David Alves updated KUDU-1703:
    Issue Type: Sub-task  (was: Improvement)
        Parent: KUDU-430

> Handle lagging replicas for snapshot reads
> ------------------------------------------
>                 Key: KUDU-1703
>                 URL: https://issues.apache.org/jira/browse/KUDU-1703
>             Project: Kudu
>          Issue Type: Sub-task
>    Affects Versions: 1.1.0
>            Reporter: David Alves
> When we fix safe time advancement, replicas will start to block on snapshot 
> scans for arbitrary amounts of time, waiting to have a consistent view of the 
> world at that timestamp before serving the scan.
> This will be a serious problem for lagging replicas, which might be several 
> seconds or even minutes behind. Moreover in the absence of writes, the same 
> will happen even for non-lagging replicas, which will have their safe times 
> updated only when the leader heartbeats.
> We need to at least make sure that:
> - Blocked scanner threads are not starving other work.
> - If the replica's safe time is lagging by a lot, the replica refuses to do 
> the scan.
> We might also consider other optimizations (like pinging the leader).

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