################
Lightning Fabric
################

Fabric is the fast and lightweight way to scale PyTorch models without boilerplate code.

- Easily switch from running on CPU to GPU (Apple Silicon, CUDA, ...), TPU, multi-GPU or even multi-node training
- State-of-the-art distributed training strategies (DDP, FSDP, DeepSpeed) and mixed precision out of the box
- Handles all the boilerplate device logic for you
- Brings useful tools to help you build a trainer (callbacks, logging, checkpoints, ...)
- Designed with multi-billion parameter models in mind

|

.. code-block:: diff

      import torch
      import torch.nn as nn
      from torch.utils.data import DataLoader, Dataset

    + from lightning.fabric import Fabric

      class PyTorchModel(nn.Module):
          ...

      class PyTorchDataset(Dataset):
          ...

    + fabric = Fabric(accelerator="cuda", devices=8, strategy="ddp")
    + fabric.launch()

    - device = "cuda" if torch.cuda.is_available() else "cpu"
      model = PyTorchModel(...)
      optimizer = torch.optim.SGD(model.parameters())
    + model, optimizer = fabric.setup(model, optimizer)
      dataloader = DataLoader(PyTorchDataset(...), ...)
    + dataloader = fabric.setup_dataloaders(dataloader)
      model.train()

      for epoch in range(num_epochs):
          for batch in dataloader:
              input, target = batch
    -         input, target = input.to(device), target.to(device)
              optimizer.zero_grad()
              output = model(input)
              loss = loss_fn(output, target)
    -         loss.backward()
    +         fabric.backward(loss)
              optimizer.step()
              lr_scheduler.step()


----


***********
Why Fabric?
***********

|
|

.. figure:: https://pl-public-data.s3.amazonaws.com/assets_lightning/fabric/PyTorch-to-Fabric-Spectrum-2.svg
   :alt: Fabric spans across a large spectrum - from raw PyTorch all the way to high-level PyTorch Lightning
   :width: 100%

|
|

Fabric differentiates itself from a fully-fledged trainer like Lightning's `Trainer <https://pytorch-lightning.readthedocs.io/en/stable/common/trainer.html>`_ in these key aspects:

**Fast to implement**
There is no need to restructure your code: Just change a few lines in the PyTorch script and you'll be able to leverage Fabric features.

**Maximum Flexibility**
Write your own training and/or inference logic down to the individual optimizer calls.
You aren't forced to conform to a standardized epoch-based training loop like the one in Lightning `Trainer <https://pytorch-lightning.readthedocs.io/en/stable/common/trainer.html>`_.
You can do flexible iteration based training, meta-learning, cross-validation and other types of optimization algorithms without digging into framework internals.
This also makes it super easy to adopt Fabric in existing PyTorch projects to speed-up and scale your models without the compromise on large refactors.
Just remember: With great power comes a great responsibility.

**Maximum Control**
The Lightning `Trainer <https://pytorch-lightning.readthedocs.io/en/stable/common/trainer.html>`_ has many built-in features to make research simpler with less boilerplate, but debugging it requires some familiarity with the framework internals.
In Fabric, everything is opt-in. Think of it as a toolbox: You take out the tools (Fabric functions) you need and leave the other ones behind.
This makes it easier to develop and debug your PyTorch code as you gradually add more features to it.
Fabric provides important tools to remove undesired boilerplate code (distributed, hardware, checkpoints, logging, ...), but leaves the design and orchestration fully up to you.


----

************
Installation
************

Fabric ships directly with Lightning. Install it with

.. code-block:: bash

    pip install lightning

For alternative ways to install, read the :doc:`installation guide <fundamentals/installation>`.

----


************
Fundamentals
************

.. raw:: html

    <div class="display-card-container">
        <div class="row">

.. displayitem::
    :header: Getting Started
    :description: Learn how to add Fabric to your PyTorch code
    :button_link: fundamentals/convert.html
    :col_css: col-md-4
    :height: 150
    :tag: basic

.. displayitem::
    :header: Accelerators
    :description: Take advantage of your hardware with a switch of a flag
    :button_link: fundamentals/accelerators.html
    :col_css: col-md-4
    :height: 150
    :tag: intermediate

.. displayitem::
    :header: Code Structure
    :description: Best practices for setting up your training script with Fabric
    :button_link: fundamentals/code_structure.html
    :col_css: col-md-4
    :height: 150
    :tag: basic

.. displayitem::
    :header: Launch Distributed Training
    :description: Launch a Python script on multiple devices and machines
    :button_link: fundamentals/launch.html
    :col_css: col-md-4
    :height: 150
    :tag: intermediate

.. displayitem::
    :header: Fabric in Notebooks
    :description: Launch on multiple devices from within a Jupyter notebook
    :button_link: fundamentals/notebooks.html
    :col_css: col-md-4
    :height: 150
    :tag: basic

.. displayitem::
    :header: Mixed Precision Training
    :description: Save memory and speed up training using mixed precision
    :button_link: fundamentals/precision.html
    :col_css: col-md-4
    :height: 150
    :tag: intermediate

.. raw:: html

        </div>
    </div>


----


**********************
Build Your Own Trainer
**********************

.. raw:: html

    <div class="display-card-container">
        <div class="row">

.. displayitem::
    :header: The LightningModule
    :description: Organize your code in a LightningModule and use it with Fabric
    :button_link: guide/lightning_module.html
    :col_css: col-md-4
    :height: 150
    :tag: basic

.. displayitem::
    :header: Callbacks
    :description: Make use of the Callback system in Fabric
    :button_link: guide/callbacks.html
    :col_css: col-md-4
    :height: 150
    :tag: basic

.. displayitem::
    :header: Logging
    :description: Learn how Fabric helps you remove boilerplate code for tracking metrics with a logger
    :button_link: guide/logging.html
    :col_css: col-md-4
    :height: 150
    :tag: basic

.. displayitem::
    :header: Checkpoints
    :description: Efficient saving and loading of model weights, training state, hyperparameters and more.
    :button_link: guide/checkpoint.html
    :col_css: col-md-4
    :height: 150
    :tag: basic

.. displayitem::
    :header: Trainer Template
    :description: Take our Fabric Trainer template and customize it for your needs
    :button_link: https://github.com/Lightning-AI/lightning/tree/master/examples/fabric/build_your_own_trainer
    :col_css: col-md-4
    :height: 150
    :tag: intermediate

.. raw:: html

        </div>
    </div>


----


***************
Advanced Topics
***************

.. raw:: html

    <div class="display-card-container">
        <div class="row">

.. displayitem::
    :header: Efficient Gradient Accumulation
    :description: Learn how to perform efficient gradient accumulation in distributed settings
    :button_link: advanced/gradient_accumulation.html
    :col_css: col-md-4
    :height: 160
    :tag: advanced

.. displayitem::
    :header: Distributed Communication
    :description: Learn all about communication primitives for distributed operation. Gather, reduce, broadcast, etc.
    :button_link: advanced/distributed_communication.html
    :col_css: col-md-4
    :height: 160
    :tag: advanced

.. displayitem::
    :header: Multiple Models and Optimizers
    :description: See how flexible Fabric is to work with multiple models and optimizers!
    :button_link: advanced/multiple_setup.html
    :col_css: col-md-4
    :height: 160
    :tag: advanced

.. raw:: html

        </div>
    </div>


----


.. raw:: html

    <div style="display:none">

.. toctree::
    :maxdepth: 1
    :name: start
    :caption: Get Started

    Fabric in 5 minutes <fundamentals/convert>
    Installation <fundamentals/installation>

.. toctree::
    :maxdepth: 1
    :name: fundamentals
    :caption: Fundamentals

    Accelerators <fundamentals/accelerators>
    Code Structure <fundamentals/code_structure>
    Launch Distributed Training <fundamentals/launch>
    Fabric in Notebooks <fundamentals/notebooks>
    Mixed Precision Training <fundamentals/precision>

.. toctree::
    :maxdepth: 1
    :name: byot
    :caption: Build Your Own Trainer

    The LightningModule <guide/lightning_module>
    Callbacks <guide/callbacks>
    Logging <guide/logging>
    Checkpoints <guide/checkpoint>
    Trainer Template <https://github.com/Lightning-AI/lightning/tree/master/examples/fabric/build_your_own_trainer>

.. toctree::
    :maxdepth: 1
    :name: advanced
    :caption: Advanced Topics

    Efficient Gradient Accumulation <advanced/gradient_accumulation>
    Distributed Communication <advanced/distributed_communication>
    Multiple Models and Optimizers <advanced/multiple_setup>

.. toctree::
    :maxdepth: 1
    :name: examples
    :caption: Examples

    Examples <examples/index>

.. toctree::
    :maxdepth: 1
    :name: api
    :caption: API Reference

    Fabric Arguments <api/fabric_args>
    Fabric Methods <api/fabric_methods>
    Utilities <api/utilities>
    Full API Reference <api_reference>


.. raw:: html

    </div>