Currently, in CQL3, you set the consistency level of an operation in
the language, eg 'SELECT * FROM foo USING CONSISTENCY QUORUM'. It now
looks like this was a mistake, and that consistency should be set at
the protocol level, i.e. as a separate parameter along with the query.
The reasoning is that the CL applies to the guarantee provided by the
operation being successful, not to the query itself. Specifically,
having the CL being part of the language means that CL is opaque to
low level client libraries without themselves parsing the CQL, which
we want to avoid. Thus,
- Those libraries can't implement automatic retries policy, where a query would be retried with a smaller CL. (I'm aware that this is often a Bad Idea, but it does have legitimate uses and not having that available is seen as a regression from the Thrift api.)
- We had to introduce
CASSANDRA-4448to allow the client to configure some form of default CL since the library can't handle that anymore, which is hackish.
- Executing prepared statements with different CL requires preparing multiple statements.
- CL only makes sense for BATCH operations as a whole, not the sub-statements within the batch. Currently CQL3 "fixes" that by validating the given CLs match, but it would be much more clear if the CL was on the protocol side.