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https://issues.apache.org/jira/browse/CASSANDRA-14459?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16551064#comment-16551064
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Joseph Lynch commented on CASSANDRA-14459:
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Ok based on the feedback I've moved the latency probes into a scheduled
executor that runs every second and sends a single message to one node needing
latency probes. If no probes are needed then they are not sent. I've gone a
step further and posted a follow up commit which replaces all the uses of
histograms with a simpler (and 10x more performant) EMA implementation. The
second patch is very optional but I know that some folks have mentioned they
don't like all the garbage that the Histograms create so I fixed it.
||trunk||trunk+EMA||
|[patch|https://github.com/apache/cassandra/compare/trunk...jolynch:CASSANDRA-14459]|[patch|https://github.com/apache/cassandra/compare/trunk...jolynch:CASSANDRA-14459-EMA]|
|[unit
tests|https://github.com/apache/cassandra/compare/trunk...jolynch:CASSANDRA-14459#diff-f1baf62fd510947bf60e9a9def69810b]
[!https://circleci.com/gh/jolynch/cassandra/tree/CASSANDRA-14459.png?circle-token=
1102a59698d04899ec971dd36e925928f7b521f5!|https://circleci.com/gh/jolynch/cassandra/tree/CASSANDRA-14459]|[unit
tests|https://github.com/apache/cassandra/compare/trunk...jolynch:CASSANDRA-14459-EMA#diff-f1baf62fd510947bf60e9a9def69810b]
[microbenchmark|https://github.com/apache/cassandra/compare/trunk...jolynch:CASSANDRA-14459-EMA#diff-f48455469e3cd6a2e225e3cb2b389d93]
[!https://circleci.com/gh/jolynch/cassandra/tree/CASSANDRA-14459.png?circle-token=
1102a59698d04899ec971dd36e925928f7b521f5!|https://circleci.com/gh/jolynch/cassandra/tree/CASSANDRA-14459-EMA]|
> DynamicEndpointSnitch should never prefer latent nodes
> ------------------------------------------------------
>
> Key: CASSANDRA-14459
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14459
> Project: Cassandra
> Issue Type: Improvement
> Components: Coordination
> Reporter: Joseph Lynch
> Assignee: Joseph Lynch
> Priority: Minor
> 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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