Building Machine Learning Estimator in TensorFlow

Understanding the Internals of TensorFlow Learn Estimators

Posted by Yuan Tang on July 8, 2016

(This blog is featured in DataScienceWeekly here and Chinese translation here (中文) by Xiatian)

Have you ever wondered what’s the magic behind the tutorials on Large-scale Linear Models and Wide & Deep Learning? I hope this post would at least point you to the right direction.

Please take a look at my previous blog posts to understanding some basics of TensorFlow Learn and its integration with other high-level TensorFlow modules.

The purpose of this post is to help you better understand the underlying principles of estimators in TensorFlow Learn and point out some tips and hints if you ever want to build your own estimator that’s suitable for your particular application. This post will be helpful when you ever wonder how everything works internally and gets overwelmed by the large codebase.

Understanding BaseEstimator and Estimator

BaseEstimator is the abstract and base class for training and evaluating TensorFlow models. It provides the basic functionalities like fit(), partial_fit(), evaluate(), and predict() by utilizing detailed logics hidden in graph_actions.py to handle model inference, evaluation, and training, as well as data_feeder.py to handle data batches fetching for different types of input (Note: in the future, DataFeeder will be replaced by learn.DataFrame). It also checks for compatibility of inputs in terms of dtypes and whether inputs are sparse using estimators.tensor_signature.

In the meantime, BaseEstimator intializes the settings for monitors, checkpointing, etc. While providing most of the logics required for building and evaluating a customized model function, it leaves implementations for _get_train_ops(), _get_eval_ops(), and _get_predict_ops() to its sub-classes, in order to give freedom to sub-classes that require custom handling. BaseEstimator is also distributed, I’ve discussed briefly in my previous blogpost here.

Estimator implemented in the module is the perfect example of how to implement those functions that are left to be overriden by sub-classes of BaseEstimator.

For example, _get_train_ops() in Estimator takes features and targets as inputs, and then returns a tuple of train Operation and loss Tensor, using the customized model function. If you want to implement your own estimator, this also gives you freedom to decide whether targets can be ignored if the estimator can be trained in unsupervised fashion.

Similarly, _get_eval_ops() lets a sub-class to use customized metrics to evaluate each training step. A list of available metrics can be found in a couple of high-level modules in TensorFlow. It should return a dictionary of Tensor object that represents the evaluation ops for the metrics specified.

_get_predict_ops() is implemented to customize predictions, e.g. probability v.s. actual prediction output. It returns a Tensor or a dictionary of Tensor object that represents prediction ops. You can then easily use super-class’s predict() to achieve functionalities like transform() similar to the one in Scikit-learn for unsupervised problems.

Examples of Estimators

LogisticRegressor

Estimator already provides most of the implementations you need. For example, LogisticRegressor only needs to provide its own metrics, such as AUC, accuracy, precision and recall, dedicated for only binary classification problems. So later a user can sub-class LogisticRegressor to implement a estimator for binary classification without much further effort.

TensorForestEstimator

A TensorForestEstimator has also been added to TensorFlow Learn recently. It hides most of the detailed implementations of Random Forests in contrib.tensor_forest while utilizing some exposed high-level components to build the estimator so users can use contrib.tensor_forest more easily.

For example, instead of passing all hyper-parameters to the contructor of TensorForestEstimator, they are passed into params in the contructor and the params are filled by params.fill() and later it will be used in Tensor Forest’s own RandomForestGraphs for constructing the whole graph.

class TensorForestEstimator(estimator.BaseEstimator):
  """An estimator that can train and evaluate a random forest."""

  def __init__(self, params, device_assigner=None, model_dir=None,
               graph_builder_class=tensor_forest.RandomForestGraphs,
               master='', accuracy_metric=None,
               tf_random_seed=None, verbose=1,
               config=None):
    self.params = params.fill()

Since there are a lot of more details for implementation of Random Forest’s inference (many of them have been written as separate kernels to speed things up), its _get_predict_ops() utilizes tensor_forest.RandomForestGraphs as its graph builder. It calls graph_builder.inference_graph to get the prediction ops.

def _get_predict_ops(self, features):
    graph_builder = self.graph_builder_class(
        self.params, device_assigner=self.device_assigner, training=False,
        **self.construction_args)
    features, spec = data_ops.ParseDataTensorOrDict(features)
    return graph_builder.inference_graph(features, data_spec=spec)

Similarly, it uses graph_builder.training_loss for the implementation of _get_train_ops(). Note that TensorForestEstimator uses functions in tensor_forest.data.data_ops module, such as ParseDataTensorOrDict and ParseLabelTensorOrDict to parse the input features and labels.

Other Examples

A new estimator for K-means clustering has been added today, located in contrib.factorization.python.ops.kmeans. Similar to TensorForestEstimator, dedicated kernels are written to highly optimize the speed and only some of the surfaced high-level components are used for the implementation of the estimator. More examples of implementing different estimators can be found in learn.estimators.

I highly recommend you taking a look to understand the underlying code structure better and start Implementing your own estimators!

Please do not hesitate to leave a message if you have any questions.

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Copyright Reserved Yuan Tang 2016
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