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From "C. Scott Andreas (JIRA)" <>
Subject [jira] [Updated] (CASSANDRA-14459) DynamicEndpointSnitch should never prefer latent nodes
Date Fri, 20 Jul 2018 20:46:00 GMT


C. Scott Andreas updated CASSANDRA-14459:
    Labels: 4.0-feature-freeze-review-requested  (was: )

> DynamicEndpointSnitch should never prefer latent nodes
> ------------------------------------------------------
>                 Key: CASSANDRA-14459
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Coordination
>            Reporter: Joseph Lynch
>            Assignee: Joseph Lynch
>            Priority: Minor
>              Labels: 4.0-feature-freeze-review-requested
>             Fix For: 4.x
> The DynamicEndpointSnitch has two unfortunate behaviors that allow it to provide latent
hosts as replicas:
>  # Loses all latency information when Cassandra restarts
>  # Clears latency information entirely every ten minutes (by default), allowing global queries
to be routed to _other datacenters_ (and local queries cross racks/azs)
> This means that the first few queries after restart/reset could be quite slow compared
to average latencies. I propose we solve this by resetting to the minimum observed latency
instead of completely clearing the samples and extending the {{isLatencyForSnitch}} idea to
a three state variable instead of two, in particular {{YES}}, {{NO}}, {{MAYBE}}. This extension
allows {{EchoMessages}} and {{PingMessages}} to send {{MAYBE}} indicating that the DS should
use those measurements if it only has one or fewer samples for a host. This fixes both problems
because on process restart we send out {{PingMessages}} / {{EchoMessages}} as part of startup,
and we would reset to effectively the RTT of the hosts (also at that point normal gossip {{EchoMessages}} have
an opportunity to add an additional latency measurement).
> This strategy also nicely deals with the "a host got slow but now it's fine" problem
that the DS resets were (afaik) designed to stop because the {{EchoMessage}} ping latency
will count only after the reset for that host. Ping latency is a more reasonable lower bound
on host latency (as opposed to status quo of zero).

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