Spark core之BlockManager

• key-value形式存储的数据块，用block ID表示
• 充当在Spark集群中的每个节点上运行的本地缓存（driver and executors)

SparkEnv实例化一个BlockManager对象blockManager

    // NB: blockManager is not valid until initialize() is called later.
    val blockManager = new BlockManager(
    					 executorId, 
    					 rpcEnv,
    					 blockManagerMaster,
      					 serializerManager, 
      					 conf, 
      					 memoryManager, 
      					 mapOutputTracker, 
      					 shuffleManager,
     					 blockTransferService, 
     					 securityManager, 
     					 numUsableCores)

调用BlockManager的主构造器（主构造器会执行类定义中的所有语句）

 // 定义变量blockManagerId，初始值为null
 var blockManagerId: BlockManagerId = _  
 ......
 blockManagerId = 
     if (idFromMaster != null) 	          
           idFromMaster 
     else id