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1、Kafka解密
背景：
目前No Receivers在企业中使用的越来越多，No Receivers具有更强的控制度，语义一致性。No Receivers是我们操作数据来源自然方式，操作数据来源使用一个封装器，且是RDD类型的。
所以Spark Streaming就产生了自定义RDD –> KafkaRDD.
源码分析：
1、KafkaRDD源码
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], //指定数据范围
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
}2、HasOffsetRanges
/**
* Represents any object that has a collection of [[OffsetRange]]s. This can be used to access the
* offset ranges in RDDs generated by the direct Kafka DStream (see
* [[KafkaUtils.createDirectStream()]]).
* {{{
* KafkaUtils.createDirectStream(...).foreachRDD { rdd =>
* val offsetRanges = rdd.asInstanceOf[HasOffsetRanges].offsetRanges
* ...
* }
* }}}
*/
trait HasOffsetRanges {
def offsetRanges: Array[OffsetRange]
}3、KafkaRDD中的compute
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)
}
}SparkStreaming一般使用KafkaUtils的createDirectStream读取数据
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)
val kc = new KafkaCluster(kafkaParams)
val fromOffsets = getFromOffsets(kc, kafkaParams, topics)
new DirectKafkaInputDStream[K, V, KD, VD, (K, V)](
ssc, kafkaParams, fromOffsets, messageHandler)
}4、通过getFromOffsets的方法获取topic的fromOffset值
[kafka] (
kc: KafkaClusterkafkaParams: []topics: []
): [TopicAndPartition] = {
reset = kafkaParams.get().map(_.toLowerCase)
result = {
topicPartitions <- kc.getPartitions(topics).right
leaderOffsets <- ((reset == ()) {
kc.getEarliestLeaderOffsets(topicPartitions)
} {
kc.getLatestLeaderOffsets(topicPartitions)
}).right
} {
leaderOffsets.map { (tplo) =>
(tplo.offset)
}
}
KafkaCluster.(result)
}createDirectStream其实生成的是DirectKafkaInputDStream对象，通过compute方法会产生KafkaRDD
(validTime: Time): Option[KafkaRDD[]] = {
untilOffsets = clamp(latestLeaderOffsets())
rdd = [](
context.sparkContextkafkaParamsuntilOffsetsmessageHandler)
offsetRanges = .map { (tpfo) =>
uo = untilOffsets(tp)
(tp.topictp.partitionfouo.offset)
}
description = offsetRanges.filter { offsetRange =>
offsetRange.fromOffset != offsetRange.untilOffset
}.map { offsetRange =>
{offsetRange.topic}{offsetRange.partition}+
{offsetRange.fromOffset}{offsetRange.untilOffset}}.mkString()
metadata = (
-> offsetRanges.toListStreamInputInfo.-> description)
inputInfo = (rdd.countmetadata)
ssc...reportInfo(validTimeinputInfo)
= untilOffsets.map(kv => kv._1 -> kv._2.offset)
(rdd)
}采用Direct的好处？
1. Direct方式没有数据缓存，因此不会出现内存溢出，但是如果采用Receiver的话就需要缓存。
2. 如果采用Receiver的方式，不方便做分布式，而Direct方式默认数据就在多台机器上。
3. 在实际操作的时候如果采用Receiver的方式的弊端是假设数据来不及处理，但是Direct就不会，因为是直接读取数据。
4. 语义一致性，Direct的方式数据一定会被执行。