PairDStreamFunctions.mapWithState

與 updateState 類似，mapWithState 可用於根據即將到來的資料建立有狀態 DStream。它需要 StateSpec：

import org.apache.spark.streaming._

object StatefulStats {
  val state = StateSpec.function(
    (key: String, current: Option[Double], state: State[StatCounter]) => {
      (current, state.getOption) match {
        case (Some(x), Some(cnt)) => state.update(cnt.merge(x))
        case (Some(x), None) => state.update(StatCounter(x))
        case (None, None) => state.update(StatCounter())
        case _ =>
      }
      
      (key, state.get)
    }
  )
}

它取得關鍵 key，當前 value 和累積 State 並返回新狀態。把這一切放在一起：

import org.apache.spark._
import org.apache.spark.streaming.dstream.DStream
import scala.collection.mutable.Queue
import org.apache.spark.util.StatCounter

object MapStateByKeyApp {
  def main(args: Array[String]) {
    val sc = new SparkContext("local", "mapWithState", new SparkConf())

    val ssc = new StreamingContext(sc, Seconds(10))
    ssc.checkpoint("/tmp/chk")

    val queue = Queue(
      sc.parallelize(Seq(("foo", 5.0), ("bar", 1.0))),
      sc.parallelize(Seq(("foo", 1.0), ("foo", 99.0))),
      sc.parallelize(Seq(("bar", 22.0), ("foo", 1.0))),
      sc.emptyRDD[(String, Double)],
      sc.parallelize(Seq(("foo", 1.0), ("bar", 1.0)))
    )

    val inputStream: DStream[(String, Double)] = ssc.queueStream(queue)

    inputStream.mapWithState(StatefulStats.state).print()

    ssc.start()
    ssc.awaitTermination()
    ssc.stop()
  }
}

最後預期輸出：

(bar,(count: 1, mean: 1.000000, stdev: 0.000000, max: 1.000000, min: 1.000000))
Time: 1469923290000 ms(foo,(count: 3, mean: 35.000000, stdev: 45.284287, max: 99.000000, min: 1.000000))
(foo,(count: 3, mean: 35.000000, stdev: 45.284287, max: 99.000000, min: 1.000000))
Time: 1469923300000 ms(bar,(count: 2, mean: 11.500000, stdev: 10.500000, max: 22.000000, min: 1.000000))
(foo,(count: 4, mean: 26.500000, stdev: 41.889736, max: 99.000000, min: 1.000000))
Time: 1469923310000 msTime: 1469923320000 ms(foo,(count: 5, mean: 21.400000, stdev: 38.830916, max: 99.000000, min: 1.000000))
(bar,(count: 3, mean: 8.000000, stdev: 9.899495, max: 22.000000, min: 1.000000))