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From "Jeremy Hanna (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-2897) Secondary indexes without read-before-write
Date Fri, 11 May 2012 01:59:48 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-2897?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13272981#comment-13272981
] 

Jeremy Hanna commented on CASSANDRA-2897:
-----------------------------------------

That said - our experience may be unique with the 3 secondary indexes.  It makes sense it
would affect the write path but it was surprising that 3 would have *that* much of an effect.
 Also the proposed implementation seems to be closer to the philosophy of Cassandra generally
- crazy fast write path (or simply don't slow the write path down), then sort out some of
it on the read path.
                
> Secondary indexes without read-before-write
> -------------------------------------------
>
>                 Key: CASSANDRA-2897
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-2897
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 0.7.0
>            Reporter: Sylvain Lebresne
>            Priority: Minor
>              Labels: secondary_index
>
> Currently, secondary index updates require a read-before-write to maintain the index
consistency. Keeping the index consistent at all time is not necessary however. We could let
the (secondary) index get inconsistent on writes and repair those on reads. This would be
easy because on reads, we make sure to request the indexed columns anyway, so we can just
skip the row that are not needed and repair the index at the same time.
> This does trade work on writes for work on reads. However, read-before-write is sufficiently
costly that it will likely be a win overall.
> There is (at least) two small technical difficulties here though:
> # If we repair on read, this will be racy with writes, so we'll probably have to synchronize
there.
> # We probably shouldn't only rely on read to repair and we should also have a task to
repair the index for things that are rarely read. It's unclear how to make that low impact
though.

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