Master实际上可以配置两个，那么在spark原生的standalone上也是支持Master主备切换的，也就是说，当Active Master节点挂掉之后，我们可以将Standby Master切换为Active Master
Spark Master的主备切换可以基于两种切换机制，一种是文件系统，一种是基于Zookeeper,基于文件系统的机制，是Active Master挂掉后，需要我们手动去切换到Standby Master上，基于Zookeeper机制，呆以实现自动切换。
所以这里说的主备切换机制，其实指的是在Active Master挂掉之后，切换到Standby Master时，Master会做哪些操作
1.使用持久化引挚(FileSystemPersistence或者是ZookeeperPersisitence)去读取持久化的storedApps,storedDriver,storedWorker,
2.判断上面的三个持久化的storedApps,storedDriver,storedWorker,
有任何一个不为空，就将持久化有Application,Driver,Worker的信息重新注册，注册到Master内部的缓存结构中。
3.将Application和Worker的状态都修改为UNKNOWN,然后向Application对应的Driver，Worker发送Standby Master的地址.
4.Driver,Worker,理论上讲，如果他们目前是正常工作的话，那么在收到Master发送来的地址后，就会返回响应给新的Master。
5.此时，Master在陆续接收到Driver,Worker发送来的响应消息之后，会使用completeRecovery()对没有收到发送响应消息的Driver,Worker进行处理，过滤掉他们的信息。如下：
// Kill off any workers and apps that didn’t respond to us.
workers.filter(.state == WorkerState.UNKNOWN).foreach(removeWorker)
apps.filter(.state == ApplicationState.UNKNOWN).foreach(finishApplication)
// Reschedule drivers which were not claimed by any workers
drivers.filter(_.worker.isEmpty).foreach { d =>
logWarning(s"Driver ${d.id} was not found after master recovery")
if (d.desc.supervise) {
logWarning(s"Re-launching ${d.id}")
relaunchDriver(d)} else {
removeDriver(d.id, DriverState.ERROR, None)
logWarning(s"Did not re-launch ${d.id} because it was not supervised")}
6.调用Master的schedule()，对正在等待调度的Driver,Application进行调度，比如在某个Worker上启动Driver,或者为Application在Worker上启动Executor。
state = RecoveryState.ALIVE
schedule()
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