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https://issues.apache.org/jira/browse/CASSANDRA-8099?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14583553#comment-14583553
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Benedict commented on CASSANDRA-8099:
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bq. Yes, it would be cleaner from that standpoint. And the use of iterators in
the first place is indeed largely carried from the existing code, I just hadn't
really though of the alternative tbh. I'll try to check next week how easily
such change is. That said, I'm not sure the use of iterators directly is that
confusing either, so if it turns hairy, I don't think it's worth blocking on
this (that is, we can very well change that later).
It does change the semantics quite a bit, since the state needed for iterating
must be constructed again each time, and is likely constructed in the caller of
.iterator(). This has both advantages and disadvantages. One advantage of an
Iterator, though, is that you cannot (easily) iterate over its contents twice.
I'm personally not so upset at the use of Iterator, since it's a continuation
of the existing approach, and Java 8 makes working with iterators a little
easier. We can, for instance, make use of the forEachRemaining() method, or
otherwise transform the iterator. I don't think there's any increased ugliness
inherent in exposing the higher-level information in the Iterator, though.
I believe [~iamaleksey] is working on a way to integrate the Java Streams API
at some point in the future, which may lead to other benefits that Iterable
cannot deliver.
Either way, I think getting this ticket in sooner than later is better, and we
can focus on how we might make the Iterator abstraction a little nicer in a
follow up.
> 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 beta 1
>
> 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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