The External Shuffle Service: Surviving a Dead Executor

There is a problem with shuffle files living on an executor's local disk: what happens to them when that executor dies or is taken away? The reduce tasks still need those buckets, but the process that wrote them is gone. Without a solution, losing an executor mid-shuffle would force the map tasks that ran on it to be recomputed, the kind of expensive cascade you want to avoid. The external shuffle service is that solution. The external shuffle service is a separate, long-lived process that runs on each worker node, independent of any single executor. Map tasks write their shuffle files, and the shuffle service, not the executor, is what serves those files to the reduce side. So even if the executor that produced the data has exited, its shuffle files are still on the node and the shuffle s

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