[GitHub] [spark] Ngone51 edited a comment on issue #28117: [SPARK-31344][CORE] Polish implementation of barrier() and allGather()

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Ngone51 edited a comment on issue #28117: [SPARK-31344][CORE] Polish implementation of barrier() and allGather()
URL: https://github.com/apache/spark/pull/28117#issuecomment-608968979
 
 
   This PR tried to address comment https://github.com/apache/spark/pull/27395#pullrequestreview-357681136 , but please note that we can not simply build `barrier()` on top of `allGather("")`, because we need a way do distinguish these two different calls to prevent following illegal case:
   
   ```scala
   rdd.barrier().mapPartitions { iter =>
      val context = BarrierTaskContext.get()
      val partitionId = context.partitionId
      if (partitionId == 0) {
        context.barrier()
      } else {
        context.allGather(partitionId.toString)
      }
     iter
    }
   ``` 
   
   ping @sarthfrey @jiangxb1987 @mengxr Please take a look, thanks!

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