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The Transform TFX Pipeline Component

The Transform TFX pipeline component performs feature engineering on tf.Examples emitted from an ExampleGen component, using a data schema created by a SchemaGen component, and emits a SavedModel. When executed, the SavedModel will accept tf.Examples emitted from an ExampleGen component and emit the transformed feature data.

  • Consumes: tf.Examples from an ExampleGen component, and a data schema from a SchemaGen component.
  • Emits: A SavedModel to a Trainer component

Configuring a Transform Component

Once your preprocessing_fn is written, it needs to be defined in a python module that is then provided to the Transform component as an input. This module will be loaded by transform and the function named preprocessing_fn will be found and used by Transform to construct the preprocessing pipeline.

transform_training = components.Transform(
    examples=examples_gen.outputs['training_examples'],
    schema=infer_schema.outputs['schema'],
    module_file=taxi_pipeline_utils,
    name='transform-training')

transform_eval = components.Transform(
    examples=examples_gen.outputs['eval_examples'],
    schema=infer_schema.outputs['schema'],
    transform_dir=transform_training.outputs['output'],
    name='transform-eval')

Transform and TensorFlow Transform

Transform makes extensive use of TensorFlow Transform for performing feature engineering on your dataset. TensorFlow Transform is a great tool for transforming feature data before it goes to your model and as a part of the training process. Common feature transformations include:

  • Embedding: converting sparse features (like the integer IDs produced by a vocabulary) into dense features by finding a meaningful mapping from high- dimensional space to low dimensional space. See the Embeddings unit in the Machine-learning Crash Course for an introduction to embeddings.
  • Vocabulary generation: converting strings or other non-numeric features into integers by creating a vocabulary that maps each unique value to an ID number.
  • Normalizing values: transforming numeric features so that they all fall within a similar range.
  • Bucketization: converting continuous-valued features into categorical features by assigning values to discrete buckets.
  • Enriching text features: producing features from raw data like tokens, n-grams, entities, sentiment, etc., to enrich the feature set.

TensorFlow Transform provides support for these and many other kinds of transformations:

  • Automatically generate a vocabulary from your latest data.

  • Perform arbitrary transformations on your data before sending it to your model. TensorFlow Transform builds transformations into the TensorFlow graph for your model so the same transformations are performed at training and inference time. You can define transformations that refer to global properties of the data, like the max value of a feature across all training instances.

You can transform your data however you like prior to running TFX. But if you do it within TensorFlow Transform, transforms become part of the TensorFlow graph. This approach helps avoid training/serving skew.

Transformations inside your modeling code use FeatureColumns. Using FeatureColumns, you can define bucketizations, integerizations that use predefined vocabularies, or any other transformations that can be defined without looking at the data.

By contrast, TensorFlow Transform is designed for transformations that require a full pass over the data to compute values that are not known in advance. For example, vocabulary generation requires a full pass over the data.

In addition to computing values using Apache Beam, TensorFlow Transform allows users to embed these values into a TensorFlow graph, which can then be loaded into the training graph. For example when normalizing features, the tft.scale_to_z_score function will compute the mean and standard deviation of a feature, and also a representation, in a TensorFlow graph, of the function that subtracts the mean and divides by the standard deviation. By emitting a TensorFlow graph, not just statistics, TensorFlow Transform simplifies the process of authoring your preprocessing pipeline.

Since the preprocessing is expressed as a graph, it can happen on the server, and it's guaranteed to be consistent between training and serving. This consistency eliminates one source of training/serving skew.

TensorFlow Transform allows users to specify their preprocessing pipeline using TensorFlow code. This means that a pipeline is constructed in the same manner as a TensorFlow graph. If only TensorFlow ops were used in this graph, the pipeline would be a pure map that accepts batches of input and returns batches of output. Such a pipeline would be equivalent to placing this graph inside your input_fn when using the tf.Estimator API. In order to specify full-pass operations such as computing quantiles, TensorFlow Transform provides special functions called analyzers that appear like TensorFlow ops, but in fact specify a deferred computation that will be done by Apache Beam, and the output inserted into the graph as a constant. While an ordinary TensorFlow op will take a single batch as its input, perform some computation on just that batch and emit a batch, an analyzer will perform a global reduction (implemented in Apache Beam) over all batches and return the result.

By combining ordinary TensorFlow ops and TensorFlow Transform analyzers, users can create complex pipelines to preprocess their data. For example the tft.scale_to_z_score function takes an input tensor and returns that tensor normalized to have mean 0 and variance 1. It does this by calling the mean and var analyzers under the hood, which will effectively generate constants in the graph equal to the mean and variance of the input tensor. It will then use TensorFlow ops to subtract the mean and divide by the standard deviation.

The TensorFlow Transform preprocessing_fn

The TFX Transform component simplifies the use of Transform by handling the API calls related to reading and writing data, and writing the output SavedModel to disk. As a TFX user, you only have to define a single function called the preprocessing_fn. In preprocessing_fn you define a series of functions that manipulate the input dict of tensors to produce the output dict of tensors. You can find helper functions like scale_to_0_1 and compute_and_apply_vocabulary the TensorFlow Transform API or use regular TensorFlow functions as shown below.

def preprocessing_fn(inputs):
  """tf.transform's callback function for preprocessing inputs.

  Args:
    inputs: map from feature keys to raw not-yet-transformed features.

  Returns:
    Map from string feature key to transformed feature operations.
  """
  outputs = {}
  for key in _DENSE_FLOAT_FEATURE_KEYS:
    # Preserve this feature as a dense float, setting nan's to the mean.
    outputs[_transformed_name(key)] = transform.scale_to_z_score(
        _fill_in_missing(inputs[key]))

  for key in _VOCAB_FEATURE_KEYS:
    # Build a vocabulary for this feature.
    outputs[_transformed_name(
        key)] = transform.compute_and_apply_vocabulary(
            _fill_in_missing(inputs[key]),
            top_k=_VOCAB_SIZE,
            num_oov_buckets=_OOV_SIZE)

  for key in _BUCKET_FEATURE_KEYS:
    outputs[_transformed_name(key)] = transform.bucketize(
        _fill_in_missing(inputs[key]), _FEATURE_BUCKET_COUNT)

  for key in _CATEGORICAL_FEATURE_KEYS:
    outputs[_transformed_name(key)] = _fill_in_missing(inputs[key])

  # Was this passenger a big tipper?
  taxi_fare = _fill_in_missing(inputs[_FARE_KEY])
  tips = _fill_in_missing(inputs[_LABEL_KEY])
  outputs[_transformed_name(_LABEL_KEY)] = tf.where(
      tf.is_nan(taxi_fare),
      tf.cast(tf.zeros_like(taxi_fare), tf.int64),
      # Test if the tip was > 20% of the fare.
      tf.cast(
          tf.greater(tips, tf.multiply(taxi_fare, tf.constant(0.2))), tf.int64))

  return outputs

Understanding the inputs to the preprocessing_fn

The preprocessing_fn describes a series of operations on tensors (that is, Tensors or SparseTensors) and so to write the preprocessing_fn correctly it is necessary to understand how your data is represented as tensors. The input to the preprocessing_fn is determined by the schema. A Schema proto contains a list of Features, and Transform converts these to a "feature spec" (sometimes called a "parsing spec") which is a dict whose keys are feature names and whose values are one of FixedLenFeature or VarLenFeature (or other options not used by TensorFlow Transform).

The rules for inferring a feature spec from the Schema are

  • Each feature with shape set will result in a tf.FixedLenFeature with shape and default_value=None. presence.min_fraction must be 1 otherwise and error will result, since when there is no default value, a tf.FixedLenFeature requires the feature to always be present.
  • Each feature with shape not set will result in a VarLenFeature.
  • Each sparse_feature will result in a tf.SparseFeature whose size and is_sorted are determined by the fixed_shape and is_sorted fields of the SparseFeature message.
  • Features used as the index_feature or value_feature of a sparse_feature will not have their own entry generated in the feature spec.
  • The correspondence between type field of the feature (or the values feature of a sparse_feature proto) and the dtype of the feature spec is given by the following table:
type dtype
schema_pb2.INT tf.int64
schema_pb2.FLOAT tf.float32
schema_pb2.BYTES tf.string

Using TensorFlow Transform to handle string labels

Usually one wants to use TensorFlow Transform to both generate a vocabulary and apply that vocabulary to convert strings to integers. When following this workflow, the input_fn constructed in the model will output the integerized string. However labels are an exception, because in order for the model to be able to map the output (integer) labels back to strings, the model needs the input_fn to output a string label, together with a list of possible values of the label. E.g. if the labels are cat and dog then the output of the input_fn should be these raw strings, and the keys ["cat", "dog"] need to be passed into the estimator as a parameter (see details below).

In order to handle the mapping of string labels to integers, you should use TensorFlow Transform to generate a vocabulary. We demonstrate this in the code snippet below:

def _preprocessing_fn(inputs):
  """Preprocess input features into transformed features."""

  ...


  education = inputs[features.RAW_LABEL_KEY]
  _ = tft.uniques(education, vocab_filename=features.RAW_LABEL_KEY)

  ...

The preprocessing function above takes the raw input feature (which will also be returned as part of the output of the preprocessing function) and calls tft.uniques on it. This results in a vocabulary being generated for education that can be accessed in the model.

The example also shows how to transform a label and then generate a vocabulary for the transformed label. In particular it takes the raw label education and converts all but the top 5 labels (by frequency) to UNKNOWN, without converting the label to an integer.

In the model code, the classifier must be given the vocabulary generated by tft.uniques as the label_vocabulary argument. This is done by first reading this vocabulary as a list with a helper function. This is shown in the snippet below. Note the example code uses the transformed label discussed above but here we show code for using the raw label.

def create_estimator(pipeline_inputs, hparams):

  ...

  tf_transform_output = trainer_util.TFTransformOutput(
      pipeline_inputs.transform_dir)

  # vocabulary_by_name() returns a Python list.
  label_vocabulary = tf_transform_output.vocabulary_by_name(
      features.RAW_LABEL_KEY)

  return tf.contrib.learn.DNNLinearCombinedClassifier(
      ...
      n_classes=len(label_vocab),
      label_vocabulary=label_vocab,
      ...)