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.