>
> This will cancel all current queries with a
> BlurException of type BACK_PRESSURE, the controller will automatically
> retry the queries that were cancelled


This is a real nice feature and most handy in production too, as a
protection gainst both intentional and un-intentional bad queries


 Since every update to the index via thrift mutates are committed and syncedto
> HDFS, it possible to have other shards follow the lead shard (the

writer)

I think I am back to data-locality we previously discussed with your "lead
shard" remark. It is now possible to pin data-nodes to a file during writes
in hadoop. https://issues.apache.org/jira/browse/HDFS-2576

We can always pin a shard to 3 shard-servers [assuming shard-server is run
in datanodes] and persist this ZK. Failures can be easily handled using
this info

The big advantage of this method is, even in case of failures short-circuit
reads are always utilized. I know you are not a big-fan of it, but still is
it worthwhile to pursue it?

One downside to this approach is, cluster balance might get skewed and need
to be properly handled in HDFS

--
Ravi

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