Spark-2.4.0源码:sparkContext
在看sparkContext之前,先回顾一下Scala的语法。Scala构造函数分主构造和辅构造函数,辅构造函数是关键字def+this定义的,而类中不在方法体也不在辅构造函数中的代码就是主构造函数,实例化对象的时候主构造函数都会被执行,例:
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class person(name String,age Int){
println("主构造函数被调用")
def this(name String,age Int){ //辅构造函数
this () //必须先调用主构造函数
this.name = name
this.age = age
}
def introduce(){
println("name :" + name + "-age :" + age)
}
}
val jack = new person("jack",2)
jack.introduce()
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运行结果:
主构造函数被调用
name :jack-age :2
切入正题,看sparkContext的主构造函数比较重要的一些代码:
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try{
...
// Create the Spark execution environment (cache, map output tracker, etc)
_env = createSparkEnv(_conf, isLocal, listenerBus)
SparkEnv.set(_env)
...
// 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))
// Create and start the scheduler
val (sched, ts) = SparkContext.createTaskScheduler(this, master, deployMode)
_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()
}
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首先:
_env = createSparkEnv(_conf, isLocal, listenerBus)
SparkEnv.set(_env)
_heartbeatReceiver = env.rpcEnv.setupEndpoint(
HeartbeatReceiver.ENDPOINT_NAME, new HeartbeatReceiver(this))
这里是在sparkContext中创建rpcEnv,并通过 setupEndpoint 向 rpcEnv 注册一个心跳的 Endpoint。
接着:
val (sched, ts) = SparkContext.createTaskScheduler(this, master, deployMode)
调的sparkContext自己的方法,创建taskScheduler,返回的是一个 (SchedulerBackend, TaskScheduler) 元组
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private def createTaskScheduler(
sc: SparkContext,
master: String,
deployMode: String): (SchedulerBackend, TaskScheduler) = {
import SparkMasterRegex._
// When running locally, don't try to re-execute tasks on failure.
val MAX_LOCAL_TASK_FAILURES = 1
master match {
//...
//standalone的提交模式
case SPARK_REGEX(sparkUrl) =>
val scheduler = new TaskSchedulerImpl(sc)
val masterUrls = sparkUrl.split(",").map("spark://" + _)
val backend = new StandaloneSchedulerBackend(scheduler, sc, masterUrls)
//调用初始化方法
scheduler.initialize(backend)
(backend, scheduler)
}
//...
}
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方法内部根据master参数判断不同的提交模式,创建不同的(SchedulerBackend, TaskScheduler) ,拿standalon模式举例,根据入参创建TaskSchedulerImpl和StandalonSchedulerBackend,再调用TaskSchedulerImpl的初始化方法,最后返回一个元组。
scheduler.initialize(backend),其实就是根据不同的schedulingMode创建不同的schedulableBuilder,它就是对Scheduleable tree的封装,负责对taskSet的调度。
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def initialize(backend: SchedulerBackend) {
this.backend = backend
schedulableBuilder = {
schedulingMode match {
case SchedulingMode.FIFO =>
new FIFOSchedulableBuilder(rootPool)
case SchedulingMode.FAIR =>
new FairSchedulableBuilder(rootPool, conf)
case _ =>
throw new IllegalArgumentException(s"Unsupported $SCHEDULER_MODE_PROPERTY: " +
s"$schedulingMode")
}
}
schedulableBuilder.buildPools()
}
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接着下面两行代码:
_dagScheduler = new DAGScheduler(this)
创建DAG有向无环图,实现类面向stage的调度机制的高层次调度层,他会为每个stage计算DAG(有向无环图),追踪RDD和stage的输出是否被物化(写入磁盘或内存),并且寻找一个最少消耗的调度机制来运行job。它会将stage作为taskSets提交到底层的TaskSchedulerImpl上来在集群运行。除了处理stage的DAG,它还负责决定运行每个task的最佳位置,基于当前的缓存状态,将最佳位置提交给底层的TaskSchedulerImpl,此外,他会处理由于每个shuffle输出文件导致的失败,在这种情况下旧的stage可能会被重新提交。一个stage内部的失败,如果不是由于shuffle文件丢失导致的失败,会被taskScheduler处理,它会多次重试每个task,还不行才会取消整个stage。
_heartbeatReceiver.ask[Boolean](TaskSchedulerIsSet)
在上面创建好了TaskScheduler和SchedulerBackend后,告诉 HeartbeatReceiver(心跳) 的监听端。
最后:
_taskScheduler.start()
在TaskSchedulerImpl的start()方法中调的是SchedulerBackend的start()方法,所以start()方法运行的是这段:
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override def start() {
super.start()
// SPARK-21159. The scheduler backend should only try to connect to the launcher when in client
// mode. In cluster mode, the code that submits the application to the Master needs to connect
// to the launcher instead.
if (sc.deployMode == "client") {
launcherBackend.connect()
}
//参数设置
val appDesc = ApplicationDescription(sc.appName, maxCores, sc.executorMemory, command,
webUrl, sc.eventLogDir, sc.eventLogCodec, coresPerExecutor, initialExecutorLimit)
client = new StandaloneAppClient(sc.env.rpcEnv, masters, appDesc, this, conf)
client.start()
launcherBackend.setState(SparkAppHandle.State.SUBMITTED)
waitForRegistration()
launcherBackend.setState(SparkAppHandle.State.RUNNING)
}
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这里创建了两个对象:AppliactionDescription和AppClient,AppliactionDescription顾名思义就是对Application的描述类,比如它需要的资源等;AppClient负责负责为application与spark集群通信。SchedulerBackend的start()最终调用了AppClient的start(),代码如下:
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def start() {
// Just launch an rpcEndpoint; it will call back into the listener.
endpoint.set(rpcEnv.setupEndpoint("AppClient", new ClientEndpoint(rpcEnv)))
}
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启动一个rpcEndPoint并回调给监听器,RPC原理可看这篇 https://www.cnblogs.com/superhedantou/p/7570692.html
最后画个大概流程图https://www.cnblogs.com/cnblogs-syui/p/10948471.html