Apache Spark源码走读之17 -- 如何进行代码跟读

欢迎转载，转载请注明出处，徽沪一郎

概要

今天不谈Spark中什么复杂的技术实现，只稍为聊聊如何进行代码跟读。众所周知，Spark使用scala进行开发，由于scala有众多的语法糖，很多时候代码跟着跟着就觉着线索跟丢掉了，另外Spark基于Akka来进行消息交互，那如何知道谁是接收方呢？

new Throwable().printStackTrace

代码跟读的时候，经常会借助于日志，针对日志中输出的每一句，我们都很想知道它们的调用者是谁。但有时苦于对spark系统的了解程度不深，或者对scala认识不够，一时半会之内无法找到答案，那么有没有什么简便的办法呢？

我的办法就是在日志出现的地方加入下面一句话

new Throwable().printStackTrace()

现在举一个实际的例子来说明问题。

比如我们在启动spark-shell之后，输入一句非常简单的sc.textFile("README.md")，会输出下述的log

14/07/05 19:53:27 INFO MemoryStore: ensureFreeSpace(32816) called with curMem=0, maxMem=308910489
14/07/05 19:53:27 INFO MemoryStore: Block broadcast_0 stored as values in memory (estimated size 32.0 KB, free 294.6 MB)
14/07/05 19:53:27 DEBUG BlockManager: Put block broadcast_0 locally took  78 ms
14/07/05 19:53:27 DEBUG BlockManager: Putting block broadcast_0 without replication took  79 ms
res0: org.apache.spark.rdd.RDD[String] = README.md MappedRDD[1] at textFile at :13

那我很想知道是第二句日志所在的tryToPut函数是被谁调用的该怎么办？

办法就是打开MemoryStore.scala，找到下述语句

logInfo("Block %s stored as %s in memory (estimated size %s, free %s)".format(
          blockId, valuesOrBytes, Utils.bytesToString(size), Utils.bytesToString(freeMemory)))

在这句话之上，添加如下语句

new Throwable().printStackTrace()

然后，重新进行源码编译

sbt/sbt assembly

再次打开spark-shell,执行sc.textFile("README.md"),就可以得到如下输出，从中可以清楚知道tryToPut的调用者是谁

14/07/05 19:53:27 INFO MemoryStore: ensureFreeSpace(32816) called with curMem=0, maxMem=308910489
14/07/05 19:53:27 WARN MemoryStore: just show the calltrace by entering some modified code
java.lang.Throwable
	at org.apache.spark.storage.MemoryStore.tryToPut(MemoryStore.scala:182)
	at org.apache.spark.storage.MemoryStore.putValues(MemoryStore.scala:76)
	at org.apache.spark.storage.MemoryStore.putValues(MemoryStore.scala:92)
	at org.apache.spark.storage.BlockManager.doPut(BlockManager.scala:699)
	at org.apache.spark.storage.BlockManager.put(BlockManager.scala:570)
	at org.apache.spark.storage.BlockManager.putSingle(BlockManager.scala:821)
	at org.apache.spark.broadcast.HttpBroadcast.(HttpBroadcast.scala:52)
	at org.apache.spark.broadcast.HttpBroadcastFactory.newBroadcast(HttpBroadcastFactory.scala:35)
	at org.apache.spark.broadcast.HttpBroadcastFactory.newBroadcast(HttpBroadcastFactory.scala:29)
	at org.apache.spark.broadcast.BroadcastManager.newBroadcast(BroadcastManager.scala:62)
	at org.apache.spark.SparkContext.broadcast(SparkContext.scala:787)
	at org.apache.spark.SparkContext.hadoopFile(SparkContext.scala:556)
	at org.apache.spark.SparkContext.textFile(SparkContext.scala:468)
	at $line5.$read$$iwC$$iwC$$iwC$$iwC.(:13)
	at $line5.$read$$iwC$$iwC$$iwC.(:18)
	at $line5.$read$$iwC$$iwC.(:20)
	at $line5.$read$$iwC.(:22)
	at $line5.$read.(:24)
	at $line5.$read$.(:28)
	at $line5.$read$.()
	at $line5.$eval$.(:7)
	at $line5.$eval$.()
	at $line5.$eval.$print()
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:483)
	at org.apache.spark.repl.SparkIMain$ReadEvalPrint.call(SparkIMain.scala:788)
	at org.apache.spark.repl.SparkIMain$Request.loadAndRun(SparkIMain.scala:1056)
	at org.apache.spark.repl.SparkIMain.loadAndRunReq$1(SparkIMain.scala:614)
	at org.apache.spark.repl.SparkIMain.interpret(SparkIMain.scala:645)
	at org.apache.spark.repl.SparkIMain.interpret(SparkIMain.scala:609)
	at org.apache.spark.repl.SparkILoop.reallyInterpret$1(SparkILoop.scala:796)
	at org.apache.spark.repl.SparkILoop.interpretStartingWith(SparkILoop.scala:841)
	at org.apache.spark.repl.SparkILoop.command(SparkILoop.scala:753)
	at org.apache.spark.repl.SparkILoop.processLine$1(SparkILoop.scala:601)
	at org.apache.spark.repl.SparkILoop.innerLoop$1(SparkILoop.scala:608)
	at org.apache.spark.repl.SparkILoop.loop(SparkILoop.scala:611)
	at org.apache.spark.repl.SparkILoop$$anonfun$process$1.apply$mcZ$sp(SparkILoop.scala:936)
	at org.apache.spark.repl.SparkILoop$$anonfun$process$1.apply(SparkILoop.scala:884)
	at org.apache.spark.repl.SparkILoop$$anonfun$process$1.apply(SparkILoop.scala:884)
	at scala.tools.nsc.util.ScalaClassLoader$.savingContextLoader(ScalaClassLoader.scala:135)
	at org.apache.spark.repl.SparkILoop.process(SparkILoop.scala:884)
	at org.apache.spark.repl.SparkILoop.process(SparkILoop.scala:982)
	at org.apache.spark.repl.Main$.main(Main.scala:31)
	at org.apache.spark.repl.Main.main(Main.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:483)
	at org.apache.spark.deploy.SparkSubmit$.launch(SparkSubmit.scala:303)
	at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:55)
	at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)
14/07/05 19:53:27 INFO MemoryStore: Block broadcast_0 stored as values in memory (estimated size 32.0 KB, free 294.6 MB)
14/07/05 19:53:27 DEBUG BlockManager: Put block broadcast_0 locally took  78 ms
14/07/05 19:53:27 DEBUG BlockManager: Putting block broadcast_0 without replication took  79 ms
res0: org.apache.spark.rdd.RDD[String] = README.md MappedRDD[1] at textFile at :13

git同步

对代码作了修改之后，如果并不想提交代码，那该如何将最新的内容同步到本地呢？

git reset --hard
git pull origin master

Akka消息跟踪

追踪消息的接收者是谁，相对来说比较容易，只要使用好grep就可以了，当然前提是要对actor model有一点点了解。

还是举个实例吧，我们知道CoarseGrainedSchedulerBackend会发送LaunchTask消息出来，那么谁是接收方呢？只需要执行以下脚本即可。

grep LaunchTask -r core/src/main

从如下的输出中，可以清楚看出CoarseGrainedExecutorBackend是LaunchTask的接收方，接收到该函数之后的业务处理，只需要去看看接收方的receive函数即可。

core/src/main/scala/org/apache/spark/executor/CoarseGrainedExecutorBackend.scala:    case LaunchTask(data) =>
core/src/main/scala/org/apache/spark/executor/CoarseGrainedExecutorBackend.scala:        logError("Received LaunchTask command but executor was null")
core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedClusterMessage.scala:  case class LaunchTask(data: SerializableBuffer) extends CoarseGrainedClusterMessage
core/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala:          executorActor(task.executorId) ! LaunchTask(new SerializableBuffer(serializedTask))

小结

今天的内容相对简单，没有技术含量，自己做个记述，免得时间久了，不记得。