Vinayak Joshi created SPARK-17843:
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             Summary: History Server gives no feedback about the application 
list being incomplete
                 Key: SPARK-17843
                 URL: https://issues.apache.org/jira/browse/SPARK-17843
             Project: Spark
          Issue Type: Improvement
          Components: Web UI
    Affects Versions: 2.0.1, 2.0.0
            Reporter: Vinayak Joshi
            Priority: Minor


A user who lands on the history server's application listing page before the 
history server's background processing has finished replaying the existing 
event logs complains why a particular  application is not included in the list. 
There is no indication on the UI about the possible reason, or that refreshing 
the page after some time may show up the application the user expects to see. 
This problem is very noticeable when there are large sized event logs that take 
a long time to finish replaying.

The usability problems with large (number or size) event logs and the history 
server have been known. Particularly SPARK-5522, SPARK-13988 and other issues 
referenced within them highlight the problems and the improvements done so far. 
To improve the history server startup time and reduce the impact of large event 
log files, the event logs are now processed (replayed) via a pool of threads. 
This allows a user to browse to the application listing page before the event 
logs have finished replaying. After history server startup, a user expects to 
see any old completed applications to appear in the application list page. But 
unless the corresponding event logs have finished replaying, the application 
wont be in the list, and the user complains. There is no feedback to the user 
about this on the UI, hence this JIRA to try and address this problem. 

Idea is to give some indication of the number of event logs that are pending 
replay to the user. Note that the way the replay is currently designed, one 
cycle of "check for logs that need to be replayed > replay the logs > update 
application info" needs to complete before a new one begins. Therefore, it 
should be possible for the FsApplicationHistoryProvider to send info about the 
number of logs that are currently pending processing. This in turn would 
address the user anxiety of not seeing the application they expect to see. 

I will be attaching a pull request with my initial take on implementing this.



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