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State in the Scala DataStream API


I'm looking at the following page of the documentation

particularly at this piece of code:

val stream: DataStream[(String, Int)] = ...

val counts: DataStream[(String, Int)] = stream
  .mapWithState((in: (String, Int), count: Option[Int]) =>
    count match {
      case Some(c) => ( (in._1, c), Some(c + in._2) )
      case None => ( (in._1, 0), Some(in._2) )

How is the state clear/purge in this case for keys that no longer appear?

Thank you,