State interface for reducing state. Elements can be added to the state, they will be
combined using a reduce function. The current state can be inspected.
The state is accessed and modified by user functions, and checkpointed consistently by the
system as part of the distributed snapshots.
The state is only accessible by functions applied on a KeyedStream. The key is automatically
supplied by the system, so the function always sees the value mapped to the key of the current
element. That way, the system can handle stream and state partitioning consistently together.
Adding the given value to the tail of this list state.
Removes the value mapped under the current key.
Returns the elements under the current key.