At its launch in 2018, TensorFlow Hub offered a single type of asset: TF1 Hub format for import into TensorFlow 1 programs.
This page explains how to use TF1 Hub format in TF1 (or the TF1 compatibility
mode of TF2) with the
hub.Module class and associated APIs. (The typical use
is to build a
tf.Graph, possibly inside a TF1
Estimator, by combining one or
more models in TF1 Hub format with
Users of TensorFlow 2 (outside TF1 compatibility mode) must use
the new API with
hub.KerasLayer. The new
API loads the new TF2 SavedModel asset type, but also has limited
support for loading TF1 Hub format into TF2.
Using a model in TF1 Hub format
Instantiating a model in TF1 Hub format
A model in TF1 Hub format is imported into a TensorFlow program by creating a
hub.Module object from a string with its URL or filesystem path, such as:
m = hub.Module("path/to/a/module_dir")
This adds the module's variables to the current TensorFlow graph. Running their initializers will read their pre-trained values from disk. Likewise, tables and other state is added to the graph.
When creating a module from a URL, the module content is downloaded and cached
in the local system temporary directory. The location where modules are cached
can be overridden using
TFHUB_CACHE_DIR environment variable. For details, see
Applying a Module
Once instantiated, a module
m can be called zero or more times like a Python
function from tensor inputs to tensor outputs:
y = m(x)
Each such call adds operations to the current TensorFlow graph to compute
x. If this involves variables with trained weights, these are
shared between all applications.
Modules can define multiple named signatures in order to allow being applied
in more than one way (similar to how Python objects have methods).
A module's documentation should describe the available
signatures. The call above applies the signature named
signature can be selected by passing its name to the optional
If a signature has multiple inputs, they must be passed as a dict, with the keys
defined by the signature. Likewise, if a signature has multiple outputs, these
can be retrieved as a dict by passing
as_dict=True, under the keys defined by
the signature (the key
"default" is for the single output returned if
as_dict=False). So the most general form of applying a Module looks like:
outputs = m(dict(apples=x1, oranges=x2), signature="fruit_to_pet", as_dict=True) y1 = outputs["cats"] y2 = outputs["dogs"]
A caller must supply all inputs defined by a signature, but there is no
requirement to use all of a module's outputs.
TensorFlow will run only those parts of the module that end up
as dependencies of a target in
tf.Session.run(). Indeed, module publishers may
choose to provide various outputs for advanced uses (like activations of
intermediate layers) along with the main outputs. Module consumers should
handle additional outputs gracefully.
Trying out alternative modules
Whenever there are multiple modules for the same task, TensorFlow Hub encourages to equip them with compatible signatures (interfaces) such that trying different ones is as easy as varying the module handle as a string-valued hyperparameter.
To this end, we maintain a collection of recommended Common Signatures for popular tasks.
Creating a New Module
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.
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
To define a new module, a publisher calls
hub.create_module_spec() with a
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.
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
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
For compatibility with TensorFlow Estimators,
modules from the latest checkpoint, just like
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.
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.
Training the variables of an imported module together with those of the model around it is called fine-tuning. Fine-tuning can result in better quality, but adds new complications. We advise consumers to look into fine-tuning only after exploring simpler quality tweaks, and only if the module publisher recommends it.
To enable fine-tuning, instantiate the module with
hub.Module(..., trainable=True) to make its variables trainable and
REGULARIZATION_LOSSES. If the module has multiple
graph variants, make sure to pick the one appropriate for training.
Usually, that's the one with tags
Choose a training regime that does not ruin the pre-trained weights, for example, a lower learning rate than for training from scratch.
To make fine-tuning easier for consumers, please be mindful of the following:
Fine-tuning needs regularization. Your module is exported with the
REGULARIZATION_LOSSEScollection, 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
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 critical hyperparameters.