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From "Jonathan Ellis (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-2897) Secondary indexes without read-before-write
Date Sun, 25 Mar 2012 05:24:39 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-2897?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13237782#comment-13237782
] 

Jonathan Ellis commented on CASSANDRA-2897:
-------------------------------------------

Doug added over on the Hypertable post,

bq. In Hypertable, the way deletes are handled is by inserting delete records (tombstones),
so during compaction the secondary index is purged of stale entries by bulk inserting a bunch
of delete records. Since Hypertable is essentially a LSM tree, bulk inserts are very efficient
and require no random i/o.

I think I understand: it generates both new, correct index entries, AND tombstones for old,
invalid ones (the column entries in the parent CF that get discarded during compaction).

That's a fair bit of work for us, to change compaction to expose more than just the surviving
value, but doable.

I like this idea, it should lower the overhead of indexes a lot, even for SSD deployments.
 (The read-before-write that the current implementation requires extra locking, as well as
the read itself.)
                
> 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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