Debug.

System is in error state but can recover and continue reliably.

System is in severe error state and has to abort.

System-level events and status, in correct states.

Disable all logging.

Request-level events, with request-specific rlevel.

System is in severe error state and cannot recover reliably.

Trace detailed system execution, e.g.

System approaching error state, or is in a correct but undesirable state.

Logger provides a common interface for logging libraries.