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From "Jason Brown (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13289) Make it possible to monitor an ideal consistency level separate from actual consistency level
Date Fri, 10 Mar 2017 18:48:04 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13289?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15905541#comment-15905541
] 

Jason Brown commented on CASSANDRA-13289:
-----------------------------------------

Some thoughts:

- maybe only instantiate {{AbstractWriteResponseHandler#responsesAndExpirations}} in {{#setIdealCLResponseHandler()}},
and thus only create the {{AtomicInteger}} when you know you are actually going to use it.
- if the ideal CL and the requested CL are the same, should we even bother capturing metrics
about it? I'm kinda mixed on it...
- what happens if the user mixes non-CAS consistency levels with CAS consistency levels (or
vice versa)? I think the behavior will be correct (we won't inadvertantly violate paxos semantics),
but the semantic difference between CAS and non-CAS requests might not be meaningful. So perhaps
ignore the idealCl if the CL types are different? wdyt?
- how will timed out message metrics be affected? We create an entry in {{MessagingService#callbacks}}
for each peer contacted for an operation (just talking reads/mutations right now), and say
the request CL is satisfied, but the idealCL doesn't hear back from some nodes. In that case
we'll increment the timeouts, {{ConnectionMetrics.totalTimeouts.mark()}}, even though they
weren't explicitly part of the user's request. It might be confusing to users or operators.
I'm not sure how hard it is to code around that, or if it's worthwhile. If we feel it's not,
perhaps we just document it in the yaml that "you may see higher than usual timeout counts".
Thoughts?
- calling it "ideal consistency level" doesn't sound quite right. Maybe something like "alternative"
or "secondary" might work. It might be good to point out that the emphasis here should be
on discovering the latencies a different CL would bring, and not necessarily the impact on
data consistency itself.



> Make it possible to monitor an ideal consistency level separate from actual consistency
level
> ---------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-13289
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13289
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: Core
>            Reporter: Ariel Weisberg
>            Assignee: Ariel Weisberg
>             Fix For: 4.0
>
>
> As an operator there are several issues related to multi-datacenter replication and consistency
you may want to have more information on from your production database.
> For instance. If your application writes at LOCAL_QUORUM how often are those writes failing
to achieve EACH_QUORUM at other data centers. If you failed your application over to one of
those data centers roughly how inconsistent might it be given the number of writes that didn't
propagate since the last incremental repair?
> You might also want to know roughly what the latency of writes would be if you switched
to a different consistency level. For instance you are writing at LOCAL_QUORUM and want to
know what would happen if you switched to EACH_QUORUM.
> The proposed change is to allow an ideal_consistency_level to be specified in cassandra.yaml
as well as get/set via JMX. If no ideal consistency level is specified no additional tracking
is done.
> if an ideal consistency level is specified then the {{AbstractWriteResponesHandler}}
will contain a delegate WriteResponseHandler that tracks whether the ideal consistency level
is met before a write times out. It also tracks the latency for achieving the ideal CL  of
successful writes.
> These two metrics would be reported on a per keyspace basis.



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