本节内容基于官方文档：
Spark Stream 缓存
Checkpoint
案例
1. Spark Stream 缓存
通过前面一系列的课程介绍，我们知道DStream是由一系列的RDD构成的，它同一般的RDD一样，也可以将流式数据持久化到内容当中，采用的同样是persisit方法，调用该方法后DStream将持久化所有的RDD数据。这对于一些需要重复计算多次或数据需要反复被使用的DStream特别有效。像reduceByWindow、reduceByKeyAndWindow等基于窗口操作的方法，它们默认都是有persisit操作的。reduceByKeyAndWindow方法源码具体如下：
def reduceByKeyAndWindow(reduceFunc: (V, V) => V,invReduceFunc: (V, V) => V,windowDuration: Duration,slideDuration: Duration,partitioner: Partitioner,filterFunc: ((K, V)) => Boolean): DStream[(K, V)] = ssc.withScope {val cleanedReduceFunc = ssc.sc.clean(reduceFunc)val cleanedInvReduceFunc = ssc.sc.clean(invReduceFunc)val cleanedFilterFunc = if (filterFunc != null) Some(ssc.sc.clean(filterFunc)) else Nonenew ReducedWindowedDStream[K, V](self, cleanedReduceFunc, cleanedInvReduceFunc, cleanedFilterFunc,windowDuration, slideDuration, partitioner) }
从上面的方法来看，它最返回的是一个ReducedWindowedDStream对象，跳到该类的源码中可以看到在其主构造函数中包含下面两段代码：
private[streaming]class ReducedWindowedDStream[K: ClassTag, V: ClassTag](parent: DStream[(K, V)],reduceFunc: (V, V) => V,invReduceFunc: (V, V) => V,filterFunc: Option[((K, V)) => Boolean],_windowDuration: Duration,_slideDuration: Duration,partitioner: Partitioner ) extends DStream[(K, V)](parent.ssc) { //省略其它非关键代码 //默认被缓存到内存当中 // Persist RDDs to memory by default as these RDDs are going to be reused. super.persist(StorageLevel.MEMORY_ONLY_SER) reducedStream.persist(StorageLevel.MEMORY_ONLY_SER)}
通过上面的代码我们可以看到，通过窗口操作产生的DStream不需要开发人员手动去调用persist方法，Spark会自动帮我们将数据缓存当内存当中。同一般的RDD类似，DStream支持的persisit级别为：
2. Checkpoint机制
通过前期对Spark Streaming的理解，我们知道，Spark Streaming应用程序如果不手动停止，则将一直运行下去，在实际中应用程序一般是24小时*7天不间断运行的，因此Streaming必须对诸如系统错误、JVM出错等与程序逻辑无关的错误（failures ）具体很强的弹性，具备一定的非应用程序出错的容错性。Spark Streaming的Checkpoint机制便是为此设计的，它将足够多的信息checkpoint到某些具备容错性的存储系统如HDFS上，以便出错时能够迅速恢复。有两种数据可以chekpoint：
（1）Metadata checkpointing 将流式计算的信息保存到具备容错性的存储上如HDFS，Metadata Checkpointing适用于当streaming应用程序Driver所在的节点出错时能够恢复，元数据包括： Configuration（配置信息） – 创建streaming应用程序的配置信息 DStream operations – 在streaming应用程序中定义的DStreaming操作 Incomplete batches – 在列队中没有处理完的作业
（2）Data checkpointing 将生成的RDD保存到外部可靠的存储当中，对于一些数据跨度为多个bactch的有状态tranformation操作来说，checkpoint非常有必要，因为在这些transformation操作生成的RDD对前一RDD有依赖，随着时间的增加，依赖链可能会非常长，checkpoint机制能够切断依赖链，将中间的RDD周期性地checkpoint到可靠存储当中，从而在出错时可以直接从checkpoint点恢复。
具体来说，metadata checkpointing主要还是从drvier失败中恢复，而Data Checkpoing用于对有状态的transformation操作进行checkpointing
Checkpointing具体的使用方式时通过下列方法：
//checkpointDirectory为checkpoint文件保存目录streamingContext.checkpoint(checkpointDirectory)3. 案例
程序来源：https://github.com/apache/spark/blob/master/examples/src/main/scala/org/apache/spark/examples/streaming/RecoverableNetworkWordCount.scala 进行了适量修改
import java.io.Fileimport java.nio.charset.Charsetimport com.google.common.io.Filesimport org.apache.spark.SparkConfimport org.apache.spark.rdd.RDDimport org.apache.spark.streaming.{Time, Seconds, StreamingContext}import org.apache.spark.util.IntParam/** * Counts words in text encoded with UTF8 received from the network every second. * * Usage: RecoverableNetworkWordCount <hostname> <port> <checkpoint-directory> <output-file> * <hostname> and <port> describe the TCP server that Spark Streaming would connect to receive * data. <checkpoint-directory> directory to HDFS-compatible file system which checkpoint data * <output-file> file to which the word counts will be appended * * <checkpoint-directory> and <output-file> must be absolute paths * * To run this on your local machine, you need to first run a Netcat server * *`$ nc -lk 9999` * * and run the example as * *`$ ./bin/run-example org.apache.spark.examples.streaming.RecoverableNetworkWordCount \ *localhost 9999 ~/checkpoint/ ~/out` * * If the directory ~/checkpoint/ does not exist (e.g. running for the first time), it will create * a new StreamingContext (will print “Creating new context” to the console). Otherwise, if * checkpoint data exists in ~/checkpoint/, then it will create StreamingContext from * the checkpoint data. * * Refer to the online documentation for more details. */object RecoverableNetworkWordCount { def createContext(ip: String, port: Int, outputPath: String, checkpointDirectory: String): StreamingContext = {//程序第一运行时会创建该条语句，如果应用程序失败，则会从checkpoint中恢复，该条语句不会执行println(“Creating new context”)val outputFile = new File(outputPath)if (outputFile.exists()) outputFile.delete()val sparkConf = new SparkConf().setAppName(“RecoverableNetworkWordCount”).setMaster(“local[4]”)// Create the context with a 1 second batch sizeval ssc = new StreamingContext(sparkConf, Seconds(1))ssc.checkpoint(checkpointDirectory)//将socket作为数据源val lines = ssc.socketTextStream(ip, port)val words = lines.flatMap(_.split(” “))val wordCounts = words.map(x => (x, 1)).reduceByKey(_ + _)wordCounts.foreachRDD((rdd: RDD[(String, Int)], time: Time) => {val counts = “Counts at time ” + time + ” ” + rdd.collect().mkString(“[“, “, “, “]”)println(counts)println(“Appending to ” + outputFile.getAbsolutePath)Files.append(counts + “\n”, outputFile, Charset.defaultCharset())})ssc } //将String转换成Int private object IntParam { def unapply(str: String): Option[Int] = {try {Some(str.toInt)} catch {case e: NumberFormatException => None} }} def main(args: Array[String]) {if (args.length != 4) {System.err.println(“You arguments were ” + args.mkString(“[“, “, “, “]”))System.err.println(“””|Usage: RecoverableNetworkWordCount <hostname> <port> <checkpoint-directory>|<output-file>. <hostname> and <port> describe the TCP server that Spark|Streaming would connect to receive data. <checkpoint-directory> directory to|HDFS-compatible file system which checkpoint data <output-file> file to which the|word counts will be appended||In local mode, <master> should be ‘local[n]’ with n > 1|Both <checkpoint-directory> and <output-file> must be absolute paths”””.stripMargin)System.exit(1)} val Array(ip, IntParam(port), checkpointDirectory, outputPath) = args//getOrCreate方法，从checkpoint中重新创建StreamingContext对象或新创建一个StreamingContext对象val ssc = StreamingContext.getOrCreate(checkpointDirectory,() => {createContext(ip, port, outputPath, checkpointDirectory)})ssc.start()ssc.awaitTermination() }}
输入参数配置如下：
运行状态图如下：
天下爱情，大抵如斯。
