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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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