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From "Sylvain Lebresne (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-5417) Push composites support in the storage engine
Date Mon, 25 Nov 2013 10:53:37 GMT


Sylvain Lebresne commented on CASSANDRA-5417:

Thanks for the review.

bq. 0xFF looks suspicious to me - should perhaps be 0xFFFF?

Correct, good catch (pushed a fix to the same branch as before).

bq. It looks like we're interning TimeUUIDType column identifiers

I don't think that's the case but we may have to be a little more precise here.  Bug notwithstanding,
we only intern names that are part of the column_metadata in thrift parlance. Now thrift does
let you declare a column_metadata that is a timeUUID (it's useless and probably nobody does
it, but it's possible) but even in that case, we'll only intern the timeUUID that were declared.
We do not intern dynamic names in particular if that is what you meant (contrarily to the
current code I might add).

bq. CellName is confusing, as it isn't a name-of-a-cell, but a Cell-with-a-name

Well, actually no. It *is* a name-of-a-cell. At that point, I need to add that what we refer
as 'cell' nowadays is what is currently the Column class.  Obviously, the naming would be
a lot more sensible if the Column class was actually named Cell, and we plan on changing that
(CASSANDRA-6063), but I didn't wanted to include it in this patch-set because it's big enough
as it is (CASSANDRA-6063 is marked 3.0 but I'm definitively for doing at least the Column->Cell
renaming along with this patch (though I'd leave the rest of CASSANDRA-6063 for later)). Anyway,
this also means than in your "nits" patch, I'd rather not do the cellNameFromByteBuffer->cellFromByteBuffer

bq. Compound(.*)CellName was also a little confusing (to me, at least), suggest either Prefixed/Clustered
and (Blank)/Unclustered

Can't say I'm overjoy with Compound myself. However, Prefixed and/or Clustered doesn't really
apply here (a "simple" cell name is not more or less prefixed/clustered than a compound one)
so I don't think it's better. The difference between Simple and Compound (using the naming
of the patch) is largely an implementation detail, namely whether the underlying encoding
starts by 'number of components' or not (I would have loved not leaking the isCompound() method
in the CellName interface in particular, to really nail the point that it's an implementation
detail (for backward compatibility) but there still is a handful of places where we kind of
need it so I've let it be).

Remove Sparse from *SparseCellName - misleading, really it is just !Dense, so leave out Dense
from the name

At the risk of sounding obtuse, I've tried that initially and I kind of like it the way it
is. Let me note that the Sparse naming doesn't really leak outside the 'composite' packages,
most of the code just use the 'isDense' method of CellName and that's it. But as far as the
implementations of CellName goes, I think SimpleCellName would suggest it's somewhat a super-class
of SimpleDenseCellName which it's not (see the point below too). We do have 4 largely orthogonal
layout and I like making that very explicit in the implementation names. Probably a matter
of personal preference though. 

bq. I would possibly suggest removing CellName interface, and having SimpleSparseCellName
be simply NamedCell

That would be wrong imo. There is no meaningful inheritance relations between the cell names
implementations (each pair of implementations do share a number of characteristics, which
is why there is a few Abstract classes to avoid code duplication, but at the concrete implementation
level there is no inheritance relation). I also like having CellName being an interface because
that allows to cleanly have a few specific implementations like the EMPTY one (and the "FakeCellName"
from ColumnSlice, though that one is largely a hack so it's probably a bad example).

bq. Also, suggest renaming CompositeBound to one of BoundedComposite

Good idea.

bq. Changed toByteBuffer to always return a ByteBuffer that cannot affect the state of the
callee, and modified callers that were using .duplicate() to no longer do so

I'd rather not. We have an implicit rule in the code base that a caller should never expect
it can modify the state of a ByteBuffer and it should call duplicate() if it needs to (one
goal being to avoid defensive duplicate() when it's not needed since most of the code is written
in a way that don't modify the ByteBuffers anyway) so the patch is consistent with the code
base in that respect. Besides, in this particular case, thrift queries will call toByteBuffer()
all the time without needing the duplication so leaving it to the caller does save some.

bq. Following code comment in CompositeType appears to be incomplete

Definitively not the most clear comment but as far as I can tell it's not incomplete. Can
you clarify what you mean by "it suggests different outcomes for same spec".

> Push composites support in the storage engine
> ---------------------------------------------
>                 Key: CASSANDRA-5417
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Sylvain Lebresne
>            Assignee: Sylvain Lebresne
>              Labels: performance
>             Fix For: 2.1
> 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 {{}} 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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