Spark异常Randomness of hash of string should be disabled via PYTHONHASHSEED

一、现象

1. 用java写的在spark on yarn 模式下正常。
2. 用python写的在spark on local 模式下正常，在spark on yarn 模式下出错。

Exception: Randomness of hash of string should be disabled via PYTHONHASHSEED
        at org.apache.spark.api.python.PythonRunner$$anon$1.read(PythonRDD.scala:193)
        at org.apache.spark.api.python.PythonRunner$$anon$1.<init>(PythonRDD.scala:234)
        at org.apache.spark.api.python.PythonRunner.compute(PythonRDD.scala:152)
        at org.apache.spark.api.python.PythonRDD.compute(PythonRDD.scala:63)
        at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:323)
        at org.apache.spark.rdd.RDD.iterator(RDD.scala:287)
        at org.apache.spark.api.python.PairwiseRDD.compute(PythonRDD.scala:390)
        at org.apache.spark.rdd.RDD.computeOrReadCheckpoint(RDD.scala:323)
        at org.apache.spark.rdd.RDD.iterator(RDD.scala:287)
        at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:96)
        at org.apache.spark.scheduler.ShuffleMapTask.runTask(ShuffleMapTask.scala:53)
        at org.apache.spark.scheduler.Task.run(Task.scala:99)
        at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:322)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        ... 1 more

二、问题原因

经过排查，发现是python 的 PYTHONHASHSEED 环境变量引起的问题。
新版的python为了安全性，在hash计算时加入随加盐，在单会话下，加的盐是一致的，所以hash出来的结果一致。而在集群多台服务器一起计算时，每台服务器有各自的会话，大家都加随机加盐，结果导致各自的hash结果不一致。从而引起程序报错。

三、解决方案

解决方法很简单，关闭随机加盐，或设置相同的盐值。
方法一：
python代码里设转置环境变量

conf = SparkConf()
conf.set("spark.executorEnv.PYTHONHASHSEED","0");

方法二:
修改spark-defaults.conf,增加

spark.executorEnv.PYTHONHASHSEED=0