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Class SequenceQueueingStateSaver

SequenceQueueingStateSaver provides access to stateful values from input.

This class is meant to be used instead of, e.g., a Queue, for splitting variable-length sequence inputs into segments of sequences with fixed length and batching them into mini-batches. It maintains contexts and state for a sequence across the segments. It can be used in conjunction with a QueueRunner (see the example below).

The SequenceQueueingStateSaver (SQSS) accepts one example at a time via the inputs input_length, input_key, input_sequences (a dict), input_context (a dict), and initial_states (a dict). The sequences, values in input_sequences, may have variable first dimension (the padded_length), though this dimension must always be a multiple of num_unroll. All other dimensions must be fixed and accessible via get_shape calls. The length prior to padding can be recorded in input_length. The context values in input_context must all have fixed and well defined dimensions. The initial state values must all have fixed and well defined dimensions.

The SQSS splits the sequences of an input example into segments of length num_unroll. Across examples minibatches of size batch_size are formed. These minibatches contain a segment of the sequences, copy the context values, and maintain state, length, and key information of the original input examples. In the first segment of an example the state is still the initial state. It can then be updated; and updated state values are accessible in subsequent segments of the same example. After each segment batch.save_state() must be called which is done by the state_saving_rnn. Without this call, the dequeue op associated with the SQSS will not run. Internally, SQSS has a queue for the input examples. Its capacity is configurable. If set smaller than batch_size then the dequeue op will block indefinitely. A small multiple of batch_size is a good rule of thumb to prevent that queue from becoming a bottleneck and slowing down training. If set too large (and note that it defaults to unbounded) memory consumption goes up. Moreover, when iterating over the same input examples multiple times reusing the same key the capacity must be smaller than the number of examples.

The prefetcher, which reads one unrolled, variable-length input sequence at a time, is accessible via prefetch_op. The underlying Barrier object is accessible via barrier. Processed minibatches, as well as state read and write capabilities are accessible via next_batch. Specifically, next_batch provides access to all of the minibatched data, including the following, see NextQueuedSequenceBatch for details:

  • total_length, length, insertion_index, key, next_key,
  • sequence (the index each minibatch entry's time segment index),
  • sequence_count (the total time segment count for each minibatch entry),
  • context (a dict of the copied minibatched context values),
  • sequences (a dict of the split minibatched variable-length sequences),
  • state (to access the states of the current segments of these entries)
  • save_state (to save the states for the next segments of these entries)

Example usage:

batch_size = 32
num_unroll = 20
lstm_size = 8
cell = tf.compat.v1.nn.rnn_cell.BasicLSTMCell(num_units=lstm_size)
initial_state_values = tf.zeros(cell.state_size, dtype=tf.float32)

raw_data = get_single_input_from_input_reader()
length, key, sequences, context = my_parser(raw_data)
assert "input" in sequences.keys()
assert "label" in context.keys()
initial_states = {"lstm_state": initial_state_value}

stateful_reader = tf.SequenceQueueingStateSaver(
    batch_size, num_unroll,
    length=length, input_key=key, input_sequences=sequences,
    input_context=context, initial_states=initial_states,

batch = stateful_reader.next_batch
inputs = batch.sequences["input"]
context_label = batch.context["label"]

inputs_by_time = tf.split(value=inputs, num_or_size_splits=num_unroll, axis=1)
assert len(inputs_by_time) == num_unroll

lstm_output, _ = tf.contrib.rnn.static_state_saving_rnn(

# Start a prefetcher in the background
sess = tf.compat.v1.Session()
num_threads = 3
queue_runner = tf.compat.v1.train.QueueRunner(
    stateful_reader, [stateful_reader.prefetch_op] * num_threads)

while True:
  # Step through batches, perform training or inference...

Note: Usually the barrier is given to a QueueRunner as in the examples above. The QueueRunner will close the barrier if the prefetch_op receives an OutOfRange Error from upstream input queues (i.e., reaches the end of the input). If the barrier is closed no further new examples are added to the SQSS. The underlying barrier might, however, still contain further unroll-steps of examples that have not undergone all iterations. To gracefully finish all examples, the flag allow_small_batch must be set to true, which causes the SQSS to issue progressively smaller mini-batches with the remaining examples.


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Creates the SequenceQueueingStateSaver.


  • batch_size: int or int32 scalar Tensor, how large minibatches should be when accessing the state() method and context, sequences, etc, properties.
  • num_unroll: Python integer, how many time steps to unroll at a time. The input sequences of length k are then split into k / num_unroll many segments.
  • input_length: An int32 scalar Tensor, the length of the sequence prior to padding. This value may be at most padded_length for any given input (see below for the definition of padded_length). Batched and total lengths of the current iteration are made accessible via the length and total_length properties. The shape of input_length (scalar) must be fully specified.
  • input_key: A string scalar Tensor, the unique key for the given input. This is used to keep track of the split minibatch elements of this input. Batched keys of the current iteration are made accessible via the key property. The shape of input_key (scalar) must be fully specified.
  • input_sequences: A dict mapping string names to Tensor values. The values must all have matching first dimension, called padded_length. The SequenceQueueingStateSaver will split these tensors along this first dimension into minibatch elements of dimension num_unroll. Batched and segmented sequences of the current iteration are made accessible via the sequences property.

    Note: padded_length may be dynamic, and may vary from input to input, but must always be a multiple of num_unroll. The remainder of the shape (other than the first dimension) must be fully specified.

  • input_context: A dict mapping string names to Tensor values. The values are treated as "global" across all time splits of the given input, and will be copied across for all minibatch elements accordingly. Batched and copied context of the current iteration are made accessible via the context property.

    Note: All input_context values must have fully defined shapes.

  • initial_states: A dict mapping string state names to multi-dimensional values (e.g. constants or tensors). This input defines the set of states that will be kept track of during computing iterations, and which can be accessed via the state and save_state methods.

    Note: All initial_state values must have fully defined shapes.

  • capacity: The max capacity of the SQSS in number of examples. Needs to be at least batch_size. Defaults to unbounded.

  • allow_small_batch: If true, the SQSS will return smaller batches when there aren't enough input examples to fill a whole batch and the end of the input has been reached (i.e., the underlying barrier has been closed).

  • name: An op name string (optional).


  • TypeError: if any of the inputs is not an expected type.
  • ValueError: if any of the input values is inconsistent, e.g. if not enough shape information is available from inputs to build the state saver.






The NextQueuedSequenceBatch providing access to batched output data.

Also provides access to the state and save_state methods. The first time this gets called, it additionally prepares barrier reads and creates NextQueuedSequenceBatch / next_batch objects. Subsequent calls simply return the previously created next_batch.

In order to access data in next_batch without blocking, the prefetch_op must have been run at least batch_size times (ideally in a separate thread, or launched via a QueueRunner). After processing a segment in next_batch(), batch.save_state() must be called which is done by the state_saving_rnn. Without this call, the dequeue op associated with the SQSS will not run.


A cached NextQueuedSequenceBatch instance.



The op used to prefetch new data into the state saver.

Running it once enqueues one new input example into the state saver. The first time this gets called, it additionally creates the prefetch_op. Subsequent calls simply return the previously created prefetch_op.

It should be run in a separate thread via e.g. a QueueRunner.


An Operation that performs prefetching.



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Closes the barrier and the FIFOQueue.

This operation signals that no more segments of new sequences will be enqueued. New segments of already inserted sequences may still be enqueued and dequeued if there is a sufficient number filling a batch or allow_small_batch is true. Otherwise dequeue operations will fail immediately.


  • cancel_pending_enqueues: (Optional.) A boolean, defaulting to False. If True, all pending enqueues to the underlying queues will be cancelled, and completing already started sequences is not possible.
  • name: Optional name for the op.


The operation that closes the barrier and the FIFOQueue.