Spark IMF传奇行动第18课：RDD持久化、广播、累加器总结

昨晚听了王家林老师的Spark IMF传奇行动第18课：RDD持久化、广播、累加器，作业是unpersist试验，阅读累加器源码看内部工作机制：

scala> val rdd = sc.parallelize(1 to 1000)
rdd: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[0] at parallelize at <console>:21

scala> rdd.persist
res0: rdd.type = ParallelCollectionRDD[0] at parallelize at <console>:21

scala> rdd.count
16/01/24 11:42:56 INFO DAGScheduler: Job 0 finished: count at <console>:24, took 1.451543 s
res1: Long = 1000

16/01/24 11:43:14 INFO DAGScheduler: Job 2 finished: count at <console>:24, took 0.094119 s
res3: Long = 1000

scala> rdd.unpersist()
16/01/24 11:43:43 INFO ParallelCollectionRDD: Removing RDD 0 from persistence list
16/01/24 11:43:43 INFO BlockManager: Removing RDD 0
res5: rdd.type = ParallelCollectionRDD[0] at parallelize at <console>:21

scala> rdd.count
16/01/24 11:44:56 INFO DAGScheduler: Job 0 finished: count at <console>:24, took 1.475321 s
res1: Long = 1000

persisit后，count执行快了许多，但unpersist后，执行又变慢了。

累加器Accumulator：全局唯一，对于Executor只能修改但不可读，只对Driver可读，只增不减

 val sum = sc.accumulator(0)
 val d1 = sc.parallelize(1 to 5)
 val result1 = d1.foreach(item => sum+= item)
 println(sum)

结果是15.

后续课程可以参照新浪微博 王家林_DT大数据梦工厂：http://weibo.com/ilovepains

王家林  中国Spark第一人,微信公共号DT_Spark

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