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https://issues.apache.org/jira/browse/CASSANDRA-14459?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16506659#comment-16506659
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Joseph Lynch edited comment on CASSANDRA-14459 at 6/8/18 11:36 PM:
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I think that we need some way to get latency measurements for hosts that have
been excluded from traffic due to high minimums. For example if during the
initial {{PingMessages}} a local DC host gets a very high measurement (e.g.
100ms) we will never send traffic to it ever. My understanding is that's why we
reset in the first place.
I'll try to come up with a solution that doesn't involve additional traffic.
was (Author: jolynch):
I think that we need some way to get latency measurements for hosts that have
been excluded from traffic due to high minimums. For example if during the
initial {{PingMessages}} a local DC host gets a very high measurement (e.g.
100ms) we will never send traffic to it ever. My understanding is that's why we
reset in the first place.
I'll work on a feedback mechanism for the {{DES}} to ask for latency probes
(which I guess would be best implemented as {{PingMessages}} since you're
concerned about {{EchoMessages}}). I see possible two designs: one where I send
the probes directly from the {{DES}} or I can have a method expressing the
desire for probes that propagates up to e.g. the {{MessagingService}}. Are
there better options?
> 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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