.. _train-tensorflow-overview:

Get Started with Distributed Training using TensorFlow/Keras
============================================================

Ray Train's `TensorFlow <https://www.tensorflow.org/>`__ integration enables you
to scale your TensorFlow and Keras training functions to many machines and GPUs.

On a technical level, Ray Train schedules your training workers
and configures ``TF_CONFIG`` for you, allowing you to run
your ``MultiWorkerMirroredStrategy`` training script. See `Distributed
training with TensorFlow <https://www.tensorflow.org/guide/distributed_training>`_
for more information.

Most of the examples in this guide use TensorFlow with Keras, but
Ray Train also works with vanilla TensorFlow.


Quickstart
-----------
.. literalinclude:: ./doc_code/tf_starter.py
  :language: python
  :start-after: __tf_train_start__
  :end-before: __tf_train_end__


Update your training function
-----------------------------

First, update your :ref:`training function <train-overview-training-function>` to support distributed
training.


.. note::
   The current TensorFlow implementation supports
   ``MultiWorkerMirroredStrategy`` (and ``MirroredStrategy``). If there are
   other strategies you wish to see supported by Ray Train, submit a `feature request on GitHub <https://github.com/ray-project/ray/issues>`_.

These instructions closely follow TensorFlow's `Multi-worker training
with Keras <https://www.tensorflow.org/tutorials/distribute/multi_worker_with_keras>`_
tutorial. One key difference is that Ray Train handles the environment
variable set up for you.

**Step 1:** Wrap your model in ``MultiWorkerMirroredStrategy``.

The `MultiWorkerMirroredStrategy <https://www.tensorflow.org/api_docs/python/tf/distribute/experimental/MultiWorkerMirroredStrategy>`_
enables synchronous distributed training. You *must* build and compile the ``Model`` within the scope of the strategy.

.. testcode::
    :skipif: True

    with tf.distribute.MultiWorkerMirroredStrategy().scope():
        model = ... # build model
        model.compile()

**Step 2:** Update your ``Dataset`` batch size to the *global* batch
size.

Set ``batch_size`` appropriately because `batch <https://www.tensorflow.org/api_docs/python/tf/data/Dataset#batch>`_
splits evenly across worker processes.

.. code-block:: diff

    -batch_size = worker_batch_size
    +batch_size = worker_batch_size * train.get_context().get_world_size()


.. warning::
    Ray doesn't automatically set any environment variables or configuration
    related to local parallelism or threading
    :ref:`aside from "OMP_NUM_THREADS" <omp-num-thread-note>`.
    If you want greater control over TensorFlow threading, use
    the ``tf.config.threading`` module (eg.
    ``tf.config.threading.set_inter_op_parallelism_threads(num_cpus)``)
    at the beginning of your ``train_loop_per_worker`` function.

Create a TensorflowTrainer
--------------------------

``Trainer``\s are the primary Ray Train classes for managing state and
execute training. For distributed TensorFlow,
use a :class:`~ray.train.tensorflow.TensorflowTrainer`
that you can setup like this:

.. testcode::
    :hide:

    train_func = lambda: None

.. testcode::

    from ray.train import ScalingConfig
    from ray.train.tensorflow import TensorflowTrainer
    # For GPU Training, set `use_gpu` to True.
    use_gpu = False
    trainer = TensorflowTrainer(
        train_func,
        scaling_config=ScalingConfig(use_gpu=use_gpu, num_workers=2)
    )

To customize the backend setup, you can pass a
:class:`~ray.train.tensorflow.TensorflowConfig`:

.. testcode::
    :skipif: True

    from ray.train import ScalingConfig
    from ray.train.tensorflow import TensorflowTrainer, TensorflowConfig

    trainer = TensorflowTrainer(
        train_func,
        tensorflow_backend=TensorflowConfig(...),
        scaling_config=ScalingConfig(num_workers=2),
    )


For more configurability, see the :py:class:`~ray.train.data_parallel_trainer.DataParallelTrainer` API.


Run a training function
-----------------------

With a distributed training function and a Ray Train ``Trainer``, you are now
ready to start training.

.. testcode::
    :skipif: True

    trainer.fit()

Load and preprocess data
------------------------

TensorFlow by default uses its own internal dataset sharding policy, as described
`in the guide <https://www.tensorflow.org/tutorials/distribute/multi_worker_with_keras#dataset_sharding>`__.
If your TensorFlow dataset is compatible with distributed loading, you don't need to
change anything.

If you require more advanced preprocessing, you may want to consider using Ray Data
for distributed data ingest. See :ref:`Ray Data with Ray Train <data-ingest-torch>`.

The main difference is that you may want to convert your Ray Data dataset shard to
a TensorFlow dataset in your training function so that you can use the Keras
API for model training.

`See this example <https://github.com/ray-project/ray/blob/master/python/ray/train/examples/tf/tensorflow_autoencoder_example.py>`__
for distributed data loading. The relevant parts are:

.. testcode::

    import tensorflow as tf
    from ray import train
    from ray.train.tensorflow import prepare_dataset_shard

    def train_func(config: dict):
        # ...

        # Get dataset shard from Ray Train
        dataset_shard = train.get_context().get_dataset_shard("train")

        # Define a helper function to build a TensorFlow dataset
        def to_tf_dataset(dataset, batch_size):
            def to_tensor_iterator():
                for batch in dataset.iter_tf_batches(
                    batch_size=batch_size, dtypes=tf.float32
                ):
                    yield batch["image"], batch["label"]

            output_signature = (
                tf.TensorSpec(shape=(None, 784), dtype=tf.float32),
                tf.TensorSpec(shape=(None, 784), dtype=tf.float32),
            )
            tf_dataset = tf.data.Dataset.from_generator(
                to_tensor_iterator, output_signature=output_signature
            )
            # Call prepare_dataset_shard to disable automatic sharding
            # (since the dataset is already sharded)
            return prepare_dataset_shard(tf_dataset)

        for epoch in range(epochs):
            # Call our helper function to build the dataset
            tf_dataset = to_tf_dataset(
                dataset=dataset_shard,
                batch_size=64,
            )
            history = multi_worker_model.fit(tf_dataset)



Report results
--------------
During training, the training loop should report intermediate results and checkpoints
to Ray Train. This reporting logs the results to the console output and appends them to
local log files. The logging also triggers :ref:`checkpoint bookkeeping <train-dl-configure-checkpoints>`.

The easiest way to report your results with Keras is by using the
:class:`~ray.train.tensorflow.keras.ReportCheckpointCallback`:

.. testcode::

    from ray.train.tensorflow.keras import ReportCheckpointCallback

    def train_func(config: dict):
        # ...
        for epoch in range(epochs):
            model.fit(dataset, callbacks=[ReportCheckpointCallback()])


This callback automatically forwards all results and checkpoints from the
Keras training function to Ray Train.


Aggregate results
~~~~~~~~~~~~~~~~~

TensorFlow Keras automatically aggregates metrics from all workers. If you wish to have more
control over that, consider implementing a `custom training loop <https://www.tensorflow.org/tutorials/distribute/custom_training>`__.


Save and load checkpoints
-------------------------

You can save :class:`Checkpoints <ray.train.Checkpoint>` by calling ``train.report(metrics, checkpoint=Checkpoint(...))`` in the
training function. This call saves the checkpoint state from the distributed
workers on the ``Trainer``, where you executed your python script.

You can access the latest saved checkpoint through the ``checkpoint`` attribute of
the :py:class:`~ray.train.Result`, and access the best saved checkpoints with the ``best_checkpoints``
attribute.

These concrete examples demonstrate how Ray Train appropriately saves checkpoints, model weights but not models, in distributed training.


.. testcode::

    import json
    import os
    import tempfile

    from ray import train
    from ray.train import Checkpoint, ScalingConfig
    from ray.train.tensorflow import TensorflowTrainer

    import numpy as np

    def train_func(config):
        import tensorflow as tf
        n = 100
        # create a toy dataset
        # data   : X - dim = (n, 4)
        # target : Y - dim = (n, 1)
        X = np.random.normal(0, 1, size=(n, 4))
        Y = np.random.uniform(0, 1, size=(n, 1))

        strategy = tf.distribute.experimental.MultiWorkerMirroredStrategy()
        with strategy.scope():
            # toy neural network : 1-layer
            model = tf.keras.Sequential([tf.keras.layers.Dense(1, activation="linear", input_shape=(4,))])
            model.compile(optimizer="Adam", loss="mean_squared_error", metrics=["mse"])

        for epoch in range(config["num_epochs"]):
            history = model.fit(X, Y, batch_size=20)

            with tempfile.TemporaryDirectory() as temp_checkpoint_dir:
                model.save(os.path.join(temp_checkpoint_dir, "model.keras"))
                checkpoint_dict = os.path.join(temp_checkpoint_dir, "checkpoint.json")
                with open(checkpoint_dict, "w") as f:
                    json.dump({"epoch": epoch}, f)
                checkpoint = Checkpoint.from_directory(temp_checkpoint_dir)

                train.report({"loss": history.history["loss"][0]}, checkpoint=checkpoint)

    trainer = TensorflowTrainer(
        train_func,
        train_loop_config={"num_epochs": 5},
        scaling_config=ScalingConfig(num_workers=2),
    )
    result = trainer.fit()
    print(result.checkpoint)

By default, checkpoints persist to local disk in the :ref:`log
directory <train-log-dir>` of each run.

Load checkpoints
~~~~~~~~~~~~~~~~

.. testcode::

    import os
    import tempfile

    from ray import train
    from ray.train import Checkpoint, ScalingConfig
    from ray.train.tensorflow import TensorflowTrainer

    import numpy as np

    def train_func(config):
        import tensorflow as tf
        n = 100
        # create a toy dataset
        # data   : X - dim = (n, 4)
        # target : Y - dim = (n, 1)
        X = np.random.normal(0, 1, size=(n, 4))
        Y = np.random.uniform(0, 1, size=(n, 1))

        strategy = tf.distribute.experimental.MultiWorkerMirroredStrategy()
        with strategy.scope():
            # toy neural network : 1-layer
            checkpoint = train.get_checkpoint()
            if checkpoint:
                with checkpoint.as_directory() as checkpoint_dir:
                    model = tf.keras.models.load_model(
                        os.path.join(checkpoint_dir, "model.keras")
                    )
            else:
                model = tf.keras.Sequential(
                    [tf.keras.layers.Dense(1, activation="linear", input_shape=(4,))]
                )
            model.compile(optimizer="Adam", loss="mean_squared_error", metrics=["mse"])

        for epoch in range(config["num_epochs"]):
            history = model.fit(X, Y, batch_size=20)

            with tempfile.TemporaryDirectory() as temp_checkpoint_dir:
                model.save(os.path.join(temp_checkpoint_dir, "model.keras"))
                extra_json = os.path.join(temp_checkpoint_dir, "checkpoint.json")
                with open(extra_json, "w") as f:
                    json.dump({"epoch": epoch}, f)
                checkpoint = Checkpoint.from_directory(temp_checkpoint_dir)

                train.report({"loss": history.history["loss"][0]}, checkpoint=checkpoint)

    trainer = TensorflowTrainer(
        train_func,
        train_loop_config={"num_epochs": 5},
        scaling_config=ScalingConfig(num_workers=2),
    )
    result = trainer.fit()
    print(result.checkpoint)


Further reading
---------------
See :ref:`User Guides <train-user-guides>` to explore more topics:

- :ref:`Experiment tracking <train-experiment-tracking-native>`
- :ref:`Fault tolerance and training on spot instances <train-fault-tolerance>`
- :ref:`Hyperparameter optimization <train-tune>`