Spark修炼之道（进阶篇）——Spark入门到精通：第十四节 Spark Streaming 缓存、Checkpoint机制

作者：周志湖
微信号：zhouzhihubeyond

主要内容

本节内容基于官方文档：http://spark.apache.org/docs/latest/streaming-programming-guide.html

1. Spark Stream 缓存
2. Checkpoint
3. 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 None
    new 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.File
import java.nio.charset.Charset

import com.google.common.io.Files

import org.apache.spark.SparkConf
import org.apache.spark.rdd.RDD
import 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 size
    val 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：

首次运行时：

//创建新的StreamingContext
Creating new context
15/11/30 07:20:32 WARN MetricsSystem: Using default name DAGScheduler for source because spark.app.id is not set.
15/11/30 07:20:33 WARN SizeEstimator: Failed to check whether UseCompressedOops is set; assuming yes
Counts at time 1448896840000 ms []
Appending to /root/out2
15/11/30 07:20:47 WARN BlockManager: Block input-0-1448896847000 replicated to only 0 peer(s) instead of 1 peers
Counts at time 1448896850000 ms [(Spark,1), (Context,1)]

手动将程序停止，然后重新运行

//这时从checkpoint目录中读取元数据信息，进行StreamingContext的恢复
Counts at time 1448897070000 ms []
Appending to /root/out2
Counts at time 1448897080000 ms []
Appending to /root/out2
Counts at time 1448897090000 ms []
Appending to /root/out2
15/11/30 07:24:58 WARN BlockManager: Block input-0-1448897098600 replicated to only 0 peer(s) instead of 1 peers
[Stage 8:>                                                          (0 + 0) / 4]Counts at time 1448897100000 ms [(Spark,1), (Context,1)]
Appending to /root/out2