
# Callbacks

Callback is an interface to do __everything else__ besides the training iterations.

Apart from the actual training iterations that minimize the cost,
you almost surely would like to do something else.
There are several places where you might want to do something else:

* Before the training has started (e.g. initialize the saver, dump the graph)
* Along with each training iteration (e.g. run some other operations in the graph)
* Between training iterations (e.g. update the progress bar, update hyperparameters)
* Between epochs (e.g. save the model, run some validation)
* After the training (e.g. send the model somewhere, send a message to your phone)

People normally would write the training loop together with these extra features.
This makes the loop lengthy, and the code for the same feature probably get separated (imagine a
feature which needs initialization in the beginning and then some actual work between iterations).

By writing callbacks to implement what to do at each place, tensorpack trainers
will call the callbacks at the proper time.
Therefore these features can be reused with one single line, as long as you are using tensorpack trainers.

For example, these are the callbacks I used when training a ResNet:

```python
callbacks=[
  # save the model every epoch
  ModelSaver(),
  # backup the model with best validation error
  MinSaver('val-error-top1'),
  # run inference on another Dataflow every epoch, compute classification error and log to monitors
  InferenceRunner(dataset_val, [
      ClassificationError('wrong-top1', 'val-error-top1'),
      ClassificationError('wrong-top5', 'val-error-top5')]),
  # schedule the learning rate based on epoch number
  ScheduledHyperParamSetter('learning_rate',
                            [(30, 1e-2), (60, 1e-3), (85, 1e-4), (95, 1e-5)]),
  # can manually change the learning rate through a file, without interrupting training
  HumanHyperParamSetter('learning_rate'),
  # send validation error to my phone through pushbullet
  SendStat('curl -u your_id_xxx: https://api.pushbullet.com/v2/pushes \\
             -d type=note -d title="validation error" \\
             -d body={val-error-top1} > /dev/null 2>&1',
             'val-error-top1'),
  # record GPU utilizations during training
  GPUUtilizationTracker(),
  # touch a file to pause the training and start a debug shell, to observe what's going on
  InjectShell(shell='ipython'),
  # estimate time until completion
  EstimatedTimeLeft()
] + [    # these callbacks are enabled by default already, though you can customize them
  # maintain those moving average summaries defined in the model (e.g. training loss, training error)
  MovingAverageSummary(),
  # draw a progress bar
  ProgressBar(),
  # run `tf.summary.merge_all` every epoch and log to monitors
  MergeAllSummaries(),
  # run ops in GraphKeys.UPDATE_OPS collection along with training, if any
  RunUpdateOps(),
],
monitors=[        # monitors are a special kind of callbacks. these are also enabled by default
  # write everything to tensorboard
  TFEventWriter(),
  # write all scalar data to a json file, for easy parsing
  JSONWriter(),
  # print all scalar data every epoch (can be configured differently)
  ScalarPrinter(),
]
```

You can see from the above snippet, that callbacks cover every detail of training, ranging from graph operations to the progress bar.
This means you can customize every part of the training to your preference, e.g. display something
different in the progress bar, evaluate part of the summaries at a different frequency, etc.
Similar concepts also exists in other frameworks, such as Keras callbacks, or
`tf.train.SessionRunHook`. But tensorpack callbacks have more functionalities in
design, and can achive much more features, as you can see above.

These features are not always necessary, but think about how messy the main loop would look like if you
were to write these logic together with the loops, and how easy your life will be if you could enable
these features with just one line when you need them.

See [list of callbacks](../modules/callbacks.html)
for a long list of tensorpack builtin callbacks.
See [Write a callback](extend/callback.html)
for details on how callbacks work, what they can do, and how to write them.
