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From "Michael Kjellman (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-14293) Speculative Retry Policy Should Support Specifying MIN/MAX of 2 PERCENTILE and FIXED Policies
Date Wed, 07 Mar 2018 06:05:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-14293?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16389067#comment-16389067
] 

Michael Kjellman commented on CASSANDRA-14293:
----------------------------------------------

Ideally we'd express this a bit nicer by having the speculative_retry table config option
as a map, but we're bound by legacy here and have no way to migrate and change this with the
current state of schema. So, ultimately we'll still need to do a bunch of string parsing but
we can at least get the functionality we need with the following function-esque syntax (examples):

max(99.9p,50ms)

MIN(50MS,90PERCENTILE)

> Speculative Retry Policy Should Support Specifying MIN/MAX of 2 PERCENTILE and FIXED
Policies
> ---------------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-14293
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-14293
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Michael Kjellman
>            Assignee: Michael Kjellman
>            Priority: Major
>
> Currently the Speculative Retry Policy takes a single string as a parameter, this can
be NONE, ALWAYS, 99PERCENTILE (PERCENTILE), 50MS (CUSTOM).
> The problem we have is when a single host goes into a bad state this drags up the percentiles.
This means if we are set to use p99 alone, we might not speculate when we intended to to
because the value at the specified percentile has gone so high.
> As a fix we need to have support for something like min(99percentile,50ms)
> this means if the normal p99 for the table is <50ms, we will still speculate at this
value and not drag the happy path tail latencies up... but if the p99th goes above what we
know we should never exceed we use that instead.



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