.. _train-jax:

Get Started with Distributed Training using JAX
===============================================

This guide provides an overview of the `JaxTrainer` in Ray Train.

What is JAX?
------------

`JAX <https://github.com/jax-ml/jax>`_ is a Python library for accelerator-oriented array computation and
program transformation, designed for high-performance numerical computing and large-scale machine learning.

JAX provides an extensible system for transforming numerical functions like `jax.grad`, `jax.jit`, and `jax.vmap`,
utilizing the XLA compiler to create highly optimized code that scales efficiently on accelerators like GPUs and TPUs.
The core power of JAX lies in its composability, allowing these transformations to be combined to build complex,
high-performance numerical programs for distributed execution.

What are TPUs?
--------------

Tensor Processing Units (TPUs), are custom-designed accelerators created by Google to optimize machine learning
workloads. Unlike general-purpose CPUs or parallel-processing GPUs, TPUs are highly specialized for the massive
matrix and tensor computations involved in deep learning, making them exceptionally efficient.

The primary advantage of TPUs is performance at scale, as they are designed to be connected into large, multi-host
configurations called “PodSlices” via a high-speed ICI interconnect, making them ideal for training large models
that are unable to fit on a single node.

To learn more about configuring TPUs with KubeRay, see :ref:`kuberay-tpu`.

JaxTrainer API
--------------

The :class:`~ray.train.v2.jax.JaxTrainer` is the core component for orchestrating distributed JAX training in Ray Train with TPUs.
It follows the Single-Program, Multi-Data (SPMD) paradigm, where your training code is executed simultaneously
across multiple workers, each running on a separate TPU virtual machine within a TPU slice. Ray automatically
handles atomically reserving a TPU multi-host slice.

The `JaxTrainer` is initialized with your training logic, defined in a `train_loop_per_worker` function, and a
`ScalingConfig` that specifies the distributed hardware layout. The `JaxTrainer` currently only supports TPU
accelerator types.

Configuring Scale and TPU
-------------------------

For TPU training, the `ScalingConfig` is where you define the specifics of your hardware slice. Key fields include:

* `use_tpu`: This is a new field added in Ray 2.49.0 to the V2 `ScalingConfig`. This boolean flag explicitly tells Ray Train to initialize the JAX backend for TPU execution.
* `topology`: This is a new field added in Ray 2.49.0 to the V2 `ScalingConfig`. Topology is a string defining the physical arrangement of the TPU chips (e.g., "4x4"). This is required for multi-host training and ensures Ray places workers correctly across the slice. For a list of supported TPU topologies by generation,
  see the `GKE documentation <https://cloud.google.com/kubernetes-engine/docs/concepts/plan-tpus#topology>`_.
* `num_workers`: Set to the number of VMs in your TPU slice. For a v4-32 slice with a 2x2x4 topology, this would be 4.
* `resources_per_worker`: A dictionary specifying the resources each worker needs. For TPUs, you typically request the number of chips per VM (Ex: {"TPU": 4}).
* `accelerator_type`: For TPUs, `accelerator_type` specifies the TPU generation you are using (e.g., "TPU-V6E"), ensuring your workload is scheduled on the desired TPU slice.

Together, these configurations provide a declarative API for defining your entire distributed JAX
training environment, allowing Ray Train to handle the complex task of launching and coordinating
workers across a TPU slice.

Quickstart
----------

For reference, the final code is as follows:

.. testcode::
    :skipif: True

    from ray.train.v2.jax import JaxTrainer
    from ray.train import ScalingConfig

    def train_func():
        # Your JAX training code here.

    scaling_config = ScalingConfig(num_workers=4, use_tpu=True, topology="4x4", accelerator_type="TPU-V6E")
    trainer = JaxTrainer(train_func, scaling_config=scaling_config)
    result = trainer.fit()

1. `train_func` is the Python code that executes on each distributed training worker.
2. :class:`~ray.train.ScalingConfig` defines the number of distributed training workers and whether to use TPUs.
3. :class:`~ray.train.v2.jax.JaxTrainer` launches the distributed training job.

Compare a JAX training script with and without Ray Train.

.. tab-set::

    .. tab-item:: JAX + Ray Train

        .. testcode::
            :skipif: True

            import jax
            import jax.numpy as jnp
            import optax
            import ray.train

            from ray.train.v2.jax import JaxTrainer
            from ray.train import ScalingConfig

            def train_func():
                """This function is run on each distributed worker."""
                key = jax.random.PRNGKey(jax.process_index())
                X = jax.random.normal(key, (100, 1))
                noise = jax.random.normal(key, (100, 1)) * 0.1
                y = 2 * X + 1 + noise

                def linear_model(params, x):
                    return x @ params['w'] + params['b']

                def loss_fn(params, x, y):
                    preds = linear_model(params, x)
                    return jnp.mean((preds - y) ** 2)

                @jax.jit
                def train_step(params, opt_state, x, y):
                    loss, grads = jax.value_and_grad(loss_fn)(params, x, y)
                    updates, opt_state = optimizer.update(grads, opt_state)
                    params = optax.apply_updates(params, updates)
                    return params, opt_state, loss

                # Initialize parameters and optimizer.
                key, w_key, b_key = jax.random.split(key, 3)
                params = {'w': jax.random.normal(w_key, (1, 1)), 'b': jax.random.normal(b_key, (1,))}
                optimizer = optax.adam(learning_rate=0.01)
                opt_state = optimizer.init(params)

                # Training loop
                epochs = 100
                for epoch in range(epochs):
                    params, opt_state, loss = train_step(params, opt_state, X, y)
                    # Report metrics back to Ray Train.
                    ray.train.report({"loss": float(loss), "epoch": epoch})

            # Define the hardware configuration for your distributed job.
            scaling_config = ScalingConfig(
                num_workers=4,
                use_tpu=True,
                topology="4x4",
                accelerator_type="TPU-V6E",
                placement_strategy="SPREAD"
            )

            # Define and run the JaxTrainer.
            trainer = JaxTrainer(
                train_loop_per_worker=train_func,
                scaling_config=scaling_config,
            )
            result = trainer.fit()
            print(f"Training finished. Final loss: {result.metrics['loss']:.4f}")

    .. tab-item:: JAX

        .. This snippet isn't tested because it doesn't use any Ray code.

        .. testcode::
            :skipif: True

            import jax
            import jax.numpy as jnp
            import optax

            # In a non-Ray script, you would manually initialize the
            # distributed environment for multi-host training.
            # import jax.distributed
            # jax.distributed.initialize()

            # Generate synthetic data.
            key = jax.random.PRNGKey(0)
            X = jax.random.normal(key, (100, 1))
            noise = jax.random.normal(key, (100, 1)) * 0.1
            y = 2 * X + 1 + noise

            # Model and loss function are standard JAX.
            def linear_model(params, x):
                return x @ params['w'] + params['b']

            def loss_fn(params, x, y):
                preds = linear_model(params, x)
                return jnp.mean((preds - y) ** 2)

            @jax.jit
            def train_step(params, opt_state, x, y):
                loss, grads = jax.value_and_grad(loss_fn)(params, x, y)
                updates, opt_state = optimizer.update(grads, opt_state)
                params = optax.apply_updates(params, updates)
                return params, opt_state, loss

            # Initialize parameters and optimizer.
            key, w_key, b_key = jax.random.split(key, 3)
            params = {'w': jax.random.normal(w_key, (1, 1)), 'b': jax.random.normal(b_key, (1,))}
            optimizer = optax.adam(learning_rate=0.01)
            opt_state = optimizer.init(params)

            # Training loop
            epochs = 100
            print("Starting training...")
            for epoch in range(epochs):
                params, opt_state, loss = train_step(params, opt_state, X, y)
                if epoch % 10 == 0:
                    print(f"Epoch {epoch}, Loss: {loss:.4f}")

            print("Training finished.")
            print(f"Learned parameters: w={params['w'].item():.4f}, b={params['b'].item():.4f}")

Set up a training function
--------------------------

Ray Train automatically initializes the JAX distributed environment on each TPU worker.
To adapt your existing JAX code, you simply need to wrap your training logic in a Python function
that can be passed to the `JaxTrainer`.

This function is the entry point that Ray will execute on each remote worker.

.. code-block:: diff

    +from ray.train.v2.jax import JaxTrainer
    +from ray.train import ScalingConfig, report

    -def main_logic()
    +def train_func():
        """This function is run on each distributed worker."""
        # ... (JAX model, data, and training step definitions) ...

        # Training loop
        for epoch in range(epochs):
            params, opt_state, loss = train_step(params, opt_state, X, y)
    -       print(f"Epoch {epoch}, Loss: {loss:.4f}")
    +       # In Ray Train, you can report metrics back to the trainer
    +       report({"loss": float(loss), "epoch": epoch})

    -if __name__ == "__main__":
    -    main_logic()
    +# Define the hardware configuration for your distributed job.
    +scaling_config = ScalingConfig(
    +    num_workers=4,
    +    use_tpu=True,
    +    topology="4x4",
    +    accelerator_type="TPU-V6E",
    +    placement_strategy="SPREAD"
    +)
    +
    +# Define and run the JaxTrainer, which executes `train_func`.
    +trainer = JaxTrainer(
    +    train_loop_per_worker=train_func,
    +    scaling_config=scaling_config
    +)
    +result = trainer.fit()

Configure persistent storage
----------------------------

Create a :class:`~ray.train.RunConfig` object to specify the path where results
(including checkpoints and artifacts) will be saved.

.. testcode::

    from ray.train import RunConfig

    # Local path (/some/local/path/unique_run_name)
    run_config = RunConfig(storage_path="/some/local/path", name="unique_run_name")

    # Shared cloud storage URI (s3://bucket/unique_run_name)
    run_config = RunConfig(storage_path="s3://bucket", name="unique_run_name")

    # Shared NFS path (/mnt/nfs/unique_run_name)
    run_config = RunConfig(storage_path="/mnt/nfs", name="unique_run_name")


.. warning::

    Specifying a *shared storage location* (such as cloud storage or NFS) is
    *optional* for single-node clusters, but it is **required for multi-node clusters.**
    Using a local path will :ref:`raise an error <multinode-local-storage-warning>`
    during checkpointing for multi-node clusters.


For more details, see :ref:`persistent-storage-guide`.

Launch a training job
---------------------

Tying it all together, you can now launch a distributed training job with a :class:`~ray.train.v2.jax.JaxTrainer`.

.. testcode::
    :skipif: True

    from ray.train import ScalingConfig

    train_func = lambda: None
    scaling_config = ScalingConfig(num_workers=4, use_tpu=True, topology="4x4", accelerator_type="TPU-V6E")
    run_config = None

.. testcode::
    :skipif: True

    from ray.train.v2.jax import JaxTrainer

    trainer = JaxTrainer(
        train_func, scaling_config=scaling_config, run_config=run_config
    )
    result = trainer.fit()

Access training results
-----------------------

After training completes, a :class:`~ray.train.Result` object is returned which contains
information about the training run, including the metrics and checkpoints reported during training.

.. testcode::
    :skipif: True

    result.metrics     # The metrics reported during training.
    result.checkpoint  # The latest checkpoint reported during training.
    result.path        # The path where logs are stored.
    result.error       # The exception that was raised, if training failed.

For more usage examples, see :ref:`train-inspect-results`.

Next steps
----------

After you have converted your JAX training script to use Ray Train:

* See :ref:`User Guides <train-user-guides>` to learn more about how to perform specific tasks.
* Browse the :doc:`Examples <examples>` for end-to-end examples of how to use Ray Train.
* Consult the :ref:`API Reference <train-api>` for more details on the classes and methods from this tutorial.