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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-5417) Push composites support in the storage engine
Date Tue, 02 Apr 2013 16:21:15 GMT
Sylvain Lebresne created CASSANDRA-5417:
-------------------------------------------

             Summary: Push composites support in the storage engine
                 Key: CASSANDRA-5417
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-5417
             Project: Cassandra
          Issue Type: Improvement
            Reporter: Sylvain Lebresne
            Assignee: Sylvain Lebresne
             Fix For: 2.0


CompositeType happens to be very useful and is now widely used: CQL3 heavily rely on it, and
super columns are now using it too internally. Besides, CompositeType has been advised as
a replacement of super columns on the thrift side for a while, so it's safe to assume that
it's generally used there too.

CompositeType has initially been introduced as just another AbstractType.  Meaning that the
storage engine has no nothing whatsoever of composites being, well, composite. This has the
following drawbacks:
* Because internally a composite value is handled as just a ByteBuffer, we end up doing a
lot of extra work. Typically, each time we compare 2 composite value, we end up "deserializing"
the components (which, while it doesn't copy data per-se because we just slice the global
ByteBuffer, still waste some cpu cycles and allocate a bunch of ByteBuffer objects). And since
compare can be called *a lot*, this is likely not negligible.
* This make CQL3 code uglier than necessary. Basically, CQL3 makes extensive use of composites,
and since it gets backs ByteBuffer from the internal columns, it always have to check if it's
actually a compositeType or not, and then split it and pick the different parts it needs.
It's only an API problem, but having things exposed as composites directly would definitively
make thinks cleaner. In particular, in most cases, CQL3 don't care whether it has a composite
with only one component or a non-really-composite value, but we still always distinguishes
both cases.  Lastly, if we do expose composites more directly internally, it's not a lot more
work to "internalize" better the different parts of the cell name that CQL3 uses (what's the
clustering key, what's the actuall CQL3 column name, what's the collection element), making
things cleaner. Last but not least, there is currently a bunch of places where methods take
a ByteBuffer as argument and it's hard to know whether it expects a cell name or a CQL3 column
name. This is pretty error prone.
* It makes it hard (or impossible) to do a number of performance improvements.  Consider CASSANDRA-4175,
I'm not really sure how you can do it properly (in memory) if cell names are just ByteBuffer
(since CQL3 column names are just one of the component in general). But we also miss oportunities
of sharing prefixes. If we were able to share prefixes of composite names in memory we would
1) lower the memory footprint and 2) potentially speed-up comparison (of the prefixes) by
checking reference equality first (also, doing prefix sharing on-disk, which is a separate
concern btw, might be easier to do if we do prefix sharing in memory).

So I suggest pushing CompositeType support inside the storage engine. What I mean by that
concretely would be change the internal {{Column.name}} from ByteBuffer to some CellName type.
A CellName would API-wise just be a list of ByteBuffer. But in practice, we'd have a specific
CellName implementation for not-really-composite names, and the truly composite implementation
will allow some prefix sharing. From an external API however, nothing would change, we would
pack the composite as usual before sending it back to the client, but at least internally,
comparison won't have to deserialize the components every time, and CQL3 code will be cleaner.


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