流式计算概述

常规计算引擎分类

1. 批处理
   • 高吞吐，低延迟
   • 面向静态数据集合的处理
   • 分钟甚至小时级别延迟
   • 比如MR, Spark
2. 流式计算
   • 面向行级别数据处理
   • 毫秒级延迟
   • 比如storm

流式计算分类

1. 面向行
   Apache Flink — 收集一堆数据，然后一行一行处理
   Storm
2. 面向micro-Batch
   Spark Streaming — 收集一堆数据，然后一起处理

流式计算通用户环节

数据源 —> 数据缓存 —> 流式引擎 —> 结果存储

流式计算计算方式

1. 固定窗口
   Spark Streaming 常规支持的方式

2. 滑动窗口( window )

3. 会话计算( mapWithStates )
   存储Spark Streaming的状态信息（类似session），可以进行过期处理

Spark Streaming编程要点

Spark Streaming: exactly once delivery
特殊情况：故障重算，推测执行等

1. 监控和管理 jobs

• where to run the driver?
  Yarn cluster mode. Driver will continue to running when the client machine goes down.
• How to restart driver ?
  set up automatic restart.
  In spark configuration (e.g. spark-defaults.conf):

spark.yarn.maxAppAttempts=2  // 重试尝试次数
spark.yarn.am.attemptFailuresValidityInterval=1h  // 重置尝试次数的时间
spark.yarn.max.executor.failures={8 * num_executors}  // executor失败的最大次数
spark.yarn.executor.failuresValidityInterval=1h // 重置失败的时间
spark.task.maxFailures=8   // task重试次数 默认是4
spark.speculation=true  //预测执行， 前提：task是幂等

• Summary
  各种Listener接口

1. 如何优雅的管理streaming app
   思路： Thread hooks – Check for an external flag every N seconds

/** * Stop the execution of the streams, with option of ensuring all received data 
    * has been processed. 
    *
    * * @param stopSparkContext if true, stops the associated SparkContext. The underlying SparkContext 
    * will be stopped regardless of whether this StreamingContext has been 
    * started. 
    * @param stopGracefully if true, stops gracefully by waiting for the processing of all 
    * received data to be completed 
    */ 
    def stop(stopSparkContext: Boolean, stopGracefully: Boolean): Unit = {
    receiverTracker.stop(processAllReceivedData) //default is to wait 10 second, grace waits until done jobGenerator.stop(processAllReceivedData) // Will use spark.streaming.gracefulStopTimeout 
    jobExecutor.shutdown() 
    val terminated = if (processAllReceivedData) { 
        jobExecutor.awaitTermination(1, TimeUnit.HOURS) // just a very large period of time 
    } else { 
        jobExecutor.awaitTermination(2, TimeUnit.SECONDS) 
    } 
    if (!terminated) { jobExecutor.shutdownNow() 
    }

How to be graceful?
• 方法一 cmd line
– $SPARK_HOME_DIR/bin/spark-submit –master $MASTER_REST_URL –kill $DRIVER_ID
– spark.streaming.stopGracefullyOnShutdown=true

private def stopOnShutdown(): Unit = { 
val stopGracefully = conf.getBoolean("spark.streaming.stopGracefullyOnShutdown", false) 
logInfo(s"Invoking stop(stopGracefully=$stopGracefully) from shutdown hook") 
// Do not stop SparkContext, let its own shutdown hook stop it 
stop(stopSparkContext = false, stopGracefully = stopGracefully) }

• 方法二 By marker file 推荐
– Touch a file when starting the app on HDFS
– Remove the file when you want to stop
– Separate thread in Spark app, calls

streamingContext.stop(stopSparkContext = true, stopGracefully = true)