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Subject [GitHub] [zookeeper] lvfangmin commented on issue #1047: [ZOOKEEPER-3500] Improving the ZAB UPTODATE semantic to only issue it to learner when there is limited lagging
Date Mon, 19 Aug 2019 17:28:50 GMT
lvfangmin commented on issue #1047: [ZOOKEEPER-3500] Improving the ZAB UPTODATE semantic to
only issue it to learner when there is limited lagging
URL: https://github.com/apache/zookeeper/pull/1047#issuecomment-522676267
 
 
   @hanm to the previous comment:
   
   >>>> I think these are the transactions queued on learner while SNAP sync is
happening?
   
   Those are the txns queued on leader while learner took snapshot when received NEWLEADER
packet.
   
   >>>> Does the stale data cause any issues? I think this is an optimization
issue instead of a correctness issue, and a client will never end up in a case where it fails
to see its last write when connect to a sync in flight server, and zookeeper's sequential
consistency is still guaranteed. If so, wondering why seeing stale data would be an issue
if the servers will finally converge?
   
   It's not correctness issue, it's just how far behind the server is when it started to serve
client traffic. If the client has last seen zxid, then it's fine, it will try to connect to
a different one, but for new clients who don't have this information (like new clients), they
may read stale data.
   
   And another improvement with this change is that we can decouple the sync timeout with
the data size and traffic, otherwise it may take the learner more than sync timeout to replay
those txns and cannot finish SYNC.
   
   >>>> I think with this change the clients will have to expect a little bit
longer recovery time in certain cases - was there any concerns / discussions around this increased
recovery time on client side and the implications?
   
   In general, this won't affect the quorum up time, but it may take longer time for the minority
server who is lagging behind too much.

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