官网翻译 ：基于batch的用于消费kafka消息的接口

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],
    leaders: Map[TopicAndPartition, (String, Int)],
    messageHandler: MessageAndMetadata[K, V] => R
  ) extends RDD[R](sc, Nil) with Logging with HasOffsetRanges {
  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
  }

入参 SparkContext,kafkaParams,offsetRanges,leaders,messageHandler
主要看一下如何产生partition的，即compute方法

 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) {//如果起始的offset相同的话，则跳过
      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)
    }
  }

再看下KafkaRDDIterator ,首先看下getNext方法

override def getNext(): R = {
      if (iter == null || !iter.hasNext) {//如果当前分区的iterator为空则获取下一个batch
        iter = fetchBatch
      }
      if (!iter.hasNext) {
        assert(requestOffset == part.untilOffset, errRanOutBeforeEnd(part))
        finished = true
        null.asInstanceOf[R]
      } else {
        val item = iter.next()
        if (item.offset >= part.untilOffset) {//如果获取的offset已经大于要消费的offset则返回异常
          assert(item.offset == part.untilOffset, errOvershotEnd(item.offset, part))
          finished = true
          null.asInstanceOf[R]
        } else {
          requestOffset = item.nextOffset
          messageHandler(new MessageAndMetadata(
            part.topic, part.partition, item.message, item.offset, keyDecoder, valueDecoder))
        }
      }
    }

继续看fetchBatch

 private def fetchBatch: Iterator[MessageAndOffset] = {
      val req = new FetchRequestBuilder()
        .addFetch(part.topic, part.partition, requestOffset, kc.config.fetchMessageMaxBytes)
        .build()
      val resp = consumer.fetch(req)
      handleFetchErr(resp)
      // kafka may return a batch that starts before the requested offset
      resp.messageSet(part.topic, part.partition)
        .iterator
        .dropWhile(_.offset < requestOffset)
    }

发送FetchRequestBuilder
consumer是创建parttion的时候创建的

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

    // The idea is to use the provided preferred host, except on task retry attempts,
    // to minimize number of kafka metadata requests //
    private def connectLeader: SimpleConsumer = {
      if (context.attemptNumber > 0) {
        kc.connectLeader(part.topic, part.partition).fold(
          errs => throw new SparkException(
            s"Couldn't connect to leader for topic ${part.topic} ${part.partition}: " +
              errs.mkString("\n")),
          consumer => consumer
        )
      } else {//不用获取leader直接访问对应host，能够减少对kafka metadata的请求
        kc.connect(part.host, part.port)
      }
    }