Hello, DisplayData is currently in an odd position: - Dataflow is the only runner that supports it at all [1] - Dataflow supports it only for primitive transforms, but discards it for composite transforms [2](which is quite problematic for very complex but commonly used composite transforms, such as TextIO.write() and BigQueryIO.write()) - There are a lot of tests testing display data of composite transforms - these tests are currently testing something that is a no-op in production; and these tests often are a maintenance burden when refactoring a transform.
What should we do about this? I see two options: - Publish guidance for library authors that Beam is not currently ready for supporting HasDisplayData on composite transforms, and you shouldn't bother implementing or testing it (though you can implement it on primitive transforms of course - which is basically just Read.from(Source) and ParDo) - Say that people should be implementing and testing this feature nevertheless, and promise that Beam runners are going to make it "worth it" (not be a no-op) in the foreseeable future. [1] https://lists.apache.org/thread.html/53b33f17d981054ed198af0351196907bf147b6acf16160dbf395b62@%3Cdev.beam.apache.org%3E [2] https://issues.apache.org/jira/browse/BEAM-366
