tensorflow Dataset API diff between make_initializable_iterator and make_one_shot_iterator

Question

I want to know the difference between make_initializable_iterator and make_one_shot_iterator.
1. Tensorflow documentations said that A "one-shot" iterator does not currently support re-initialization. What exactly does that mean?
2. Are the following 2 snippets equivalent?
Use make_initializable_iterator

iterator = data_ds.make_initializable_iterator()
data_iter = iterator.get_next()
sess = tf.Session()
sess.run(tf.global_variables_initializer())
for e in range(1, epoch+1):
    sess.run(iterator.initializer)
    while True:
        try:
            x_train, y_train = sess.run([data_iter])
            _, cost = sess.run([train_op, loss_op], feed_dict={X: x_train,
                                                               Y: y_train})
        except tf.errors.OutOfRangeError:   
            break
sess.close()

Use make_one_shot_iterator

iterator = data_ds.make_one_shot_iterator()
data_iter = iterator.get_next()
sess = tf.Session()
sess.run(tf.global_variables_initializer())
for e in range(1, epoch+1):
    while True:
        try:
            x_train, y_train = sess.run([data_iter])
            _, cost = sess.run([train_op, loss_op], feed_dict={X: x_train,
                                                               Y: y_train})
        except tf.errors.OutOfRangeError:   
            break
sess.close()

Answer 1

Suppose you want to use the same code to do your training and validation. You might like to use the same iterator, but initialized to point to different datasets; something like the following:

def _make_batch_iterator(filenames):
    dataset = tf.data.TFRecordDataset(filenames)
    ...
    return dataset.make_initializable_iterator()


filenames = tf.placeholder(tf.string, shape=[None])
iterator = _make_batch_iterator(filenames)

with tf.Session() as sess:
    for epoch in range(num_epochs):

        # Initialize iterator with training data
        sess.run(iterator.initializer,
                 feed_dict={filenames: ['training.tfrecord']})

        _train_model(...)

        # Re-initialize iterator with validation data
        sess.run(iterator.initializer,
                 feed_dict={filenames: ['validation.tfrecord']})

        _validate_model(...)

With a one-shot iterator, you can't re-initialize it like this.