本文我们将详细分析SparkContext的源码。
我们先来看一下SparkConf：
SparkConf
SparkContext实例化的时候需要传进一个SparkConf作为参数，SparkConf描述整个Spark应用程序的配置信息，如果和系统默认的配置冲突就会覆盖系统默认的设置。我们经常会在单元测试的时候使用new SparkConf(fasle)(如果不传入参数，默认是true)实例化SparkConf，这样就不会加载“conf/”下默认的配置，这样无论在什么样的集群环境中运行单元测试，其配置都是一样的，不会随着环境的变化而变化。另外在程序运行的时候不能修改SparkConf，数据结构通过ConcurrentHashMap进行维护，只能通过clone的方式读取Spark的配置信息。最后需要说明的是SparkConf可以进行链式的调用，即：
new SparkConf().setMaster("local").setAppName("TestApp")因为这些方法在设置完配置信息后最终都返回了自己，即SparkConf本身，SparkConf的部分源码如下：
// 用来存储key-value的配置信息private val settings = new ConcurrentHashMap[String, String]()// 默认会加载“spark.*”格式的配置信息if (loadDefaults) { // Load any spark.* system properties
for ((key, value) <- Utils.getSystemProperties if key.startsWith("spark.")) {
set(key, value)
}
}/** Set a configuration variable. */def set(key: String, value: String): SparkConf = { if (key == null) { throw new NullPointerException("null key")
} if (value == null) { throw new NullPointerException("null value for " + key)
}
logDeprecationWarning(key)
settings.put(key, value) // 这里我们可以清楚的看到，每次进行设置后都会返回SparkConf自身，所以可以进行链式的调用
this}SparkContext
SparkContext是整个Spark功能的入口，代表了应用程序与整个集群的连接点，通过SparkContext可以创建RDD、Accumulators和Broadcast。
Spark应用程序是通过SparkContext发布到Spark集群的，并且Spark程序的运行都是在SparkContext为核心的调度指挥下进行的，SparkContext崩溃或者结束就代表Spark应用程序执行结束，可见SparkContext在Spark中是多么的重要，下面我们结合源码进行详细分析(只选取重要部分)：
// 方便开发人员查看调用的信息// The call site where this SparkContext was constructed.private val creationSite: CallSite = Utils.getCallSite()// 是否允许存在多个SparkContext，默认是false// If true, log warnings instead of throwing exceptions when multiple SparkContexts are activeprivate val allowMultipleContexts: Boolean =
config.getBoolean("spark.driver.allowMultipleContexts", false)// In order to prevent multiple SparkContexts from being active at the same time, mark this// context as having started construction.// NOTE: this must be placed at the beginning of the SparkContext constructor.SparkContext.markPartiallyConstructed(this, allowMultipleContexts)val startTime = System.currentTimeMillis()// 判断上下文状态private[spark] val stopped: AtomicBoolean = new AtomicBoolean(false)
...// 消息通信相关的内容，我们会单独进行说明// An asynchronous listener bus for Spark eventsprivate[spark] val listenerBus = new LiveListenerBus...// 追踪所有执行持久化的RDD// Keeps track of all persisted RDDsprivate[spark] val persistentRdds = new TimeStampedWeakValueHashMap[Int, RDD[_]]
...// 传递给executors的环境变量信息// Environment variables to pass to our executors.private[spark] val executorEnvs = HashMap[String, String]()// 配置SPARK_USER// Set SPARK_USER for user who is running SparkContext.val sparkUser = Utils.getCurrentUserName()同时还有几个重载的构造方法，我们不进行一一说明，下面我们来看SparkContext中最重要的一个部分，即try里面的内容(大部分初始化的工作都在这里面，因为内容较多，大家可以根据具体的功能点进行查看)：
try { // 读取SparkConf的信息，即Spark的配置信息，并检查是否有非法的配置信息
_conf = config.clone()
_conf.validateSettings() // 判断是否配置了Master，没有的话抛出异常
if (!_conf.contains("spark.master")) { throw new SparkException("A master URL must be set in your configuration")
} // 判断是否配置了AppName，没有的话抛出异常
if (!_conf.contains("spark.app.name")) { throw new SparkException("An application name must be set in your configuration")
} // Yarn模式下的判断
// System property spark.yarn.app.id must be set if user code ran by AM on a YARN cluster
// yarn-standalone is deprecated, but still supported
if ((master == "yarn-cluster" || master == "yarn-standalone") &&
!_conf.contains("spark.yarn.app.id")) { throw new SparkException("Detected yarn-cluster mode, but isn't running on a cluster. " + "Deployment to YARN is not supported directly by SparkContext. Please use spark-submit.")
} if (_conf.getBoolean("spark.logConf", false)) {
logInfo("Spark configuration:\n" + _conf.toDebugString)
}
// 设置Driver的host和port
// Set Spark driver host and port system properties
_conf.setIfMissing("spark.driver.host", Utils.localHostName())
_conf.setIfMissing("spark.driver.port", "0")
_conf.set("spark.executor.id", SparkContext.DRIVER_IDENTIFIER)
_jars = _conf.getOption("spark.jars").map(_.split(",")).map(_.filter(_.size != 0)).toSeq.flatten
_files = _conf.getOption("spark.files").map(_.split(",")).map(_.filter(_.size != 0))
.toSeq.flatten
_eventLogDir = if (isEventLogEnabled) { // 这里需要进行设置，否则默认路径是“/tmp/spark-events”，防止系统自动清除
val unresolvedDir = conf.get("spark.eventLog.dir", EventLoggingListener.DEFAULT_LOG_DIR)
.stripSuffix("/") Some(Utils.resolveURI(unresolvedDir))
} else { None
}
_eventLogCodec = { val compress = _conf.getBoolean("spark.eventLog.compress", false) if (compress && isEventLogEnabled) { Some(CompressionCodec.getCodecName(_conf)).map(CompressionCodec.getShortName)
} else { None
}
}
_conf.set("spark.externalBlockStore.folderName", externalBlockStoreFolderName) if (master == "yarn-client") System.setProperty("SPARK_YARN_MODE", "true") // "_jobProgressListener" should be set up before creating SparkEnv because when creating
// "SparkEnv", some messages will be posted to "listenerBus" and we should not miss them.
_jobProgressListener = new JobProgressListener(_conf) // 关于ListenerBus我们会单独分析
listenerBus.addListener(jobProgressListener) // Create the Spark execution environment (cache, map output tracker, etc)
// 创建SparkEnv
_env = createSparkEnv(_conf, isLocal, listenerBus) SparkEnv.set(_env) // 元数据清理器
_metadataCleaner = new MetadataCleaner(MetadataCleanerType.SPARK_CONTEXT, this.cleanup, _conf) // 监控job和Stage的执行
_statusTracker = new SparkStatusTracker(this) // 显示Stage的执行进度，从statusTracker定期拉取活动状态的Stages的进度，将在Stage至少运行500ms后显示，如果多个Stage在同一时间执行，则它们的状态将会合并到一起，在一行中显示，每200ms更新一次
_progressBar = if (_conf.getBoolean("spark.ui.showConsoleProgress", true) && !log.isInfoEnabled) { Some(new ConsoleProgressBar(this))
} else { None
} // Spark UI部分
_ui = if (conf.getBoolean("spark.ui.enabled", true)) { Some(SparkUI.createLiveUI(this, _conf, listenerBus, _jobProgressListener,
_env.securityManager, appName, startTime = startTime))
} else { // For tests, do not enable the UI
None
} // Bind the UI before starting the task scheduler to communicate
// the bound port to the cluster manager properly
_ui.foreach(_.bind())
_hadoopConfiguration = SparkHadoopUtil.get.newConfiguration(_conf) // 添加Jar包的依赖
// Add each JAR given through the constructor
if (jars != null) {
jars.foreach(addJar)
} if (files != null) {
files.foreach(addFile)
}
// 获取executor的内存配置信息，如果没有设置，默认就是1G
_executorMemory = _conf.getOption("spark.executor.memory")
.orElse(Option(System.getenv("SPARK_EXECUTOR_MEMORY")))
.orElse(Option(System.getenv("SPARK_MEM"))
.map(warnSparkMem))
.map(Utils.memoryStringToMb)
.getOrElse(1024) // Convert java options to env vars as a work around
// since we can't set env vars directly in sbt.
for { (envKey, propKey) <- Seq(("SPARK_TESTING", "spark.testing"))
value <- Option(System.getenv(envKey)).orElse(Option(System.getProperty(propKey)))} {
executorEnvs(envKey) = value
} Option(System.getenv("SPARK_PREPEND_CLASSES")).foreach { v =>
executorEnvs("SPARK_PREPEND_CLASSES") = v
} // Mesos相关的配置
// The Mesos scheduler backend relies on this environment variable to set executor memory.
// TODO: Set this only in the Mesos scheduler.
executorEnvs("SPARK_EXECUTOR_MEMORY") = executorMemory + "m"
executorEnvs ++= _conf.getExecutorEnv
executorEnvs("SPARK_USER") = sparkUser // We need to register "HeartbeatReceiver" before "createTaskScheduler" because Executor will
// retrieve "HeartbeatReceiver" in the constructor. (SPARK-6640)
_heartbeatReceiver = env.rpcEnv.setupEndpoint( HeartbeatReceiver.ENDPOINT_NAME, new HeartbeatReceiver(this)) // 下面就是SparkContext中最重要的部分，即创建一系列调度器
// Create and start the scheduler
val (sched, ts) = SparkContext.createTaskScheduler(this, master)
_schedulerBackend = sched
_taskScheduler = ts
_dagScheduler = new DAGScheduler(this)
_heartbeatReceiver.ask[Boolean](TaskSchedulerIsSet) // start TaskScheduler after taskScheduler sets DAGScheduler reference in DAGScheduler's
// constructor
_taskScheduler.start() // 初始化应用程序Id信息
_applicationId = _taskScheduler.applicationId()
_applicationAttemptId = taskScheduler.applicationAttemptId()
_conf.set("spark.app.id", _applicationId)
_ui.foreach(_.setAppId(_applicationId))
_env.blockManager.initialize(_applicationId) // 统计系统相关内容
// The metrics system for Driver need to be set spark.app.id to app ID.
// So it should start after we get app ID from the task scheduler and set spark.app.id.
metricsSystem.start() // Attach the driver metrics servlet handler to the web ui after the metrics system is started.
metricsSystem.getServletHandlers.foreach(handler => ui.foreach(_.attachHandler(handler)))
_eventLogger = if (isEventLogEnabled) { val logger = new EventLoggingListener(_applicationId, _applicationAttemptId, _eventLogDir.get,
_conf, _hadoopConfiguration)
logger.start()
listenerBus.addListener(logger) Some(logger)
} else { None
} // Optionally scale number of executors dynamically based on workload. Exposed for testing.
val dynamicAllocationEnabled = Utils.isDynamicAllocationEnabled(_conf) if (!dynamicAllocationEnabled && _conf.getBoolean("spark.dynamicAllocation.enabled", false)) {
logWarning("Dynamic Allocation and num executors both set, thus dynamic allocation disabled.")
}
_executorAllocationManager = if (dynamicAllocationEnabled) { Some(new ExecutorAllocationManager(this, listenerBus, _conf))
} else { None
}
_executorAllocationManager.foreach(_.start())
_cleaner = if (_conf.getBoolean("spark.cleaner.referenceTracking", true)) { Some(new ContextCleaner(this))
} else { None
}
_cleaner.foreach(_.start())
setupAndStartListenerBus()
postEnvironmentUpdate()
postApplicationStart() // Post init
_taskScheduler.postStartHook()
_env.metricsSystem.registerSource(_dagScheduler.metricsSource)
_env.metricsSystem.registerSource(new BlockManagerSource(_env.blockManager))
_executorAllocationManager.foreach { e =>
_env.metricsSystem.registerSource(e.executorAllocationManagerSource)
} // Make sure the context is stopped if the user forgets about it. This avoids leaving
// unfinished event logs around after the JVM exits cleanly. It doesn't help if the JVM
// is killed, though.
_shutdownHookRef = ShutdownHookManager.addShutdownHook( ShutdownHookManager.SPARK_CONTEXT_SHUTDOWN_PRIORITY) { () =>
logInfo("Invoking stop() from shutdown hook")
stop()
}
} catch { case NonFatal(e) =>
logError("Error initializing SparkContext.", e) try {
stop()
} catch { case NonFatal(inner) =>
logError("Error stopping SparkContext after init error.", inner)
} finally { throw e
}
}其实SparkContext中最主要的三大核心对象就是DAGScheduler、TaskScheduler、SchedulerBackend，我们会在接下来的文章中详细进行分析。
SparkEnv
下面我们再来分析一下SparkEnv，SparkEnv保存了一个正在运行的Spark实例(Master或者Worker)的运行时环境信息，包括序列化(serializer)、Akka的actor system(虽然1.6.x默认使用的是Netty但是有一些历史遗留代码，spark2.x开始已经不在依赖Akka)、BlockManager、MapOutPutTracker(Shuffle过程中非常重要)等，跟SparkContext一样，这些具体的功能点我们会用单独的文章分别进行说明，现在我们简单过滤一下SparkEnv的源码：
先来看createDriverEnv和createExecutorEnv，顾名思义，这两个方法就是分别创建Driver和Executor的运行时环境。
/**
* Create a SparkEnv for the driver.
*/private[spark] def createDriverEnv(
conf: SparkConf,
isLocal: Boolean,
listenerBus: LiveListenerBus,
numCores: Int,
mockOutputCommitCoordinator: Option[OutputCommitCoordinator] = None): SparkEnv = {
assert(conf.contains("spark.driver.host"), "spark.driver.host is not set on the driver!")
assert(conf.contains("spark.driver.port"), "spark.driver.port is not set on the driver!") val hostname = conf.get("spark.driver.host") val port = conf.get("spark.driver.port").toInt
create(
conf, SparkContext.DRIVER_IDENTIFIER,
hostname,
port,
isDriver = true,
isLocal = isLocal,
numUsableCores = numCores,
listenerBus = listenerBus,
mockOutputCommitCoordinator = mockOutputCommitCoordinator
)
}/**
* Create a SparkEnv for an executor.
* In coarse-grained mode, the executor provides an actor system that is already instantiated.
*/private[spark] def createExecutorEnv(
conf: SparkConf,
executorId: String,
hostname: String,
port: Int,
numCores: Int,
isLocal: Boolean): SparkEnv = { val env = create(
conf,
executorId,
hostname,
port,
isDriver = false,
isLocal = isLocal,
numUsableCores = numCores
) SparkEnv.set(env)
env
}可以看到这两个方法最终都调用了create()方法：
private def create(
conf: SparkConf,
executorId: String,
hostname: String,
port: Int,
isDriver: Boolean,
isLocal: Boolean,
numUsableCores: Int,
listenerBus: LiveListenerBus = null,
mockOutputCommitCoordinator: Option[OutputCommitCoordinator] = None): SparkEnv = { // Listener bus仅使用在Driver上，所以要进行判断
// Listener bus is only used on the driver
if (isDriver) {
assert(listenerBus != null, "Attempted to create driver SparkEnv with null listener bus!")
} // 实例化SecurityManager
val securityManager = new SecurityManager(conf) // 这里是Spark1.6.x中遗留的一部分关于Akka的代码，spark2.x中已经完全移除
// Create the ActorSystem for Akka and get the port it binds to.
val actorSystemName = if (isDriver) driverActorSystemName else executorActorSystemName // 创建rpcEnv
val rpcEnv = RpcEnv.create(actorSystemName, hostname, port, conf, securityManager,
clientMode = !isDriver) val actorSystem: ActorSystem = if (rpcEnv.isInstanceOf[AkkaRpcEnv]) {
rpcEnv.y[AkkaRpcEnv].actorSystem
} else { val actorSystemPort = if (port == 0 || rpcEnv.address == null) {
port
} else {
rpcEnv.address.port + 1
} // Create a ActorSystem for legacy codes
AkkaUtils.createActorSystem(
actorSystemName + "ActorSystem",
hostname,
actorSystemPort,
conf,
securityManager
)._1
} // Figure out which port Akka actually bound to in case the original port is 0 or occupied.
// In the non-driver case, the RPC env's address may be null since it may not be listening
// for incoming connections.
// 设置Driver或者Executor的端口号
if (isDriver) {
conf.set("spark.driver.port", rpcEnv.address.port.toString)
} else if (rpcEnv.address != null) {
conf.set("spark.executor.port", rpcEnv.address.port.toString)
}
// 根据给定的类的名字进行实例化操作
// Create an instance of the class with the given name, possibly initializing it with our conf
def instantiateClass[T](className: String): T = { val cls = Utils.classForName(className) // Look for a constructor taking a SparkConf and a boolean isDriver, then one taking just
// SparkConf, then one taking no arguments
try {
cls.getConstructor(classOf[SparkConf], java.lang.Boolean.TYPE)
.newInstance(conf, new java.lang.Boolean(isDriver))
.asInstanceOf[T]
} catch { case _: NoSuchMethodException => try {
cls.getConstructor(classOf[SparkConf]).newInstance(conf).asInstanceOf[T]
} catch { case _: NoSuchMethodException =>
cls.getConstructor().newInstance().asInstanceOf[T]
}
}
} // Create an instance of the class named by the given SparkConf property, or defaultClassName
// if the property is not set, possibly initializing it with our conf
def instantiateClassFromConf[T](propertyName: String, defaultClassName: String): T = {
instantiateClass[T](conf.get(propertyName, defaultClassName))
}
// 设置序列化器，可以看到默认使用的是Java的序列化器
val serializer = instantiateClassFromConf[Serializer]( "spark.serializer", "org.apache.spark.serializer.JavaSerializer")
logDebug(s"Using serializer: ${serializer.getClass}") val closureSerializer = instantiateClassFromConf[Serializer]( "spark.closure.serializer", "org.apache.spark.serializer.JavaSerializer") def registerOrLookupEndpoint(
name: String, endpointCreator: => RpcEndpoint): RpcEndpointRef = { if (isDriver) {
logInfo("Registering " + name)
rpcEnv.setupEndpoint(name, endpointCreator)
} else { RpcUtils.makeDriverRef(name, conf, rpcEnv)
}
}
// 如果是Driver实例化MapOutputTrackerMaster，如果是Executor实例化MapOutputTrackerWorker，在Shuffle时会详细说明
// 说明MapOutputTracker也是Master／Slaves的结构
val mapOutputTracker = if (isDriver) { new MapOutputTrackerMaster(conf)
} else { new MapOutputTrackerWorker(conf)
}
// 实例化向MapOutputTrackerMasterEndpoint并向MapOutputTracker注册
// Have to assign trackerActor after initialization as MapOutputTrackerActor
// requires the MapOutputTracker itself
mapOutputTracker.trackerEndpoint = registerOrLookupEndpoint(MapOutputTracker.ENDPOINT_NAME, new MapOutputTrackerMasterEndpoint(
rpcEnv, mapOutputTracker.asInstanceOf[MapOutputTrackerMaster], conf)) // Shuffle的配置信息，默认使用的是SortShuffleManager
// Let the user specify short names for shuffle managers
val shortShuffleMgrNames = Map( "hash" -> "org.apache.spark.shuffle.hash.HashShuffleManager", "sort" -> "org.apache.spark.shuffle.sort.SortShuffleManager", "tungsten-sort" -> "org.apache.spark.shuffle.sort.SortShuffleManager") val shuffleMgrName = conf.get("spark.shuffle.manager", "sort") val shuffleMgrClass = shortShuffleMgrNames.getOrElse(shuffleMgrName.toLowerCase, shuffleMgrName) // 实例化ShuffleManager，默认是SortShuffleManager
val shuffleManager = instantiateClass[ShuffleManager](shuffleMgrClass) // 是否使用原始的MemoryManager，即StaticMemoryManager
val useLegacyMemoryManager = conf.getBoolean("spark.memory.useLegacyMode", false) // 默认使用的是UnifiedMemoryManager内存管理器
val memoryManager: MemoryManager = if (useLegacyMemoryManager) { new StaticMemoryManager(conf, numUsableCores)
} else { UnifiedMemoryManager(conf, numUsableCores)
} // 实例化BlockTransferService，默认的实现方式是Netty，即NettyBlockTransferService
val blockTransferService = new NettyBlockTransferService(conf, securityManager, numUsableCores) // 下面是BlockManager相关的初始化过程，我们会用单独的文章说明
val blockManagerMaster = new BlockManagerMaster(registerOrLookupEndpoint( BlockManagerMaster.DRIVER_ENDPOINT_NAME, new BlockManagerMasterEndpoint(rpcEnv, isLocal, conf, listenerBus)),
conf, isDriver) // NB: blockManager is not valid until initialize() is called later.
val blockManager = new BlockManager(executorId, rpcEnv, blockManagerMaster,
serializer, conf, memoryManager, mapOutputTracker, shuffleManager,
blockTransferService, securityManager, numUsableCores) // 实例化BroadcastManager
val broadcastManager = new BroadcastManager(isDriver, conf, securityManager) // 实例化cacheManager
val cacheManager = new CacheManager(blockManager) // 统计系统
val metricsSystem = if (isDriver) { // Don't start metrics system right now for Driver.
// We need to wait for the task scheduler to give us an app ID.
// Then we can start the metrics system.
MetricsSystem.createMetricsSystem("driver", conf, securityManager)
} else { // We need to set the executor ID before the MetricsSystem is created because sources and
// sinks specified in the metrics configuration file will want to incorporate this executor's
// ID into the metrics they report.
conf.set("spark.executor.id", executorId) val ms = MetricsSystem.createMetricsSystem("executor", conf, securityManager)
ms.start()
ms
}
// 设置sparkFiles的存储目录，用来下载依赖。local模式下是一个临时的目录，分布式模式下是Executor的工作目录
// Set the sparkFiles directory, used when downloading dependencies. In local mode,
// this is a temporary directory; in distributed mode, this is the executor's current working
// directory.
val sparkFilesDir: String = if (isDriver) { Utils.createTempDir(Utils.getLocalDir(conf), "userFiles").getAbsolutePath
} else { "."
}
// outputCommitCoordinator相关的初始化及注册部分
val outputCommitCoordinator = mockOutputCommitCoordinator.getOrElse { new OutputCommitCoordinator(conf, isDriver)
} val outputCommitCoordinatorRef = registerOrLookupEndpoint("OutputCommitCoordinator", new OutputCommitCoordinatorEndpoint(rpcEnv, outputCommitCoordinator))
outputCommitCoordinator.coordinatorRef = Some(outputCommitCoordinatorRef) val envInstance = new SparkEnv(
executorId,
rpcEnv,
actorSystem,
serializer,
closureSerializer,
cacheManager,
mapOutputTracker,
shuffleManager,
broadcastManager,
blockTransferService,
blockManager,
securityManager,
sparkFilesDir,
metricsSystem,
memoryManager,
outputCommitCoordinator,
conf) // Add a reference to tmp dir created by driver, we will delete this tmp dir when stop() is
// called, and we only need to do it for driver. Because driver may run as a service, and if we
// don't delete this tmp dir when sc is stopped, then will create too many tmp dirs.
if (isDriver) {
envInstance.driverTmpDirToDelete = Some(sparkFilesDir)
}
envInstance
}本文只是带领大家浏览了一下SparkContext和SparkEnv的源码，具体的模块会进行单独的分析。
本文为原创，欢迎转载，转载请注明出处、作者，谢谢！
作者：sun4lower
链接：https://www.jianshu.com/p/5800df3e2d5f
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