Exporing models in the TF1 Hub format

You can read more about this format in TF1 Hub format

Compatibility note

The TF1 Hub format is geared towards TensorFlow 1. It is only partially supported by TF Hub in TensorFlow 2. Please do consider publishing in the new TF2 SavedModel format instead by following the Exporting a model guide.

The TF1 Hub format is similar to the SavedModel format of TensorFlow 1 on a syntactic level (same file names and protocol messages) but semantically different to allow for module reuse, composition and re-training (e.g., different storage of resource initializers, different tagging conventions for metagraphs). The easiest way to tell them apart on disk is the presence or absence of the tfhub_module.pb file.

General approach

To define a new module, a publisher calls hub.create_module_spec() with a function module_fn. This function constructs a graph representing the module's internal structure, using tf.placeholder() for inputs to be supplied by the caller. Then it defines signatures by calling hub.add_signature(name, inputs, outputs) one or more times.

For example:

def module_fn():
  inputs = tf.placeholder(dtype=tf.float32, shape=[None, 50])
  layer1 = tf.layers.dense(inputs, 200)
  layer2 = tf.layers.dense(layer1, 100)
  outputs = dict(default=layer2, hidden_activations=layer1)
  # Add default signature.
  hub.add_signature(inputs=inputs, outputs=outputs)

spec = hub.create_module_spec(module_fn)

The result of hub.create_module_spec() can be used, instead of a path, to instantiate a module object within a particular TensorFlow graph. In such case, there is no checkpoint, and the module instance will use the variable initializers instead.

Any module instance can be serialized to disk via its export(path, session) method. Exporting a module serializes its definition together with the current state of its variables in session into the passed path. This can be used when exporting a module for the first time, as well as when exporting a fine tuned module.

For compatibility with TensorFlow Estimators, hub.LatestModuleExporter exports modules from the latest checkpoint, just like tf.estimator.LatestExporter exports the entire model from the latest checkpoint.

Module publishers should implement a common signature when possible, so that consumers can easily exchange modules and find the best one for their problem.

Real example

Take a look at our text embedding module exporter for a real-world example of how to create a module from a common text embedding format.

Advice for Publishers

To make fine-tuning easier for consumers, please be mindful of the following:

  • Fine-tuning needs regularization. Your module is exported with the REGULARIZATION_LOSSES collection, which is what puts your choice of tf.layers.dense(..., kernel_regularizer=...) etc. into what the consumer gets from tf.losses.get_regularization_losses(). Prefer this way of defining L1/L2 regularization losses.

  • In the publisher model, avoid defining L1/L2 regularization via the l1_ and l2_regularization_strength parameters of tf.train.FtrlOptimizer, tf.train.ProximalGradientDescentOptimizer, and other proximal optimizers. These are not exported alongside the module, and setting regularization strengths globally may not be appropriate for the consumer. Except for L1 regularization in wide (i.e. sparse linear) or wide & deep models, it should be possible to use individual regularization losses instead.

  • If you use dropout, batch normalization, or similar training techniques, set their hyperparameters to values that make sense across many expected uses. The dropout rate may have to be adjusted to the target problem's propensity to overfitting. In batch normalization, the momentum (a.k.a. decay coefficient) should be small enough to enable fine-tuning with small datasets and/or large batches. For advanced consumers, consider adding a signature that exposes control over critcial hyperparameters.