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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7296) Add CL.COORDINATOR_ONLY
Date Mon, 10 Oct 2016 09:31:21 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7296?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15561789#comment-15561789
] 

Sylvain Lebresne commented on CASSANDRA-7296:
---------------------------------------------

I can certainly agree that having the dynamic snitch getting in the way of local queries is
not always desirable, but I'm less convinced that adding a new consistency level is the cleanest
way to circumvent that.

First, I'm not even entirely sure than letting the dynamic snitch bypass the coordinator if
it's a replica is a good idea in the first place. Everyone more or less agree that doing token-aware
routing is a good thing nowadays, and it's certainly confusing that the dynamic snitch may
screw that up. If the dynamic snitch was a perfect and instantaneous view of latencies, then
that could make sense, but it's not. Anyway, I think it's worth at least evaluating making
even the dynamic snitch always pick the local node if it's a replica, as I'm not sure the
benefit of not doing so outweigh the confusion it creates.

But short of that, why not directly attack the problem we're trying to solve and add a (protocol)
option to queries to force the behavior ("always pick the coordinator as one replica if it's
one"). That sounds less confusing to me than have a new CL that will confuse newcomers (as
the difference with ONE is somewhat subtle for a newcomer). As a bonus, it would also work
for CL > ONE (since again, it'll just be about forcing the dynamic snitch to pick the coordinator
if it's a replica).

We could also have a table option to do the same: force the dynamic snitch to pick the coordinator
if it's a replica for all queries on that table, which would be a tad more convenient for
request pinning (of course, I get that for troubleshooting you still want to per-query option).


> Add CL.COORDINATOR_ONLY
> -----------------------
>
>                 Key: CASSANDRA-7296
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7296
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Tupshin Harper
>
> For reasons such as CASSANDRA-6340 and similar, it would be nice to have a read that
never gets distributed, and only works if the coordinator you are talking to is an owner of
the row.



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