HeartSaVioR commented on issue #26821: [SPARK-20656][CORE]Support Incremental 
parsing of event logs in SHS
URL: https://github.com/apache/spark/pull/26821#issuecomment-563780160
 
 
   > Instead, in this PR, I am not closing the store, whenever there are 
changes in event logs or invalidate cache.
   
   This patch is simpler because this doesn't take "restarting SHS" into 
account. Restarting SHS will lose the information. And for now we may not want 
to tracking line offset in SHS's KV store (`listing`) since the line offset is 
only effective during single run of SHS.
   
   When you consider restoring KV store & state of listeners during restarting 
of SHS, you will have to store the snapshot of KV store into somewhere (that's 
why SPARK-29111 came in) and then you have to concern about compatibility of 
snapshot (entities in KV store including live entities on listeners) across 
Spark versions. That's why I had to change the design and introduce SPARK-29779 
instead of snapshotting.
   
   We've already gone through bunch of discussions because it is not that 
simple in reality as it seems; so please go through these PRs as well as design 
docs. 
   
   I guess the patch can be reviewed right now if the community prefers to have 
a solution which works within single SHS run first (though this may conflict 
with compaction #26416 which needs some arrangement), but if the community 
wants to have a solution which covers more cases, SPARK-28870 seems to be the 
way to go. (It doesn't mean this patch will not be valid - this patch could 
cover SPARK-29261 with some modification.)

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