spark streaming checkpoint

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

http://blog.csdn.net/wisgood/article/details/55667612

http://www.cnblogs.com/dt-zhw/p/5664663.html

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 + "
", 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()
  }
}