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A class bounding the processing state Position during queries. This can be used to specify that a query should fail if the locally available partition isn't caught up to the specified bound. "Unbounded" places no restrictions on the current location of the partition.

EDIT: The KIP initially proposed to have a "latest" bound, which would require the query to run on a RUNNING Active task, but that has been moved to StateQueryRequest#requireActive. This more cleanly separates responsibilities, since StateStores are responsible for enforcing the PositionBound and "latest" indicates that only a running active replica should reply to the queryKafka Streams is responsible for handling the other details of StateQueryRequest. The problem was that the store actually doesn't have visibility into the state or type of task that contains it, so it was unable to take responsibility for the "latest" bound. It's important in particular to keep the StateStore's responsibilities clear because that is an extension point for Kafka Streams users who implement their own stores.

@Evolving
public interface PositionBound {

  // Create a new Position bound representing "no bound"
  static PositionBound unbounded();

  // Create a new, empty Position
  static PositionBound at(Position position);

  // Check whether this is an "unbounded" Position bound
  boolean isUnbounded();

  // Get the Position (if it's not unbounded or latest)
  Position position();
}

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