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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-2897) Secondary indexes without read-before-write
Date Thu, 30 Aug 2012 14:00:17 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-2897?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13444965#comment-13444965
] 

Jonathan Ellis edited comment on CASSANDRA-2897 at 8/31/12 12:59 AM:
---------------------------------------------------------------------

bq. the test was definitely failing until I got dummyColumn figured out 

FTR, the problem was

{{IColumn dummyColumn = new Column(liveColumn.name(), column.value(), column.timestamp());}}

Here, {{column}} is the column from the index row, so column.value is always empty.  To actually
delete the old index entry we need to use the column value that was indexed:

{{IColumn dummyColumn = new Column(baseColumnName, indexedValue, column.timestamp());}}

There was a corresponding bug for the non-composite case as well.

(Edit: liveColumn.name() == baseColumnName, just thought the latter was a bit more clear.)
                
      was (Author: jbellis):
    bq. the test was definitely failing until I got dummyColumn figured out 

FTR, the problem was

{{IColumn dummyColumn = new Column(liveColumn.name(), column.value(), column.timestamp());}}

Here, {{column}} is the column from the index row, so column.value is always empty.  To actually
delete the old index entry we need to use the column value that was indexed:

{{IColumn dummyColumn = new Column(baseColumnName, indexedValue, column.timestamp());}}

There was a corresponding bug for the non-composite case as well.
                  
> 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
>            Assignee: Sam Tunnicliffe
>            Priority: Minor
>              Labels: secondary_index
>             Fix For: 1.2.0 beta 1
>
>         Attachments: 0001-CASSANDRA-2897-Secondary-indexes-without-read-before-w.txt,
0002-CASSANDRA-2897-Secondary-indexes-without-read-before-w.txt, 0003-CASSANDRA-2897.txt,
2897-apply-cleanup.txt, 2897-v4.txt, 41ec9fc-2897.txt
>
>
> 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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