The api.ddl package defines all Data Definition operations that hector supports which include: A note should be taken that by server design, it is required that all DDL operations are to be performed in a serialized manner.
Cassandra cannot support two DDL operations issued at the same time. The first operation needs to end before the next operation can take place and Hecotr does not try to protect its users from it, it's the responsibility of hector's users to prevent from multiple schema changes to happen at the same time.
See Live Schema Updates for more details.