sarthfrey opened a new pull request #27395: [SPARK-30667][CORE] Add allGather method to BarrierTaskContext URL: https://github.com/apache/spark/pull/27395 ### What changes were proposed in this pull request? The `allGather` method is added to the `BarrierTaskContext`. This method contains the same functionality as the `BarrierTaskContext.barrier` method; it blocks the task until all tasks make the call, at which time they may continue execution. In addition, the `allGather` method takes an input message. Upon returning from the `allGather` the task receives a list of all the messages sent by all the tasks that made the `allGather` call. ### Why are the changes needed? There are many situations where having the tasks communicate in a synchronized way is useful. One simple example is if each task needs to start a server to serve requests from one another; first the tasks must find a free port (the result of which is undetermined beforehand) and then start making requests, but to do so they each must know the port chosen by the other task. An `allGather` method would allow them to inform each other of the port they will run on. ### Does this PR introduce any user-facing change? Yes, an `BarrierTaskContext.allGather` method will be available through the Scala, Java, and Python APIs. ### How was this patch tested? Most of the code path is already covered by tests to the `barrier` method, since this PR includes a refactor so that much code is shared by the `barrier` and `allGather` methods. However, a test is added to assert that an all gather on each tasks partition ID will return a list of every partition ID. An example through the Python API: ```python >>> from pyspark import BarrierTaskContext >>> >>> def f(iterator): ... context = BarrierTaskContext.get() ... return [context.allGather('{}'.format(context.partitionId()))] ... >>> sc.parallelize(range(4), 4).barrier().mapPartitions(f).collect()[0] [u'3', u'1', u'0', u'2'] ```
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