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https://issues.apache.org/jira/browse/CASSANDRA-14459?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16501791#comment-16501791
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Jason Brown commented on CASSANDRA-14459:
-----------------------------------------
Took a quick look, and on the whole I like where this goes. The biggest problem
I have is {{DynamicEndpointSnitch#reset(boolean)}} gets a snapshot of the
existing {{reservoir}}. Unfortunately, generating that that snapshot is *very*
heavyweight (copies a bunch of data, creates a bunch of garbage), just to get
one value and then throw it all away. You should consider a different mechanism
for getting the min value. Perhaps instead of {{DynamicEndpointSnitch#samples}}
being defined as {{ConcurrentHashMap<InetAddressAndPort,
ExponentiallyDecayingReservoir>}}, maybe the value class is something like:
{code}
static class Holder
{
private final ExponentiallyDecayingReservoir res;
private volatile long minValue;
void update(long val)
{
res.update(...)
// It's probably ok if there's a race on minValue. better a
small/irrelevant race than any real coordination
if (val < minValue)
minValue = val;
}
}
{code}
Suit to taste, if you find this useful.
> 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
>
> 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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