# Checkpoints

This document examines how to save and restore TensorFlow models built with
Estimators. TensorFlow provides two model formats:

*   checkpoints, which is a format dependent on the code that created
    the model.
*   SavedModel, which is a format independent of the code that created
    the model.

This document focuses on checkpoints. For details on SavedModel, see the
@{$saved_model$Saving and Restoring} chapter of the
*TensorFlow Programmer's Guide*.


## Sample code

This document relies on the same Iris classification example detailed in
<!-- TODO (barryr): fill in link when module settles down. --> 
@{$premade_estimators$Getting Started with TensorFlow}.
To download and access the example, invoke the following two commands:

```shell
git clone https://github.com/tensorflow/models/
cd models/samples/core/get_started
```

Most of the code snippets in this document are minor variations
on `premade_estimator.py`.


## Saving partially-trained models

Estimators automatically write the following to disk:

*   **checkpoints**, which are versions of the model created during training.
*   **event files**, which contain information that
    [TensorBoard](https://developers.google.com/machine-learning/glossary/#TensorBoard)
    uses to create visualizations.

To specify the top-level directory in which the Estimator stores its
information, assign a value to the optional `model_dir` argument of any
Estimator's constructor.  For example, the following code sets the `model_dir`
argument to the `models/iris` directory:

```python
classifier = tf.estimator.DNNClassifier(
    feature_columns=my_feature_columns,
    hidden_units=[10, 10],
    n_classes=3,
    model_dir='models/iris')
```

Suppose you call the Estimator's `train` method. For example:


```python
classifier.train(
        input_fn=lambda:train_input_fn(train_x, train_y, batch_size=100),
                steps=200)
```

As suggested by the following diagrams, the first call to `train`
adds checkpoints and other files to the `model_dir` directory:

<div style="width:80%; margin:auto; margin-bottom:10px; margin-top:20px;">
<img style="width:100%" src="../images/first_train_calls.png">
</div>
<div style="text-align: center">
The first call to train().
</div>


To see the objects in the created `model_dir` directory on a
UNIX-based system, just call `ls` as follows:

```none
$ ls -1 models/iris
checkpoint
events.out.tfevents.timestamp.hostname
graph.pbtxt
model.ckpt-1.data-00000-of-00001
model.ckpt-1.index
model.ckpt-1.meta
model.ckpt-200.data-00000-of-00001
model.ckpt-200.index
model.ckpt-200.meta
```

The preceding `ls` command shows that the Estimator created checkpoints
at steps 1 (the start of training) and 200 (the end of training).


### Default checkpoint directory

If you don't specify `model_dir` in an Estimator's constructor, the Estimator
writes checkpoint files to a temporary directory chosen by Python's
[tempfile.mkdtemp](https://docs.python.org/3/library/tempfile.html#tempfile.mkdtemp)
function. For example, the following Estimator constructor does *not* specify
the `model_dir` argument:

```python
classifier = tf.estimator.DNNClassifier(
    feature_columns=my_feature_columns,
    hidden_units=[10, 10],
    n_classes=3)

print(classifier.model_dir)
```

The `tempfile.mkdtemp` function picks a secure, temporary directory
appropriate for your operating system. For example, a typical temporary
directory on macOS might be something like the following:

```None
/var/folders/0s/5q9kfzfj3gx2knj0vj8p68yc00dhcr/T/tmpYm1Rwa
```

### Checkpointing Frequency

By default, the Estimator saves
[checkpoints](https://developers.google.com/machine-learning/glossary/#checkpoint)
in the `model_dir` according to the following schedule:

*   Writes a checkpoint every 10 minutes (600 seconds).
*   Writes a checkpoint when the `train` method starts (first iteration)
    and completes (final iteration).
*   Retains only the 5 most recent checkpoints in the directory.

You may alter the default schedule by taking the following steps:

1.  Create a @{tf.estimator.RunConfig$`RunConfig`} object that defines the
    desired schedule.
2.  When instantiating the Estimator, pass that `RunConfig` object to the
    Estimator's `config` argument.

For example, the following code changes the checkpointing schedule to every
20 minutes and retains the 10 most recent checkpoints:

```python
my_checkpointing_config = tf.estimator.RunConfig(
    save_checkpoints_secs = 20*60,  # Save checkpoints every 20 minutes.
    keep_checkpoint_max = 10,       # Retain the 10 most recent checkpoints.
)

classifier = tf.estimator.DNNClassifier(
    feature_columns=my_feature_columns,
    hidden_units=[10, 10],
    n_classes=3,
    model_dir='models/iris',
    config=my_checkpointing_config)
```

## Restoring your model

The first time you call an Estimator's `train` method, TensorFlow saves a
checkpoint to the `model_dir`. Each subsequent call to the Estimator's
`train`, `eval`, or `predict` method causes the following:

1.  The Estimator builds the model's
    [graph](https://developers.google.com/machine-learning/glossary/#graph)
    by running the `model_fn()`.  (For details on the `model_fn()`, see
    @{$custom_estimators$Creating Custom Estimators.})
2.  The Estimator initializes the weights of the new model from the data
    stored in the most recent checkpoint.

In other words, as the following illustration suggests, once checkpoints
exist, TensorFlow rebuilds the model each time you call `train()`,
`evaluate()`, or `predict()`.

<div style="width:80%; margin:auto; margin-bottom:10px; margin-top:20px;">
<img style="width:100%" src="../images/subsequent_calls.png">
</div>
<div style="text-align: center">
Subsequent calls to train(), evaluate(), or predict()
</div>


### Avoiding a bad restoration

Restoring a model's state from a checkpoint only works if the model
and checkpoint are compatible.  For example, suppose you trained a
`DNNClassifier` Estimator containing two hidden layers,
each having 10 nodes:

```python
classifier = tf.estimator.DNNClassifier(
    feature_columns=feature_columns,
    hidden_units=[10, 10],
    n_classes=3,
    model_dir='models/iris')

classifier.train(
    input_fn=lambda:train_input_fn(train_x, train_y, batch_size=100),
        steps=200)
```

After training (and, therefore, after creating checkpoints in `models/iris`),
imagine that you changed the number of neurons in each hidden layer from 10 to
20 and then attempted to retrain the model:

``` python
classifier2 = tf.estimator.DNNClassifier(
    feature_columns=my_feature_columns,
    hidden_units=[20, 20],  # Change the number of neurons in the model.
    n_classes=3,
    model_dir='models/iris')

classifier.train(
    input_fn=lambda:train_input_fn(train_x, train_y, batch_size=100),
        steps=200)
```

Since the state in the checkpoint is incompatible with the model described
in `classifier2`, retraining fails with the following error:

```None
...
InvalidArgumentError (see above for traceback): tensor_name =
dnn/hiddenlayer_1/bias/t_0/Adagrad; shape in shape_and_slice spec [10]
does not match the shape stored in checkpoint: [20]
```

To run experiments in which you train and compare slightly different
versions of a model, save a copy of the code that created each
`model-dir`, possibly by creating a separate git branch for each version.
This separation will keep your checkpoints recoverable.

## Summary

Checkpoints provide an easy automatic mechanism for storing and restoring
models created by Estimators.  See the @{$saved_model$Saving and Restoring}
chapter of the *TensorFlow Programmer's Guide* for details on:

*   Saving and restoring models created by low-level TensorFlow APIs.
*   Saving and restoring models in the SavedModel format, which is a
    language-neutral, recoverable, serialization format.
