这一块是Spark闭包的问题，不懂自行看文档吧！
重点：Spark Streaming的foreachRDD运行在Driver端，而foreach和foreachPartion运行在Worker节点。
备注：对数据的向外输出，还是用foreach**算子好，不要用Map**算子，因为Map还要返回一个RDD。
Finally, this can be further optimized by reusing connection objects across multiple RDDs/batches. One can maintain a static pool of connection objects than can be reused as RDDs of multiple batches are pushed to the external system, thus further reducing the overheads.
最后还可以优化：多个RDD或者批次重用连接对象。可以建一个静态连接池，然后RDD向外Push数据时候可以重用，降低负载！
