[GitHub] [spark] viirya commented on pull request #30427: [SPARK-33224][SS] Add watermark gap information into SS UI page

[GitBox <gi...@apache.org>]

viirya commented on pull request #30427:
URL: https://github.com/apache/spark/pull/30427#issuecomment-730834615


   > Watermark is the one of major metrics the end users need to track for stateful queries. Watermark defines "when" the output will be emitted for append mode, hence knowing how much gap between wall clock and watermark (input data) is very helpful to make expectation of the output.
   
   Hmm, my question is, watermark should be derived from event time instead of processing time (I think it should be wall clock here?). In the examples, looks like the event time is as processing time, IIUC. So once the event time from data is different processing time, is this graph still useful?


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