# RLlib Core Concepts¶

In this section, we’ll cover three key concepts in RLlib: Policies, Samples, and Trainers.

## Policies¶

Policies are a core concept in RLlib. In a nutshell, policies are Python classes that define how an agent acts in an environment. Rollout workers query the policy to determine agent actions. In a gym environment, there is a single agent and policy. In vector envs, policy inference is for multiple agents at once, and in multi-agent, there may be multiple policies, each controlling one or more agents:

Policies can be implemented using any framework. However, for TensorFlow and PyTorch, RLlib has build_tf_policy and build_torch_policy helper functions that let you define a trainable policy with a functional-style API, for example:

def policy_gradient_loss(policy, model, dist_class, train_batch):
logits, _ = model.from_batch(train_batch)
action_dist = dist_class(logits, model)
return -tf.reduce_mean(
action_dist.logp(train_batch["actions"]) * train_batch["rewards"])

# <class 'ray.rllib.policy.tf_policy_template.MyTFPolicy'>
MyTFPolicy = build_tf_policy(
name="MyTFPolicy",


## Sample Batches¶

Whether running in a single process or large cluster, all data interchange in RLlib is in the form of sample batches. Sample batches encode one or more fragments of a trajectory. Typically, RLlib collects batches of size rollout_fragment_length from rollout workers, and concatenates one or more of these batches into a batch of size train_batch_size that is the input to SGD.

A typical sample batch looks something like the following when summarized. Since all values are kept in arrays, this allows for efficient encoding and transmission across the network:

{ 'action_logp': np.ndarray((200,), dtype=float32, min=-0.701, max=-0.685, mean=-0.694),
'actions': np.ndarray((200,), dtype=int64, min=0.0, max=1.0, mean=0.495),
'dones': np.ndarray((200,), dtype=bool, min=0.0, max=1.0, mean=0.055),
'infos': np.ndarray((200,), dtype=object, head={}),
'new_obs': np.ndarray((200, 4), dtype=float32, min=-2.46, max=2.259, mean=0.018),
'obs': np.ndarray((200, 4), dtype=float32, min=-2.46, max=2.259, mean=0.016),
'rewards': np.ndarray((200,), dtype=float32, min=1.0, max=1.0, mean=1.0),
't': np.ndarray((200,), dtype=int64, min=0.0, max=34.0, mean=9.14)}


In multi-agent mode, sample batches are collected separately for each individual policy.

## Training¶

Policies each define a learn_on_batch() method that improves the policy given a sample batch of input. For TF and Torch policies, this is implemented using a loss function that takes as input sample batch tensors and outputs a scalar loss. Here are a few example loss functions:

RLlib Trainer classes coordinate the distributed workflow of running rollouts and optimizing policies. Trainer classes leverage parallel iterators to implement the desired computation pattern. The following figure shows synchronous sampling, the simplest of these patterns:

Synchronous Sampling (e.g., A2C, PG, PPO)

RLlib uses Ray actors to scale training from a single core to many thousands of cores in a cluster. You can configure the parallelism used for training by changing the num_workers parameter. Check out our scaling guide for more details here.