[jira] [Commented] (CASSANDRA-14459) DynamicEndpointSnitch should never prefer latent nodes

["Blake Eggleston (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-14459?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16840757#comment-16840757 ] 

Blake Eggleston commented on CASSANDRA-14459:
---------------------------------------------

I finished my first round of review of the implementation. I still need to spend some time looking at the tests and docs.

Some general notes:
 * I don't think we need a dynamicsnitch package, the 2 implementations could just live in the locator package.
 * I can't find anywhere that we validate the various dynamic snitch values? I didn't look super hard, but if we're not, it would be good to check people are using sane values on startup and jmx update.
 * It's probably a good time to rebase onto current trunk.

ScheduledExecutors:
 * {{getOrCreateSharedExecutor}} could be private
 * shutdown times accumulate in the wait section. ie: if we have 2 executors with a shutdown time of 2 seconds, we'll wait up to 4 seconds for them to stop.
 * we should validate shutdown hasn't been called in {{getOrCreateSharedExecutor}}
 * I don't see any benefit to making this class lock free. Making the {{getOrCreateSharedExecutor}} and {{shutdownAndWait}} methods synchronized and just using a normal hash map for {{executors}} would make this class easier to reason about. As it stands, you could argue there's some raciness with creating and shutting down at the same time, although it's unlikely to be a problem in practice. I do think I might be borderline nit picking here though.

StorageService
 * {{doLocalReadTest}} method is unused

MessagingService
 * Fix class declaration indentation
 * remove unused import

DynamicEndpointSnitch
 * I don't think we need to check {{logger.isTraceEnabled}} before calling {{logger.trace()}}? At least I don't see any toString computation that would happen in the calls.
 * The class hierarchy could be improved a bit. There's code in {{DynamicEndpointSnitchHistogram}} that's also used in the legacy snitch, and code in {{DynamicEndpointSnitch}} that's only used in {{DynamicEndpointSnitchHistogram}}. The boundary between DynamicEndpointSnitch and DynamicEndpointSnitchHistogram in particular feels kind of arbitrary.

DynamicEndpointLegacySnitch
 * If we're going to keep the old behavior around as a failsafe (and we should), I think we should avoid improving it by changing the reset behavior. Only resetting under some situations intuitively feels like the right thing to do, but it would suck if there were unforeseen problems with it that made it a regression from the 3.x behavior.

> DynamicEndpointSnitch should never prefer latent nodes
> ------------------------------------------------------
>
>                 Key: CASSANDRA-14459
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-14459
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Legacy/Coordination
>            Reporter: Joseph Lynch
>            Assignee: Joseph Lynch
>            Priority: Low
>              Labels: 4.0-feature-freeze-review-requested, pull-request-available
>             Fix For: 4.x
>
>          Time Spent: 25.5h
>  Remaining Estimate: 0h
>
> 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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