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From "Daniel Doubleday (Commented) (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-1956) Convert row cache to row+filter cache
Date Wed, 02 Nov 2011 16:35:42 GMT


Daniel Doubleday commented on CASSANDRA-1956:

As I wrote earlier I'm a little sceptical that a query cache like this will be useful in many
cases but since there is something going on here and Jonathan asked for a wish list:

Would you guys consider making the row cache a little more pluggable? This would allow us
to maintain custom implementation more easily. Also I think that the core code could benefit
as well moving some ifs out of CFS.

Instead of implementing the control flow in CFS and using the cache as a map you could introduce
an RowCache instance that would act more like a service layer like:

interface RowCache {

    // returns filtered rows - ready to serve. reads the row from cfs if necessary.
    ColumnFamily getRow(CFS store, QueryFilter filter, int gcBefore);
    // notify the cache of a mutation. it can update or invalidate 
    void apply(CFS store, DK key, CF cf);

This way CFS would need no knowledge wether a cache is able to update or only invalidate.
And when it invalidates wether it has to invalidate the row or just portions of it. Also there
would be no expectation about the internal caching format. The row cache could do whatever
it likes. 

In CFS there would be only the cache reference. No distinction between old row cache, query
cache, off-heap-cache, my-awesome-very-specialized-cache would be necessary.
> Convert row cache to row+filter cache
> -------------------------------------
>                 Key: CASSANDRA-1956
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Stu Hood
>            Assignee: Vijay
>            Priority: Minor
>             Fix For: 1.1
>         Attachments: 0001-1956-cache-updates-v0.patch, 0001-row-cache-filter.patch, 0002-1956-updates-to-thrift-and-avro-v0.patch
> Changing the row cache to a row+filter cache would make it much more useful. We currently
have to warn against using the row cache with wide rows, where the read pattern is typically
a peek at the head, but this usecase would be perfect supported by a cache that stored only
columns matching the filter.
> Possible implementations:
> * (copout) Cache a single filter per row, and leave the cache key as is
> * Cache a list of filters per row, leaving the cache key as is: this is likely to have
some gotchas for weird usage patterns, and it requires the list overheard
> * Change the cache key to "rowkey+filterid": basically ideal, but you need a secondary
index to lookup cache entries by rowkey so that you can keep them in sync with the memtable
> * others?

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