# Exporting and Importing a MetaGraph

A [`MetaGraph`](https://www.tensorflow.org/code/tensorflow/core/protobuf/meta_graph.proto) contains both a TensorFlow GraphDef
as well as associated metadata necessary for running computation in a
graph when crossing a process boundary.  It can also be used for long
term storage of graphs.  The MetaGraph contains the information required
to continue training, perform evaluation, or run inference on a previously trained graph.

The APIs for exporting and importing the complete model are in
the @{tf.train.Saver} class:
@{tf.train.export_meta_graph}
and
@{tf.train.import_meta_graph}.

## What's in a MetaGraph

The information contained in a MetaGraph is expressed as a
[`MetaGraphDef`](https://www.tensorflow.org/code/tensorflow/core/protobuf/meta_graph.proto)
protocol buffer. It contains the following fields:

* [`MetaInfoDef`](https://www.tensorflow.org/code/tensorflow/core/protobuf/meta_graph.proto) for meta information, such as version and other user information.
* [`GraphDef`](https://www.tensorflow.org/code/tensorflow/core/framework/graph.proto) for describing the graph.
* [`SaverDef`](https://www.tensorflow.org/code/tensorflow/core/protobuf/saver.proto) for the saver.
* [`CollectionDef`](https://www.tensorflow.org/code/tensorflow/core/protobuf/meta_graph.proto)
map that further describes additional components of the model such as
@{$python/state_ops$`Variables`},
@{tf.train.QueueRunner}, etc.

In order for a Python object to be serialized
to and from `MetaGraphDef`, the Python class must implement `to_proto()` and
`from_proto()` methods, and register them with the system using
`register_proto_function`. For example:

  ```Python
  def to_proto(self, export_scope=None):

    """Converts a `Variable` to a `VariableDef` protocol buffer.

    Args:
      export_scope: Optional `string`. Name scope to remove.

    Returns:
      A `VariableDef` protocol buffer, or `None` if the `Variable` is not
      in the specified name scope.
    """
    if (export_scope is None or
        self._variable.name.startswith(export_scope)):
      var_def = variable_pb2.VariableDef()
      var_def.variable_name = ops.strip_name_scope(
          self._variable.name, export_scope)
      var_def.initializer_name = ops.strip_name_scope(
          self.initializer.name, export_scope)
      var_def.snapshot_name = ops.strip_name_scope(
          self._snapshot.name, export_scope)
      if self._save_slice_info:
        var_def.save_slice_info_def.MergeFrom(self._save_slice_info.to_proto(
            export_scope=export_scope))
      return var_def
    else:
      return None

  @staticmethod
  def from_proto(variable_def, import_scope=None):
    """Returns a `Variable` object created from `variable_def`."""
    return Variable(variable_def=variable_def, import_scope=import_scope)

  ops.register_proto_function(ops.GraphKeys.GLOBAL_VARIABLES,
                              proto_type=variable_pb2.VariableDef,
                              to_proto=Variable.to_proto,
                              from_proto=Variable.from_proto)
  ```

## Exporting a Complete Model to MetaGraph

The API for exporting a running model as a MetaGraph is `export_meta_graph()`.

  ```Python
  def export_meta_graph(filename=None, collection_list=None, as_text=False):
    """Writes `MetaGraphDef` to save_path/filename.

    Args:
      filename: Optional meta_graph filename including the path.
      collection_list: List of string keys to collect.
      as_text: If `True`, writes the meta_graph as an ASCII proto.

    Returns:
      A `MetaGraphDef` proto.
    """
  ```

  A `collection` can contain any Python objects that users would like to
  be able to uniquely identify and easily retrieve. These objects can be
  special operations in the graph, such as `train_op`, or hyper parameters,
  such as "learning rate".  Users can specify the list of collections
  they would like to export.  If no `collection_list` is specified,
  all collections in the model will be exported.

  The API returns a serialized protocol buffer. If `filename` is
  specified, the protocol buffer will also be written to a file.

  Here are some of the typical usage models:

  * Export the default running graph:

  ```Python
  # Build the model
  ...
  with tf.Session() as sess:
    # Use the model
    ...
  # Export the model to /tmp/my-model.meta.
  meta_graph_def = tf.train.export_meta_graph(filename='/tmp/my-model.meta')
  ```

  * Export the default running graph and only a subset of the collections.

  ```Python
  meta_graph_def = tf.train.export_meta_graph(
      filename='/tmp/my-model.meta',
      collection_list=["input_tensor", "output_tensor"])
  ```


The MetaGraph is also automatically exported via the `save()` API in
@{tf.train.Saver}.


## Import a MetaGraph

The API for importing a MetaGraph file into a graph is `import_meta_graph()`.

Here are some of the typical usage models:

* Import and continue training without building the model from scratch.

  ```Python
  ...
  # Create a saver.
  saver = tf.train.Saver(...variables...)
  # Remember the training_op we want to run by adding it to a collection.
  tf.add_to_collection('train_op', train_op)
  sess = tf.Session()
  for step in xrange(1000000):
      sess.run(train_op)
      if step % 1000 == 0:
          # Saves checkpoint, which by default also exports a meta_graph
          # named 'my-model-global_step.meta'.
          saver.save(sess, 'my-model', global_step=step)
  ```

  Later we can continue training from this saved `meta_graph` without building
  the model from scratch.

  ```Python
  with tf.Session() as sess:
    new_saver = tf.train.import_meta_graph('my-save-dir/my-model-10000.meta')
    new_saver.restore(sess, 'my-save-dir/my-model-10000')
    # tf.get_collection() returns a list. In this example we only want the
    # first one.
    train_op = tf.get_collection('train_op')[0]
    for step in xrange(1000000):
      sess.run(train_op)
  ```

* Import and extend the graph.

  For example, we can first build an inference graph, export it as a meta graph:

  ```Python
  # Creates an inference graph.
  # Hidden 1
  images = tf.constant(1.2, tf.float32, shape=[100, 28])
  with tf.name_scope("hidden1"):
    weights = tf.Variable(
        tf.truncated_normal([28, 128],
                            stddev=1.0 / math.sqrt(float(28))),
        name="weights")
    biases = tf.Variable(tf.zeros([128]),
                         name="biases")
    hidden1 = tf.nn.relu(tf.matmul(images, weights) + biases)
  # Hidden 2
  with tf.name_scope("hidden2"):
    weights = tf.Variable(
        tf.truncated_normal([128, 32],
                            stddev=1.0 / math.sqrt(float(128))),
        name="weights")
    biases = tf.Variable(tf.zeros([32]),
                         name="biases")
    hidden2 = tf.nn.relu(tf.matmul(hidden1, weights) + biases)
  # Linear
  with tf.name_scope("softmax_linear"):
    weights = tf.Variable(
        tf.truncated_normal([32, 10],
                            stddev=1.0 / math.sqrt(float(32))),
        name="weights")
    biases = tf.Variable(tf.zeros([10]),
                         name="biases")
    logits = tf.matmul(hidden2, weights) + biases
    tf.add_to_collection("logits", logits)

  init_all_op = tf.global_variables_initializer()

  with tf.Session() as sess:
    # Initializes all the variables.
    sess.run(init_all_op)
    # Runs to logit.
    sess.run(logits)
    # Creates a saver.
    saver0 = tf.train.Saver()
    saver0.save(sess, 'my-save-dir/my-model-10000')
    # Generates MetaGraphDef.
    saver0.export_meta_graph('my-save-dir/my-model-10000.meta')
  ```

  Then later import it and extend it to a training graph.

  ```Python
  with tf.Session() as sess:
    new_saver = tf.train.import_meta_graph('my-save-dir/my-model-10000.meta')
    new_saver.restore(sess, 'my-save-dir/my-model-10000')
    # Addes loss and train.
    labels = tf.constant(0, tf.int32, shape=[100], name="labels")
    batch_size = tf.size(labels)
    logits = tf.get_collection("logits")[0]
    loss = tf.losses.sparse_softmax_cross_entropy(labels=labels,
                                                  logits=logits)

    tf.summary.scalar('loss', loss)
    # Creates the gradient descent optimizer with the given learning rate.
    optimizer = tf.train.GradientDescentOptimizer(0.01)

    # Runs train_op.
    train_op = optimizer.minimize(loss)
    sess.run(train_op)
  ```

* Import a graph with preset devices.

  Sometimes an exported meta graph is from a training environment that the
  importer doesn't have. For example, the model might have been trained
  on GPUs, or in a distributed environment with replicas. When importing
  such models, it's useful to be able to clear the device settings in
  the graph so that we can run it on locally available devices. This can
  be achieved by calling `import_meta_graph` with the `clear_devices`
  option set to `True`.

  ```Python
  with tf.Session() as sess:
    new_saver = tf.train.import_meta_graph('my-save-dir/my-model-10000.meta',
        clear_devices=True)
    new_saver.restore(sess, 'my-save-dir/my-model-10000')
    ...
  ```

* Import within the default graph.

  Sometimes you might want to run `export_meta_graph` and `import_meta_graph`
  in codelab using the default graph. In that case, you need to reset
  the default graph by calling `tf.reset_default_graph()` first before
  running import.

  ```Python
  meta_graph_def = tf.train.export_meta_graph()
  ...
  tf.reset_default_graph()
  ...
  tf.train.import_meta_graph(meta_graph_def)
  ...
  ```

* Retrieve Hyper Parameters

  ```Python
  filename = ".".join([tf.train.latest_checkpoint(train_dir), "meta"])
  tf.train.import_meta_graph(filename)
  hparams = tf.get_collection("hparams")
  ```
