For an overview of IDL operation declarations, see "IDL operation declarations".
The optional oneway keyword specifies that when a caller invokes the operation, no reply is expected or received. The invocation semantics of a oneway operation are best-effort, which does not guarantee delivery of the call. Best-effort implies that the operation is invoked "at most once". A oneway operation must not have any output parameters and must have a return type of void. A oneway operation also must not include a raises expression.
If the oneway keyword is not specified, then the operation has at-most-once invocation semantics if an exception is raised and it has exactly-once semantics if the operation succeeds. This means that an operation that raises an exception is implemented zero or one times and an operation that succeeds is implemented exactly once.