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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8099) Refactor and modernize the storage engine
Date Fri, 13 Mar 2015 22:21:47 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8099?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14361197#comment-14361197
] 

Sylvain Lebresne commented on CASSANDRA-8099:
---------------------------------------------

I've just rebased and (force) pushed the current version of the patch to [the usual branch|https://github.com/pcmanus/cassandra/tree/8099_engine_refactor].
 It still doesn't handle thrift and misses backward compatibility code for the internal messages
(and I'll start working on those) but it's basically complete otherwise. It passes all the
CQL tests (unit and dtests) we have in particular.  Also seems to be passing other dtests
(that don't use thrift) but I'll admit I haven't had the patience to run them all locally
and jenkins since to be in a bad mood recently, so a couple might require attention but that's
likely minor.  Also, I haven't taken the time to upgrade most of our unit tests and this will
be done next with the help of some others, but hopefully the CQL tests and dtests exercise
the code changed enough that they shouldn't be major surprises.

Overall the missing parts are sufficiently isolated that I think initial review can be started.
I've actually written [here|https://github.com/pcmanus/cassandra/blob/8099_engine_refactor/guide_8099.md]
some kind of overview/guide for the sake of making diving into the patch easier.  I'll be
happy to update it if there is something missing that would help.


> 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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