Currently, UDT are serialized into a single value. For 3.0, we want to change that somewhat and allow updating individual subfields:
CASSANDRA-7423 (and ultimately, we'll probably allow querying subpart of UDT to some extend). Also for 3.0, we want to allow some nesting of collections (CASSANDRA-7826).
However, migrating the currently serialized UDT would be challenging. Besides that, even with nested collections, we probably won't be able to support nesting within map keys and sets without serializing (at the very least, not initially). Also, it can be useful in some specific case to have UDT or collections for PK columns, even if those are serialized.
So we need a better way to distinguish when a composite types (collections & UDT) are serialized (which imply you can't update subpart of the value, you have to rewrite it fully) and when they are not. The suggestion is then to introduce a new keyword, frozen, to indicate that a type is serialized:
A big advantage is that it makes the downside (you can't update the value without rewriting it all) clear and upfront.
Now, as of 2.1, we only support frozen UDT, and so we should make this clear by 1) adding the frozen keyword and 2) don't allow use of UDT unless they are "frozen" (since that's all we really support). This is what this ticket proposes to do. And this should be done in 2.1.0 or this will be a breaking change.
We will have a follow-up ticket that will extend frozen to collection, but this is less urgent since this will be strictly an improvement.
I'll note that in term of syntax, serialized was suggested as an alternative to frozen. I personally have a minor preference for serialized but it was argued that it had a "sequential" connotation which frozen don't have. Changing that is still up for discussion, but we need to reach a decision quickly.