15、Spark Streaming源码解读之No Receivers彻底思考

在前几期文章里讲了带Receiver的Spark Streaming 应用的相关源码解读，但是现在开发Spark Streaming的应用越来越多的采用No Receivers（Direct Approach）的方式，No Receiver的方式的优势： 
1. 更强的控制自由度 
2. 语义一致性 

其实No Receivers的方式更符合我们读取数据，操作数据的思路的。因为Spark 本身是一个计算框架，他底层会有数据来源，如果没有Receivers，我们直接操作数据来源，这其实是一种更自然的方式。 如果要操作数据来源，肯定要有一个封装器，这个封装器一定是RDD类型。 以直接访问Kafka中的数据为例，看一下源码中直接读写Kafka中数据的例子代码：

object DirectKafkaWordCount {
  def main(args: Array[String]) {
    if (args.length < 2) {
      System.err.println(s"""
        |Usage: DirectKafkaWordCount <brokers> <topics>
        |  <brokers> is a list of one or more Kafka brokers
        |  <topics> is a list of one or more kafka topics to consume from
        |
        """.stripMargin)
      System.exit(1)
    }

    StreamingExamples.setStreamingLogLevels()

    val Array(brokers, topics) = args

    // Create context with 2 second batch interval
    val sparkConf = new SparkConf().setAppName("DirectKafkaWordCount")
    val ssc = new StreamingContext(sparkConf, Seconds(2))

    // Create direct kafka stream with brokers and topics
    val topicsSet = topics.split(",").toSet
    val kafkaParams = Map[String, String]("metadata.broker.list" -> brokers)
    val messages = KafkaUtils.createDirectStream[String, String, StringDecoder, StringDecoder](
      ssc, kafkaParams, topicsSet)

    // Get the lines, split them into words, count the words and print
    val lines = messages.map(_._2)
    val words = lines.flatMap(_.split(" "))
    val wordCounts = words.map(x => (x, 1L)).reduceByKey(_ + _)
    wordCounts.print()

    // Start the computation
    ssc.start()
    ssc.awaitTermination()
  }
}

Spark streaming 会将数据源封装成一个RDD，也就是KafkaRDD：

/**
 * A batch-oriented interface for consuming from Kafka.
 * Starting and ending offsets are specified in advance,
 * so that you can control exactly-once semantics.
 * @param kafkaParams Kafka <a href="http://kafka.apache.org/documentation.html#configuration">
 * configuration parameters</a>. Requires "metadata.broker.list" or "bootstrap.servers" to be set
 * with Kafka broker(s) specified in host1:port1,host2:port2 form.
 * @param offsetRanges offset ranges that define the Kafka data belonging to this RDD
 * @param messageHandler function for translating each message into the desired type
 */
private[kafka]
class KafkaRDD[
  K: ClassTag,
  V: ClassTag,
  U <: Decoder[_]: ClassTag,
  T <: Decoder[_]: ClassTag,
  R: ClassTag] private[spark] (
    sc: SparkContext,
    kafkaParams: Map[String, String],
    val offsetRanges: Array[OffsetRange], //该RDD的数据偏移量
    leaders: Map[TopicAndPartition, (String, Int)],
    messageHandler: MessageAndMetadata[K, V] => R
  ) extends RDD[R](sc, Nil) with Logging with HasOffsetRanges

可以看到KafkaRDD 混入了HasOffsetRanges，它是一个trait：

trait HasOffsetRanges {
  def offsetRanges: Array[OffsetRange]
}

其中OffsetRange,标识了RDD的数据的主题、分区、开始偏移量和结束偏移量：

inal class OffsetRange private(
    val topic: String,
    val partition: Int,
    val fromOffset: Long,
    val untilOffset: Long) extends Serializable

回到KafkaRDD，看一下KafkaRDD的getPartitions方法：

  override def getPartitions: Array[Partition] = {
    offsetRanges.zipWithIndex.map { case (o, i) =>
        val (host, port) = leaders(TopicAndPartition(o.topic, o.partition))
        new KafkaRDDPartition(i, o.topic, o.partition, o.fromOffset, o.untilOffset, host, port)
    }.toArray
  }

返回KafkaRDDPartition：

private[kafka]
class KafkaRDDPartition(
  val index: Int,
  val topic: String,
  val partition: Int,
  val fromOffset: Long,
  val untilOffset: Long,
  val host: String,
  val port: Int
) extends Partition {
  /** Number of messages this partition refers to */
  def count(): Long = untilOffset - fromOffset
}

KafkaRDDPartition清晰的描述了数据的具体位置，每个KafkaRDDPartition分区的数据交给KafkaRDD的compute方法计算：

  override def compute(thePart: Partition, context: TaskContext): Iterator[R] = {
    val part = thePart.asInstanceOf[KafkaRDDPartition]
    assert(part.fromOffset <= part.untilOffset, errBeginAfterEnd(part))
    if (part.fromOffset == part.untilOffset) {
      log.info(s"Beginning offset ${part.fromOffset} is the same as ending offset " +
        s"skipping ${part.topic} ${part.partition}")
      Iterator.empty
    } else {
      new KafkaRDDIterator(part, context)
    }
  }

KafkaRDD的compute方法返回了KafkaIterator对象：

 private class KafkaRDDIterator(
      part: KafkaRDDPartition,
      context: TaskContext) extends NextIterator[R] {

    context.addTaskCompletionListener{ context => closeIfNeeded() }

    log.info(s"Computing topic ${part.topic}, partition ${part.partition} " +
      s"offsets ${part.fromOffset} -> ${part.untilOffset}")

    val kc = new KafkaCluster(kafkaParams)
    val keyDecoder = classTag[U].runtimeClass.getConstructor(classOf[VerifiableProperties])
      .newInstance(kc.config.props)
      .asInstanceOf[Decoder[K]]
    val valueDecoder = classTag[T].runtimeClass.getConstructor(classOf[VerifiableProperties])
      .newInstance(kc.config.props)
      .asInstanceOf[Decoder[V]]
    val consumer = connectLeader
    var requestOffset = part.fromOffset
    var iter: Iterator[MessageAndOffset] = null
//..................
}

KafkaIterator中创建了一个KakfkaCluster对象用于与Kafka集群进行交互，获取数据。

回到开头的例子，我们使用 KafkaUtils.createDirectStream 创建了InputDStream：

    val messages = KafkaUtils.createDirectStream[String, String, StringDecoder, StringDecoder](
      ssc, kafkaParams, topicsSet)

看一下createDirectStream源码：

  def createDirectStream[
    K: ClassTag,
    V: ClassTag,
    KD <: Decoder[K]: ClassTag,
    VD <: Decoder[V]: ClassTag] (
      ssc: StreamingContext,
      kafkaParams: Map[String, String],
      topics: Set[String]
  ): InputDStream[(K, V)] = {
    val messageHandler = (mmd: MessageAndMetadata[K, V]) => (mmd.key, mmd.message)
//创建KakfaCluster对象
    val kc = new KafkaCluster(kafkaParams)
//更具kc的信息获取数据偏移量
    val fromOffsets = getFromOffsets(kc, kafkaParams, topics)
    new DirectKafkaInputDStream[K, V, KD, VD, (K, V)](
      ssc, kafkaParams, fromOffsets, messageHandler)
  }

首先通过KafkaCluster从Kafka集群获取信息，创建DirectKafkaInputDStream对象返回

DirectKafkaInputDStream的compute方法源码：

 override def compute(validTime: Time): Option[KafkaRDD[K, V, U, T, R]] = {
    //计算最近的数据终止偏移量
    val untilOffsets = clamp(latestLeaderOffsets(maxRetries))
//利用数据的偏移量创建KafkaRDD
    val rdd = KafkaRDD[K, V, U, T, R](
      context.sparkContext, kafkaParams, currentOffsets, untilOffsets, messageHandler)

    // Report the record number and metadata of this batch interval to InputInfoTracker.
    val offsetRanges = currentOffsets.map { case (tp, fo) =>
      val uo = untilOffsets(tp)
      OffsetRange(tp.topic, tp.partition, fo, uo.offset)
    }
    val description = offsetRanges.filter { offsetRange =>
      // Don't display empty ranges.
      offsetRange.fromOffset != offsetRange.untilOffset
    }.map { offsetRange =>
      s"topic: ${offsetRange.topic}\tpartition: ${offsetRange.partition}\t" +
        s"offsets: ${offsetRange.fromOffset} to ${offsetRange.untilOffset}"
    }.mkString("\n")
    // Copy offsetRanges to immutable.List to prevent from being modified by the user
    val metadata = Map(
      "offsets" -> offsetRanges.toList,
      StreamInputInfo.METADATA_KEY_DESCRIPTION -> description)
    val inputInfo = StreamInputInfo(id, rdd.count, metadata)
    ssc.scheduler.inputInfoTracker.reportInfo(validTime, inputInfo)

    currentOffsets = untilOffsets.map(kv => kv._1 -> kv._2.offset)
    Some(rdd)
  }

可以看到DirectKafkaInputDStream的compute方法中，首先从Kafka集群获取数据的偏移量，然后利用获取偏移量创建RDD，这个Receiver的RDD创建方式不同

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