I've thought about this some more. It would be useful for Cassandra to support
user-defined "guardrails" (or constraints, whatever you want to call them),
that could be applied per keyspace or table. Whether a user or an operator is
considered the owner of a table depends on the organization deploying
Cassandra, so allowing both parties to protect their tables against mis-use
seems good to me, especially for large multi-tenant clusters with diverse
workloads.
For example, it would be really useful if a user could set the
Guardrails.{read,write}ConsistencyLevels for their tables, or declare whether
all operations should be over LWTs to avoid mixing regular and LWT workloads.
I'm hesitant about adding lots of expression syntax to the CONSTRAINT clause. I
think I'd prefer a function calling syntax that represents:
1. Whether the constraint is system / keyspace / table scoped
2. Where in query processing the constraint is checked
3. What is executed by the check
