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https://issues.apache.org/jira/browse/CASSANDRA-8099?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14358672#comment-14358672
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Benedict commented on CASSANDRA-8099:
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I think it's a shame this patch wasn't attempted at least a little more
incrementally. It looks to me that changing the serialization formats,
memtables and iterator implementations could have been done in different
patches at least, and it might have made review safer and easier, and given us
more time to digest the changes. I understand that this might have introduced
some extra development burden, although that might have been reaped back in
many fewer days spent rebasing. Having an initial patch to change the
abstractions I think would have helped to reduce the burden on the rest of the
project, and perhaps helped parallelize the work as well.
I'm a little concerned that the result is that we won't give each of the pretty
major decisions that have been made the time they need to be assessed properly,
especially now we're ramping up for release (and hence low on time). I'm not
necessarily suggesting we split it, as I can imagine that would be soul
crushingly unpleasant for [~slebresne] and introduce a delay, but I am
generally ill at ease with the scope of the changes and our ability to vet
them. I'm also worried I'm finding myself saying "too close to release to
question this decision" - which seems a problematic mode to be merging any
patch under.
> Refactor and modernize the storage engine
> -----------------------------------------
>
> Key: CASSANDRA-8099
> URL: https://issues.apache.org/jira/browse/CASSANDRA-8099
> Project: Cassandra
> Issue Type: Improvement
> Reporter: Sylvain Lebresne
> Assignee: Sylvain Lebresne
> Fix For: 3.0
>
> Attachments: 8099-nit
>
>
> The current storage engine (which for this ticket I'll loosely define as "the
> code implementing the read/write path") is suffering from old age. One of the
> main problem is that the only structure it deals with is the cell, which
> completely ignores the more high level CQL structure that groups cell into
> (CQL) rows.
> This leads to many inefficiencies, like the fact that during a reads we have
> to group cells multiple times (to count on replica, then to count on the
> coordinator, then to produce the CQL resultset) because we forget about the
> grouping right away each time (so lots of useless cell names comparisons in
> particular). But outside inefficiencies, having to manually recreate the CQL
> structure every time we need it for something is hindering new features and
> makes the code more complex that it should be.
> Said storage engine also has tons of technical debt. To pick an example, the
> fact that during range queries we update {{SliceQueryFilter.count}} is pretty
> hacky and error prone. Or the overly complex ways {{AbstractQueryPager}} has
> to go into to simply "remove the last query result".
> So I want to bite the bullet and modernize this storage engine. I propose to
> do 2 main things:
> # Make the storage engine more aware of the CQL structure. In practice,
> instead of having partitions be a simple iterable map of cells, it should be
> an iterable list of row (each being itself composed of per-column cells,
> though obviously not exactly the same kind of cell we have today).
> # Make the engine more iterative. What I mean here is that in the read path,
> we end up reading all cells in memory (we put them in a ColumnFamily object),
> but there is really no reason to. If instead we were working with iterators
> all the way through, we could get to a point where we're basically
> transferring data from disk to the network, and we should be able to reduce
> GC substantially.
> Please note that such refactor should provide some performance improvements
> right off the bat but it's not it's primary goal either. It's primary goal is
> to simplify the storage engine and adds abstraction that are better suited to
> further optimizations.
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