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From "Brandon Williams (JIRA)" <j...@apache.org>
Subject [jira] Commented: (CASSANDRA-981) Dynamic endpoint snitch
Date Thu, 27 May 2010 21:35:37 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-981?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12872380#action_12872380
] 

Brandon Williams commented on CASSANDRA-981:
--------------------------------------------

It looks like the phi algorithm can work for this.  It's simple to replace timestamps with
latency values, and then use an arbitrary value when calculating the phi score for a host.
 The main problem is calculating phi is less than free, and doing so for each request to determine
the least latent endpoint will be too expensive.  We can get around that by doing it up front
when we add a sample and keeping a cache of the scores, but that means we'll have to add samples
fairly infrequently, or at least batch the updates so the mean only has to be calculated once
per batch.

> Dynamic endpoint snitch
> -----------------------
>
>                 Key: CASSANDRA-981
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-981
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: Core
>            Reporter: Jonathan Ellis
>             Fix For: 0.7
>
>
> An endpoint snitch that automatically and dynamically infers "distance" to other machines
without having to explicitly configure rack and datacenter positions solves two problems:
> The killer feature here is adapting to things like compaction or a failing-but-not-yet-dead
disk.  This is important, since when we are doing reads we pick the "closest" replica for
actually reading data from (and only read md5s from other replicas).  This means that if the
closest replica by network topology is temporarily slow due to compaction (for instance),
we'll have to block for its reply even if we get the other replies much much faster.
> Not having to manually re-sync your configuration with your network topology when changes
(adding machines) are made is a nice bonus.

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