[jira] [Created] (FLINK-3538) DataStream join API does not enforce consistent usage

[Shang Yuanchun (Jira)]

Till Rohrmann created FLINK-3538:
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             Summary: DataStream join API does not enforce consistent usage
                 Key: FLINK-3538
                 URL: https://issues.apache.org/jira/browse/FLINK-3538
             Project: Flink
          Issue Type: Improvement
          Components: DataStream API, Scala API
    Affects Versions: 1.0.0
            Reporter: Till Rohrmann


In the Scala DataStream API the {{join}} operation does not enforce that the user has specified a {{KeySelector}} for both input sides before applying a window function. Moreover, the order of the {{where}} and {{equalTo}} clause is not fixed and it is possible to specify multiple {{where}} and {{equalTo}} clauses. In the latter case, it is not clear which {{KeySelector}} will eventually be used by the system.

So the following Flink programs compile without a compilation problem (the first two lines will only fail at runtime):
{code}
inputA.join(inputB).equalTo{x => x}.window(TumblingProcessingTimeWindows.of(Time.seconds(10)))
      .apply(new DefaultFlatJoinFunction[String, String]()).print()

inputA.join(inputB).where{x => x}.window(TumblingProcessingTimeWindows.of(Time.seconds(10)))
      .apply(new DefaultFlatJoinFunction[String, String]()).print()

inputA.join(inputB).equalTo{x => x}.where{x => x}.where{x => "1"}.equalTo{x => "42"}.window(TumblingProcessingTimeWindows.of(Time.seconds(10)))
      .apply(new DefaultFlatJoinFunction[String, String]()).print()
{code}

This is unlike the Java DataStream API where a clear pattern of {{join}} then {{where}} and then {{equalTo}} is enforced. I would propose to do the same for the Scala API.



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