RLlib Models, Preprocessors, and Action Distributions

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.

_images/rllib-components.svg

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.

Default Behaviours

Built-in 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 and Dict observations flattened (these are unflattened and accessible via the input_dict parameter in custom models). Note that for Atari, RLlib defaults to using the DeepMind preprocessors, which are also used by the OpenAI baselines library.

Built-in Models

After preprocessing raw environment outputs, these preprocessed observations are then fed through a policy’s model. RLlib picks default models based on a simple heuristic: A vision network (TF or Torch) for observations that have a shape of length larger than 2 (for example, (84 x 84 x 3)), and a fully connected network (TF or Torch) for everything else. These models can be configured via the model config key, documented in the model catalog. Note that for the vision network case, 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. Thereby, always make sure that the last Conv2D output has an output shape of [B, 1, 1, X] ([B, X, 1, 1] for Torch), where B=batch and X=last Conv2D layer’s number of filters, so that RLlib can flatten it. An informative error will be thrown if this is not the case.

In addition, if you set "model": {"use_lstm": true}, the model output will be further processed by an LSTM cell (TF or Torch). More generally, RLlib supports the use of recurrent models for its policy gradient algorithms (A3C, PPO, PG, IMPALA), and RNN support is built into its policy evaluation utilities. For custom RNN/LSTM setups, see the Recurrent Models. section below.

Built-in Model Parameters

The following is a list of the built-in model hyperparameters:

MODEL_DEFAULTS: ModelConfigDict = {
    # === Built-in options ===
    # Number of hidden layers for fully connected net
    "fcnet_hiddens": [256, 256],
    # Nonlinearity for fully connected net (tanh, relu)
    "fcnet_activation": "tanh",
    # Filter config. List of [out_channels, kernel, stride] for each filter
    "conv_filters": None,
    # Nonlinearity for built-in convnet
    "conv_activation": "relu",
    # For DiagGaussian action distributions, make the second half of the model
    # outputs floating bias variables instead of state-dependent. This only
    # has an effect is using the default fully connected net.
    "free_log_std": False,
    # Whether to skip the final linear layer used to resize the hidden layer
    # outputs to size `num_outputs`. If True, then the last hidden layer
    # should already match num_outputs.
    "no_final_linear": False,
    # Whether layers should be shared for the value function.
    "vf_share_layers": True,

    # == LSTM ==
    # Whether to wrap the model with an LSTM.
    "use_lstm": False,
    # Max seq len for training the LSTM, defaults to 20.
    "max_seq_len": 20,
    # Size of the LSTM cell.
    "lstm_cell_size": 256,
    # Whether to feed a_{t-1}, r_{t-1} to LSTM.
    "lstm_use_prev_action_reward": False,
    # Experimental (only works with `_use_trajectory_view_api`=True):
    # Whether the LSTM is time-major (TxBx..) or batch-major (BxTx..).
    "_time_major": False,

    # == Atari ==
    # Whether to enable framestack for Atari envs
    "framestack": True,
    # Final resized frame dimension
    "dim": 84,
    # (deprecated) Converts ATARI frame to 1 Channel Grayscale image
    "grayscale": False,
    # (deprecated) Changes frame to range from [-1, 1] if true
    "zero_mean": True,

    # === Options for custom models ===
    # Name of a custom model to use
    "custom_model": None,
    # Extra options to pass to the custom classes. These will be available to
    # the Model's constructor in the model_config field. Also, they will be
    # attempted to be passed as **kwargs to ModelV2 models. For an example,
    # see rllib/models/[tf|torch]/attention_net.py.
    "custom_model_config": {},
    # Name of a custom action distribution to use.
    "custom_action_dist": None,
    # Custom preprocessors are deprecated. Please use a wrapper class around
    # your environment instead to preprocess observations.
    "custom_preprocessor": None,
}

TensorFlow Models

Note

TFModelV2 replaces the previous rllib.models.Model class, which did not support Keras-style reuse of variables. The rllib.models.Model class (aka “ModelV1”) is deprecated and should no longer be used.

Custom TF models should subclass TFModelV2 to implement the __init__() and forward() methods. Forward takes in a dict of tensor inputs (the observation obs, prev_action, and prev_reward, is_training), optional RNN state, and returns the model output of size num_outputs and the new state. You can also override extra methods of the model such as value_function to implement a custom value branch. Additional supervised / self-supervised losses can be added via the custom_loss method:

class ray.rllib.models.tf.tf_modelv2.TFModelV2(obs_space: <Mock name='mock.spaces.Space' id='140277527236336'>, action_space: <Mock name='mock.spaces.Space' id='140277527236336'>, num_outputs: int, model_config: dict, name: str)[source]

TF version of ModelV2.

Note that this class by itself is not a valid model unless you implement forward() in a subclass.

__init__(obs_space: <Mock name='mock.spaces.Space' id='140277527236336'>, action_space: <Mock name='mock.spaces.Space' id='140277527236336'>, num_outputs: int, model_config: dict, name: str)[source]

Initialize a TFModelV2.

Here is an example implementation for a subclass MyModelClass(TFModelV2):

def __init__(self, *args, **kwargs):
    super(MyModelClass, self).__init__(*args, **kwargs)
    input_layer = tf.keras.layers.Input(...)
    hidden_layer = tf.keras.layers.Dense(...)(input_layer)
    output_layer = tf.keras.layers.Dense(...)(hidden_layer)
    value_layer = tf.keras.layers.Dense(...)(hidden_layer)
    self.base_model = tf.keras.Model(
        input_layer, [output_layer, value_layer])
    self.register_variables(self.base_model.variables)
forward(input_dict: Dict[str, Any], state: List[Any], seq_lens: Any)

Call the model with the given input tensors and state.

Any complex observations (dicts, tuples, etc.) will be unpacked by __call__ before being passed to forward(). To access the flattened observation tensor, refer to input_dict[“obs_flat”].

This method can be called any number of times. In eager execution, each call to forward() will eagerly evaluate the model. In symbolic execution, each call to forward creates a computation graph that operates over the variables of this model (i.e., shares weights).

Custom models should override this instead of __call__.

Parameters
  • input_dict (dict) – dictionary of input tensors, including “obs”, “obs_flat”, “prev_action”, “prev_reward”, “is_training”, “eps_id”, “agent_id”, “infos”, and “t”.

  • state (list) – list of state tensors with sizes matching those returned by get_initial_state + the batch dimension

  • seq_lens (Tensor) – 1d tensor holding input sequence lengths

Returns

The model output tensor of size

[BATCH, num_outputs], and the new RNN state.

Return type

(outputs, state)

Examples

>>> def forward(self, input_dict, state, seq_lens):
>>>     model_out, self._value_out = self.base_model(
...         input_dict["obs"])
>>>     return model_out, state
value_function() → Any

Returns the value function output for the most recent forward pass.

Note that a forward call has to be performed first, before this methods can return anything and thus that calling this method does not cause an extra forward pass through the network.

Returns

value estimate tensor of shape [BATCH].

custom_loss(policy_loss: Any, loss_inputs: Dict[str, Any]) → Any

Override to customize the loss function used to optimize this model.

This can be used to incorporate self-supervised losses (by defining a loss over existing input and output tensors of this model), and supervised losses (by defining losses over a variable-sharing copy of this model’s layers).

You can find an runnable example in examples/custom_loss.py.

Parameters
  • policy_loss (Union[List[Tensor],Tensor]) – List of or single policy loss(es) from the policy.

  • loss_inputs (dict) – map of input placeholders for rollout data.

Returns

List of or scalar tensor for the

customized loss(es) for this model.

Return type

Union[List[Tensor],Tensor]

metrics() → Dict[str, Any]

Override to return custom metrics from your model.

The stats will be reported as part of the learner stats, i.e.,
info:
learner:
model:

key1: metric1 key2: metric2

Returns

Dict of string keys to scalar tensors.

update_ops() → List[Any][source]

Return the list of update ops for this model.

For example, this should include any BatchNorm update ops.

register_variables(variables: List[Any]) → None[source]

Register the given list of variables with this model.

variables(as_dict: bool = False) → List[Any][source]

Returns the list (or a dict) of variables for this model.

Parameters

as_dict (bool) – Whether variables should be returned as dict-values (using descriptive keys).

Returns

The list (or dict if as_dict is

True) of all variables of this ModelV2.

Return type

Union[List[any],Dict[str,any]]

trainable_variables(as_dict: bool = False) → List[Any][source]

Returns the list of trainable variables for this model.

Parameters

as_dict (bool) – Whether variables should be returned as dict-values (using descriptive keys).

Returns

The list (or dict if as_dict is

True) of all trainable (tf)/requires_grad (torch) variables of this ModelV2.

Return type

Union[List[any],Dict[str,any]]

Once implemented, 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
from ray.rllib.models.tf.tf_modelv2 import TFModelV2

class MyModelClass(TFModelV2):
    def __init__(self, obs_space, action_space, num_outputs, model_config, name): ...
    def forward(self, input_dict, state, seq_lens): ...
    def value_function(self): ...

ModelCatalog.register_custom_model("my_model", MyModelClass)

ray.init()
trainer = ppo.PPOTrainer(env="CartPole-v0", config={
    "model": {
        "custom_model": "my_model",
        # Extra kwargs to be passed to your model's c'tor.
        "custom_model_config": {},
    },
})

See the keras model example for a full example of a TF custom model. You can also reference the unit tests for Tuple and Dict spaces, which show how to access nested observation fields.

PyTorch Models

Similarly, you can create and register custom PyTorch models. See these examples of fully connected, convolutional, and recurrent torch models.

class ray.rllib.models.torch.torch_modelv2.TorchModelV2(obs_space: <Mock name='mock.spaces.Space' id='140277527236336'>, action_space: <Mock name='mock.spaces.Space' id='140277527236336'>, num_outputs: int, model_config: dict, name: str)[source]

Torch version of ModelV2.

Note that this class by itself is not a valid model unless you inherit from nn.Module and implement forward() in a subclass.

__init__(obs_space: <Mock name='mock.spaces.Space' id='140277527236336'>, action_space: <Mock name='mock.spaces.Space' id='140277527236336'>, num_outputs: int, model_config: dict, name: str)[source]

Initialize a TorchModelV2.

Here is an example implementation for a subclass MyModelClass(TorchModelV2, nn.Module):

def __init__(self, *args, **kwargs):
    TorchModelV2.__init__(self, *args, **kwargs)
    nn.Module.__init__(self)
    self._hidden_layers = nn.Sequential(...)
    self._logits = ...
    self._value_branch = ...
forward(input_dict: Dict[str, Any], state: List[Any], seq_lens: Any)

Call the model with the given input tensors and state.

Any complex observations (dicts, tuples, etc.) will be unpacked by __call__ before being passed to forward(). To access the flattened observation tensor, refer to input_dict[“obs_flat”].

This method can be called any number of times. In eager execution, each call to forward() will eagerly evaluate the model. In symbolic execution, each call to forward creates a computation graph that operates over the variables of this model (i.e., shares weights).

Custom models should override this instead of __call__.

Parameters
  • input_dict (dict) – dictionary of input tensors, including “obs”, “obs_flat”, “prev_action”, “prev_reward”, “is_training”, “eps_id”, “agent_id”, “infos”, and “t”.

  • state (list) – list of state tensors with sizes matching those returned by get_initial_state + the batch dimension

  • seq_lens (Tensor) – 1d tensor holding input sequence lengths

Returns

The model output tensor of size

[BATCH, num_outputs], and the new RNN state.

Return type

(outputs, state)

Examples

>>> def forward(self, input_dict, state, seq_lens):
>>>     model_out, self._value_out = self.base_model(
...         input_dict["obs"])
>>>     return model_out, state
value_function() → Any

Returns the value function output for the most recent forward pass.

Note that a forward call has to be performed first, before this methods can return anything and thus that calling this method does not cause an extra forward pass through the network.

Returns

value estimate tensor of shape [BATCH].

custom_loss(policy_loss: Any, loss_inputs: Dict[str, Any]) → Any

Override to customize the loss function used to optimize this model.

This can be used to incorporate self-supervised losses (by defining a loss over existing input and output tensors of this model), and supervised losses (by defining losses over a variable-sharing copy of this model’s layers).

You can find an runnable example in examples/custom_loss.py.

Parameters
  • policy_loss (Union[List[Tensor],Tensor]) – List of or single policy loss(es) from the policy.

  • loss_inputs (dict) – map of input placeholders for rollout data.

Returns

List of or scalar tensor for the

customized loss(es) for this model.

Return type

Union[List[Tensor],Tensor]

metrics() → Dict[str, Any]

Override to return custom metrics from your model.

The stats will be reported as part of the learner stats, i.e.,
info:
learner:
model:

key1: metric1 key2: metric2

Returns

Dict of string keys to scalar tensors.

get_initial_state() → List[numpy.ndarray]

Get the initial recurrent state values for the model.

Returns

List of np.array objects containing the initial

hidden state of an RNN, if applicable.

Return type

List[np.ndarray]

Examples

>>> def get_initial_state(self):
>>>    return [
>>>        np.zeros(self.cell_size, np.float32),
>>>        np.zeros(self.cell_size, np.float32),
>>>    ]

Once implemented, the model can then be registered and used in place of a built-in model:

import torch.nn as nn

import ray
from ray.rllib.agents import ppo
from ray.rllib.models import ModelCatalog
from ray.rllib.models.torch.torch_modelv2 import TorchModelV2

class CustomTorchModel(TorchModelV2):
    def __init__(self, obs_space, action_space, num_outputs, model_config, name): ...
    def forward(self, input_dict, state, seq_lens): ...
    def value_function(self): ...

ModelCatalog.register_custom_model("my_model", CustomTorchModel)

ray.init()
trainer = ppo.PPOTrainer(env="CartPole-v0", config={
    "framework": "torch",
    "model": {
        "custom_model": "my_model",
        # Extra kwargs to be passed to your model's c'tor.
        "custom_model_config": {},
    },
})

See the torch model examples for various examples on how to build a custom Torch model (including recurrent ones). You can also reference the unit tests for Tuple and Dict spaces, which show how to access nested observation fields.

Recurrent Models

Instead of using the use_lstm: True option, it can be preferable to use a custom recurrent model. This provides more control over postprocessing of the LSTM output and can also allow the use of multiple LSTM cells to process different portions of the input. For an RNN model it is preferred to subclass RecurrentNetwork (either the TF or Torch versions) and to implement __init__(), get_initial_state(), and forward_rnn(). You can check out the rnn_model.py models as examples to implement your own (either TF or Torch):

class ray.rllib.models.tf.recurrent_net.RecurrentNetwork(obs_space: <Mock name='mock.spaces.Space' id='140277527236336'>, action_space: <Mock name='mock.spaces.Space' id='140277527236336'>, num_outputs: int, model_config: dict, name: str)[source]

Helper class to simplify implementing RNN models with TFModelV2.

Instead of implementing forward(), you can implement forward_rnn() which takes batches with the time dimension added already.

Here is an example implementation for a subclass MyRNNClass(RecurrentNetwork):

def __init__(self, *args, **kwargs):
    super(MyModelClass, self).__init__(*args, **kwargs)
    cell_size = 256

    # Define input layers
    input_layer = tf.keras.layers.Input(
        shape=(None, obs_space.shape[0]))
    state_in_h = tf.keras.layers.Input(shape=(256, ))
    state_in_c = tf.keras.layers.Input(shape=(256, ))
    seq_in = tf.keras.layers.Input(shape=(), dtype=tf.int32)

    # Send to LSTM cell
    lstm_out, state_h, state_c = tf.keras.layers.LSTM(
        cell_size, return_sequences=True, return_state=True,
        name="lstm")(
            inputs=input_layer,
            mask=tf.sequence_mask(seq_in),
            initial_state=[state_in_h, state_in_c])
    output_layer = tf.keras.layers.Dense(...)(lstm_out)

    # Create the RNN model
    self.rnn_model = tf.keras.Model(
        inputs=[input_layer, seq_in, state_in_h, state_in_c],
        outputs=[output_layer, state_h, state_c])
    self.register_variables(self.rnn_model.variables)
    self.rnn_model.summary()
__init__(obs_space: <Mock name='mock.spaces.Space' id='140277527236336'>, action_space: <Mock name='mock.spaces.Space' id='140277527236336'>, num_outputs: int, model_config: dict, name: str)

Initialize a TFModelV2.

Here is an example implementation for a subclass MyModelClass(TFModelV2):

def __init__(self, *args, **kwargs):
    super(MyModelClass, self).__init__(*args, **kwargs)
    input_layer = tf.keras.layers.Input(...)
    hidden_layer = tf.keras.layers.Dense(...)(input_layer)
    output_layer = tf.keras.layers.Dense(...)(hidden_layer)
    value_layer = tf.keras.layers.Dense(...)(hidden_layer)
    self.base_model = tf.keras.Model(
        input_layer, [output_layer, value_layer])
    self.register_variables(self.base_model.variables)
forward_rnn(inputs, state, seq_lens)[source]

Call the model with the given input tensors and state.

Parameters
  • inputs (dict) – observation tensor with shape [B, T, obs_size].

  • state (list) – list of state tensors, each with shape [B, T, size].

  • seq_lens (Tensor) – 1d tensor holding input sequence lengths.

Returns

The model output tensor of shape

[B, T, num_outputs] and the list of new state tensors each with shape [B, size].

Return type

(outputs, new_state)

Sample implementation for the MyRNNClass example:

def forward_rnn(self, inputs, state, seq_lens):
    model_out, h, c = self.rnn_model([inputs, seq_lens] + state)
    return model_out, [h, c]
get_initial_state()[source]

Get the initial recurrent state values for the model.

Returns

list of np.array objects, if any

Sample implementation for the MyRNNClass example:

def get_initial_state(self):
    return [
        np.zeros(self.cell_size, np.float32),
        np.zeros(self.cell_size, np.float32),
    ]

Attention Networks/Transformers

RLlib now also has experimental built-in support for attention/transformer nets (the GTrXL model in particular). Here is an example script on how to use these with some of our algorithms. There is also a test case, which confirms their learning capabilities in PPO and IMPALA.

Batch Normalization

You can use tf.layers.batch_normalization(x, training=input_dict["is_training"]) to add batch norm layers to your custom model: code example. RLlib will automatically run the update ops for the batch norm layers during optimization (see tf_policy.py and multi_gpu_impl.py for the exact handling of these updates).

In case RLlib does not properly detect the update ops for your custom model, you can override the update_ops() method to return the list of ops to run for updates.

Custom Preprocessors

Warning

Custom preprocessors are deprecated, since they sometimes conflict with the built-in preprocessors for handling complex observation spaces. Please use wrapper classes around your environment instead of preprocessors.

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 import ModelCatalog
from ray.rllib.models.preprocessors import Preprocessor

class MyPreprocessorClass(Preprocessor):
    def _init_shape(self, obs_space, options):
        return new_shape  # can vary depending on inputs

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

ModelCatalog.register_custom_preprocessor("my_prep", MyPreprocessorClass)

ray.init()
trainer = ppo.PPOTrainer(env="CartPole-v0", config={
    "model": {
        "custom_preprocessor": "my_prep",
        # Extra kwargs to be passed to your model's c'tor.
        "custom_model_config": {},
    },
})

Custom Models on Top of Built-In Ones

A common use case is to construct a custom model on top of one of RLlib’s built-in ones (e.g. a special output head on top of an fcnet, or an action + observation concat operation at the beginning or after a conv2d stack). Here is an example of how to construct a dueling layer head (for DQN) on top of an RLlib default model (either a Conv2D or an FCNet):

class DuelingQModel(TFModelV2):  # or: TorchModelV2
    """A simple, hard-coded dueling head model."""
    def __init__(obs_space, action_space, num_outputs, model_config, name):
        # Pass num_outputs=None into super constructor (so that no action/
        # logits output layer is built).
        # Alternatively, you can pass in num_outputs=[last layer size of
        # config[model][fcnet_hiddens]] AND set no_last_linear=True, but
        # this seems more tedious as you will have to explain users of this
        # class that num_outputs is NOT the size of your Q-output layer.
        super(DuelingQModel, self).__init__(
            obs_space, action_space, None, model_config, name)
        # Now: self.num_outputs contains the last layer's size, which
        # we can use to construct the dueling head.

        # Construct advantage head ...
        self.A = tf.keras.layers.Dense(num_outputs)
        # torch:
        # self.A = SlimFC(
        #     in_size=self.num_outputs, out_size=num_outputs)

        # ... and value head.
        self.V = tf.keras.layers.Dense(1)
        # torch:
        # self.V = SlimFC(in_size=self.num_outputs, out_size=1)

    def get_q_values(self, inputs):
        # Calculate q-values following dueling logic:
        v = self.V(inputs)  # value
        a = self.A(inputs)  # advantages (per action)
        advantages_mean = tf.reduce_mean(a, 1)
        advantages_centered = a - tf.expand_dims(advantages_mean, 1)
        return v + advantages_centered  # q-values

In order to construct an instance of the above model, you can still use the catalog get_model_v2 convenience method:

dueling_model = ModelCatalog.get_model_v2(
    obs_space=[obs_space],
    action_space=[action_space],
    num_outputs=[num q-value (per action) outs],
    model_config=config["model"],
    framework="tf",  # or: "torch"
    model_interface=DuelingQModel,
    name="dueling_q_model"
)

Now, with the model object, you can get the underlying intermediate output (before the dueling head) by calling dueling_model directly (out = dueling_model([input_dict])), and then passing out into your custom get_q_values method: q_values = dueling_model.get_q_values(out).

Custom Action Distributions

Similar to custom models and preprocessors, you can also specify a custom action distribution class as follows. The action dist class is passed a reference to the model, which you can use to access model.model_config or other attributes of the model. This is commonly used to implement autoregressive action outputs.

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

class MyActionDist(ActionDistribution):
    @staticmethod
    def required_model_output_shape(action_space, model_config):
        return 7  # controls model output feature vector size

    def __init__(self, inputs, model):
        super(MyActionDist, self).__init__(inputs, model)
        assert model.num_outputs == 7

    def sample(self): ...
    def logp(self, actions): ...
    def entropy(self): ...

ModelCatalog.register_custom_action_dist("my_dist", MyActionDist)

ray.init()
trainer = ppo.PPOTrainer(env="CartPole-v0", config={
    "model": {
        "custom_action_dist": "my_dist",
    },
})

Supervised Model Losses

You can mix supervised losses into any RLlib algorithm through custom models. For example, you can add an imitation learning loss on expert experiences, or a self-supervised autoencoder loss within the model. These losses can be defined over either policy evaluation inputs, or data read from offline storage.

TensorFlow: To add a supervised loss to a custom TF model, you need to override the custom_loss() method. This method takes in the existing policy loss for the algorithm, which you can add your own supervised loss to before returning. For debugging, you can also return a dictionary of scalar tensors in the metrics() method. Here is a runnable example of adding an imitation loss to CartPole training that is defined over a offline dataset.

PyTorch: There is no explicit API for adding losses to custom torch models. However, you can modify the loss in the policy definition directly. Like for TF models, offline datasets can be incorporated by creating an input reader and calling reader.next() in the loss forward pass.

Self-Supervised Model Losses

You can also use the custom_loss() API to add in self-supervised losses such as VAE reconstruction loss and L2-regularization.

Variable-length / Complex Observation Spaces

RLlib supports complex and variable-length observation spaces, including gym.spaces.Tuple, gym.spaces.Dict, and rllib.utils.spaces.Repeated. The handling of these spaces is transparent to the user. RLlib internally will insert preprocessors to insert padding for repeated elements, flatten complex observations into a fixed-size vector during transit, and unpack the vector into the structured tensor before sending it to the model. The flattened observation is available to the model as input_dict["obs_flat"], and the unpacked observation as input_dict["obs"].

To enable batching of struct observations, RLlib unpacks them in a StructTensor-like format. In summary, repeated fields are “pushed down” and become the outer dimensions of tensor batches, as illustrated in this figure from the StructTensor RFC.

_images/struct-tensor.png
For further information about complex observation spaces, see:

Variable-length / Parametric Action Spaces

Custom models can be used to work with environments where (1) the set of valid actions varies per step, and/or (2) the number of valid actions is very large. The general idea is that the meaning of actions can be completely conditioned on the observation, i.e., the a in Q(s, a) becomes just a token in [0, MAX_AVAIL_ACTIONS) that only has meaning in the context of s. This works with algorithms in the DQN and policy-gradient families and can be implemented as follows:

  1. The environment should return a mask and/or list of valid action embeddings as part of the observation for each step. To enable batching, the number of actions can be allowed to vary from 1 to some max number:

class MyParamActionEnv(gym.Env):
    def __init__(self, max_avail_actions):
        self.action_space = Discrete(max_avail_actions)
        self.observation_space = Dict({
            "action_mask": Box(0, 1, shape=(max_avail_actions, )),
            "avail_actions": Box(-1, 1, shape=(max_avail_actions, action_embedding_sz)),
            "real_obs": ...,
        })
  1. A custom model can be defined that can interpret the action_mask and avail_actions portions of the observation. Here the model computes the action logits via the dot product of some network output and each action embedding. Invalid actions can be masked out of the softmax by scaling the probability to zero:

class ParametricActionsModel(TFModelV2):
    def __init__(self,
                 obs_space,
                 action_space,
                 num_outputs,
                 model_config,
                 name,
                 true_obs_shape=(4,),
                 action_embed_size=2):
        super(ParametricActionsModel, self).__init__(
            obs_space, action_space, num_outputs, model_config, name)
        self.action_embed_model = FullyConnectedNetwork(...)

    def forward(self, input_dict, state, seq_lens):
        # Extract the available actions tensor from the observation.
        avail_actions = input_dict["obs"]["avail_actions"]
        action_mask = input_dict["obs"]["action_mask"]

        # Compute the predicted action embedding
        action_embed, _ = self.action_embed_model({
            "obs": input_dict["obs"]["cart"]
        })

        # Expand the model output to [BATCH, 1, EMBED_SIZE]. Note that the
        # avail actions tensor is of shape [BATCH, MAX_ACTIONS, EMBED_SIZE].
        intent_vector = tf.expand_dims(action_embed, 1)

        # Batch dot product => shape of logits is [BATCH, MAX_ACTIONS].
        action_logits = tf.reduce_sum(avail_actions * intent_vector, axis=2)

        # Mask out invalid actions (use tf.float32.min for stability)
        inf_mask = tf.maximum(tf.log(action_mask), tf.float32.min)
        return action_logits + inf_mask, state

Depending on your use case it may make sense to use just the masking, just action embeddings, or both. For a runnable example of this in code, check out parametric_actions_cartpole.py. Note that since masking introduces tf.float32.min values into the model output, this technique might not work with all algorithm options. For example, algorithms might crash if they incorrectly process the tf.float32.min values. The cartpole example has working configurations for DQN (must set hiddens=[]), PPO (must disable running mean and set vf_share_layers=True), and several other algorithms. Not all algorithms support parametric actions; see the feature compatibility matrix.

Autoregressive Action Distributions

In an action space with multiple components (e.g., Tuple(a1, a2)), you might want a2 to be conditioned on the sampled value of a1, i.e., a2_sampled ~ P(a2 | a1_sampled, obs). Normally, a1 and a2 would be sampled independently, reducing the expressivity of the policy.

To do this, you need both a custom model that implements the autoregressive pattern, and a custom action distribution class that leverages that model. The autoregressive_action_dist.py example shows how this can be implemented for a simple binary action space. For a more complex space, a more efficient architecture such as a MADE is recommended. Note that sampling a N-part action requires N forward passes through the model, however computing the log probability of an action can be done in one pass:

class BinaryAutoregressiveOutput(ActionDistribution):
    """Action distribution P(a1, a2) = P(a1) * P(a2 | a1)"""

    @staticmethod
    def required_model_output_shape(self, model_config):
        return 16  # controls model output feature vector size

    def sample(self):
        # first, sample a1
        a1_dist = self._a1_distribution()
        a1 = a1_dist.sample()

        # sample a2 conditioned on a1
        a2_dist = self._a2_distribution(a1)
        a2 = a2_dist.sample()

        # return the action tuple
        return TupleActions([a1, a2])

    def logp(self, actions):
        a1, a2 = actions[:, 0], actions[:, 1]
        a1_vec = tf.expand_dims(tf.cast(a1, tf.float32), 1)
        a1_logits, a2_logits = self.model.action_model([self.inputs, a1_vec])
        return (Categorical(a1_logits, None).logp(a1) + Categorical(
            a2_logits, None).logp(a2))

    def _a1_distribution(self):
        BATCH = tf.shape(self.inputs)[0]
        a1_logits, _ = self.model.action_model(
            [self.inputs, tf.zeros((BATCH, 1))])
        a1_dist = Categorical(a1_logits, None)
        return a1_dist

    def _a2_distribution(self, a1):
        a1_vec = tf.expand_dims(tf.cast(a1, tf.float32), 1)
        _, a2_logits = self.model.action_model([self.inputs, a1_vec])
        a2_dist = Categorical(a2_logits, None)
        return a2_dist

class AutoregressiveActionsModel(TFModelV2):
    """Implements the `.action_model` branch required above."""

    def __init__(self, obs_space, action_space, num_outputs, model_config,
                 name):
        super(AutoregressiveActionsModel, self).__init__(
            obs_space, action_space, num_outputs, model_config, name)
        if action_space != Tuple([Discrete(2), Discrete(2)]):
            raise ValueError(
                "This model only supports the [2, 2] action space")

        # Inputs
        obs_input = tf.keras.layers.Input(
            shape=obs_space.shape, name="obs_input")
        a1_input = tf.keras.layers.Input(shape=(1, ), name="a1_input")
        ctx_input = tf.keras.layers.Input(
            shape=(num_outputs, ), name="ctx_input")

        # Output of the model (normally 'logits', but for an autoregressive
        # dist this is more like a context/feature layer encoding the obs)
        context = tf.keras.layers.Dense(
            num_outputs,
            name="hidden",
            activation=tf.nn.tanh,
            kernel_initializer=normc_initializer(1.0))(obs_input)

        # P(a1 | obs)
        a1_logits = tf.keras.layers.Dense(
            2,
            name="a1_logits",
            activation=None,
            kernel_initializer=normc_initializer(0.01))(ctx_input)

        # P(a2 | a1)
        # --note: typically you'd want to implement P(a2 | a1, obs) as follows:
        # a2_context = tf.keras.layers.Concatenate(axis=1)(
        #     [ctx_input, a1_input])
        a2_context = a1_input
        a2_hidden = tf.keras.layers.Dense(
            16,
            name="a2_hidden",
            activation=tf.nn.tanh,
            kernel_initializer=normc_initializer(1.0))(a2_context)
        a2_logits = tf.keras.layers.Dense(
            2,
            name="a2_logits",
            activation=None,
            kernel_initializer=normc_initializer(0.01))(a2_hidden)

        # Base layers
        self.base_model = tf.keras.Model(obs_input, context)
        self.register_variables(self.base_model.variables)
        self.base_model.summary()

        # Autoregressive action sampler
        self.action_model = tf.keras.Model([ctx_input, a1_input],
                                           [a1_logits, a2_logits])
        self.action_model.summary()
        self.register_variables(self.action_model.variables)

Note

Not all algorithms support autoregressive action distributions; see the feature compatibility matrix.