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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8099) Refactor and modernize the storage engine
Date Fri, 12 Jun 2015 15:16:05 GMT

    [ 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:
-------------------------------------

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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