDPGPolicyGraph

DDPGAgent._policy_graph = CustomDDPGPolicyGraph
agent = DDPGAgent(...)

That’s it. In this example we overrode existing methods of the existing DDPG policy graph, i.e., _build_q_network, _build_p_network, _build_action_network, _build_actor_critic_loss, but you can also replace the entire graph class entirely.

Model-Based Rollouts

With a custom policy graph, you can also perform model-based rollouts and optionally incorporate the results of those rollouts as training data. For example, suppose you wanted to extend PGPolicyGraph for model-based rollouts. This involves overriding the compute_actions method of that policy graph:

class ModelBasedPolicyGraph(PGPolicyGraph):
     def compute_actions(self,
        # compute a batch of actions based on the current obs_batch
        # and state of each episode (i.e., for multiagent). You can do
        # whatever is needed here, e.g., MCTS rollouts.
        return action_batch

If you want take this rollouts data and append it to the sample batch, use the add_extra_batch() method of the episode objects passed in. For an example of this, see the testReturningModelBasedRolloutsData unit test.