Tensorflow : Graph is finalized and cannot be modified

Question

I am trying to save variables through checkpoints to introduce fault tolerance to my program. I am trying to achieve this by using the MonitoredTrainingSession function. The following is my configuration:-

import tensorflow as tf

global_step = tf.Variable(10, trainable=False, name='global_step')
x = tf.constant(2)

with tf.device("/job:local/task:0"):
    y1 = tf.Variable(x + 300)

with tf.device("/job:local/task:1"):
    y2 = tf.Variable(x**2)

with tf.device("/job:local/task:2"):
    y3 = tf.Variable(5*x)

with tf.device("/job:local/task:3"):
    y0 = tf.Variable(x - 66)
    y = y0 + y1 + y2 + y3

model = tf.global_variables_initializer()
saver = tf.train.Saver(sharded=True)

chief = tf.train.ChiefSessionCreator(scaffold=None, master='grpc://localhost:2222', config=None, checkpoint_dir='/home/tensorflow/codes/checkpoints')
summary_hook = tf.train.SummarySaverHook(save_steps=None, save_secs=10, output_dir='/home/tensorflow/codes/savepoints', summary_writer=None, scaffold=None, summary_op=tf.summary.tensor_summary(name="y", tensor=y))
saver_hook = tf.train.CheckpointSaverHook(checkpoint_dir='/home/tensorflow/codes/checkpoints', save_secs=None, save_steps=True, saver=saver, checkpoint_basename='model.ckpt', scaffold=None)

# with tf.train.MonitoredSession(session_creator=ChiefSessionCreator,hooks=[saver_hook, summary_hook]) as sess:

with tf.train.MonitoredTrainingSession(master='grpc://localhost:2222', is_chief=True, checkpoint_dir='/home/tensorflow/codes/checkpoints',
    scaffold=None, hooks=[saver_hook,summary_hook], chief_only_hooks=None, save_checkpoint_secs=None, save_summaries_steps=True, config=None) as sess:

    while not sess.should_stop():
        sess.run(tf.global_variables_initializer())

    while not sess.should_stop():
        result = sess.run(y)
        print(result)

I get the following RuntimeError which I am unable to resolve:-

Traceback (most recent call last):
  File "add_1.py", line 39, in <module>
    sess.run(tf.global_variables_initializer())
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/ops/variables.py", line 1187, in global_variables_initializer
    return variables_initializer(global_variables())
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/ops/variables.py", line 1169, in variables_initializer
    return control_flow_ops.group(*[v.initializer for v in var_list], name=name)
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/ops/control_flow_ops.py", line 2773, in group
    deps.append(_GroupControlDeps(dev, ops_on_device[dev]))
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/ops/control_flow_ops.py", line 2721, in _GroupControlDeps
    return no_op(name=name)
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/ops/gen_control_flow_ops.py", line 186, in no_op
    result = _op_def_lib.apply_op("NoOp", name=name)
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/framework/op_def_library.py", line 759, in apply_op
    op_def=op_def)
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/framework/ops.py", line 2199, in create_op
    self._check_not_finalized()
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/framework/ops.py", line 1925, in _check_not_finalized
    raise RuntimeError("Graph is finalized and cannot be modified.")
RuntimeError: Graph is finalized and cannot be modified.

Answer 1

This may not be recommended for your use case, but it is possible to unfinalize a Graph:

sess.graph._unsafe_unfinalize()

Answer 2

Since your aim is to use MonitoredTrainingSession to get you checkpointing, the usage is much simpler than your example:

import tensorflow as tf

global_step = tf.contrib.framework.get_or_create_global_step()
x = tf.constant(2)
y1 = x + 300
y2 = x**2
y3 = x * 5
y0 = x - 66
y = y0 + y1 + y2 + y3
step = tf.assign_add(global_step, 1)

with tf.train.MonitoredTrainingSession(checkpoint_dir='/tmp/checkpoints') as sess:
    while not sess.should_stop():
        result, i = sess.run([y, step])
        print(result, i)

• The hooks for saving/restoring checkpoints are created by MonitoredTrainingSession for you.
• If you pass in save_checkpoint_secs you can change the frequency of checkpointing from the 10 minute default. I find a higher frequency isn't worth it: saving checkpoints isn't free, so very frequent checkpointing will end up slowing training down.
• The ChiefSessionCreator and gRPC config is only needed for distributed running (see here for a description of the concepts. Similarly with assigning ops to specific devices - make sure you really need to do this before using it as it can slow things down if you're not careful.
• You don't need to wrap the result of operations on tensors with tf.Variable() - they already are variables.
• You can pass save_summaries_steps for monitoring training with tensorboard, but by default that'll happen every 100 steps anyway.

Answer 3

If you want to initialize the graph on loop, you can use the function to create new graph on top of loop.

import tensorflow as tf

tf.reset_default_graph()
tf.Graph().as_default()

Answer 4

The root cause for your error seems to be that MonitoredTrainingSession has finalized (frozen) the graph and your tf.global_variable_initializer() is no longer able to modify it.

Having said that, there are multiple things that require attention:

1) Why do you try to repeatedly initialize all variables here?

while not sess.should_stop():
    sess.run(tf.global_variables_initializer())

2) It seems some of your code is already included in MonitoredTrainingSession, e.g. ChiefSessionCreator. Can you please take another look at the code (https://github.com/tensorflow/tensorflow/blob/master/tensorflow/python/training/monitored_session.py#L243) or search for its sample usage and see how MonitoredTrainingSession is supposed to be used?