Apache Spark源码走读：如何进行代码跟读
概要转载的，做个标记
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 took78 ms14/07/05 19:53:27 DEBUG BlockManager: Putting block broadcast_0 without replication took79 msres0: org.apache.spark.rdd.RDD = README.md MappedRDD 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 codejava.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 took78 ms14/07/05 19:53:27 DEBUG BlockManager: Putting block broadcast_0 without replication took79 msres0: org.apache.spark.rdd.RDD = README.md MappedRDD at textFile at :13git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 --hardgit pull origin masterAkka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 CoarseGrainedClusterMessagecore/src/main/scala/org/apache/spark/scheduler/cluster/CoarseGrainedSchedulerBackend.scala: executorActor(task.executorId) ! LaunchTask(new SerializableBuffer(serializedTask))
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