HeartSaVioR commented on issue #26127: [SPARK-29348][SQL] Add observable 
Metrics for Streaming queries
URL: https://github.com/apache/spark/pull/26127#issuecomment-542994970
 
 
   1) StreamingQueryListener doesn't work with continuous mode. Please refer 
#24537 which proposes to support StreamingQueryListener in continuous mode. (I 
haven't had a look in details though... I'd spend my time on continuous mode 
once we are clear it's still making progress.)
   
   2) In continuous mode, the task is never completed - the patch seems to do 
something when task is completed which will never happen.
   
   ```
         // Only publish the value of the accumulator when the task has 
completed. This is done by
         // updating a task local accumulator ('updater') which will be merged 
with the actual
         // accumulator as soon as the task completes. This avoids the 
following problems during the
         // heartbeat:
         // - Correctness issues due to partially completed/visible updates.
         // - Performance issues due to excessive serialization.
   ```

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