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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-9708) Serialize ClusteringPrefixes in batches
Date Thu, 09 Jul 2015 19:41:04 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-9708?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14621131#comment-14621131
] 

Benedict commented on CASSANDRA-9708:
-------------------------------------

This ticket utilises the vint encoding, and by doing so ironically reduces the size of some
serializations so that they are < 9 bytes, triggering assertion errors stating we haven't
enough room to read vints. 

I've opted to fix this problem in this ticket, since it's the first cause of the issue, but
the approach warrants discussion. [~aweisberg]: your input would be welcome. I've opted to
simply extend the buffer to at least 9 bytes, since, on reading, we expect to always know
when we're done consuming an input (so having some extra bytes would not be a problem), and
by reading too much we cannot negatively impact anyone using the portion of the buffer we
extend into. This seems safe to me, but obviously if we ever depend on EOF for safe consumption
this will break things. I doubt this is something we would ever depend upon, though, since
it would be prone to corruption.

> Serialize ClusteringPrefixes in batches
> ---------------------------------------
>
>                 Key: CASSANDRA-9708
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-9708
>             Project: Cassandra
>          Issue Type: Sub-task
>          Components: Core
>            Reporter: Benedict
>            Assignee: Benedict
>             Fix For: 3.0.0 rc1
>
>
> Typically we will have very few clustering prefixes to serialize, however in theory they
are not constrained (or are they, just to a very large number?). Currently we encode a fat
header for all values up front (two bits per value), however those bits will typically be
zero, and typically we will have only a handful (perhaps 1 or 2) of values.
> This patch modifies the encoding to batch the prefixes in groups of up to 32, along with
a header that is vint encoded. Typically this will result in a single byte per batch, but
will consume up to 9 bytes if some of the values have their flags set. If we have more than
32 columns, we just read another header. This means we incur no garbage, and compress the
data on disk in many cases where we have more than 4 clustering components.
> I do wonder if we shouldn't impose a limit on clustering columns, though: If you have
more than a handful merge performance is going to disintegrate. 32 is probably well in excess
of what we should be seeing in the wild anyway.



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