The control session records all SQL calls of the job, including start, end, and LOAD checkpoints. The main and auxiliary SQL sessions open on all Teradata systems specified in the routing applicable to the session.
If a connection attempt fails on any system due to an authentication failure, Unity closes the connection on all Teradata systems and returns an error to the Bulk Load client. If a system is unavailable, Unity resets these connections and replays any missed SQL statements as part of the recovery process when the system becomes available.
Unity rejects the control session if it is configured for read only, since the session cannot perform any writes.