1. A method for locking a nomination item in a multi-user distribution scheduling system, comprising: creating at least one nomination item associated with a voyage, the voyage including at least one available supply object, at least one available demand object and an available transportation object; storing the nomination item in a database; creating a lock object, associated with the nomination item, to prevent concurrent modification of the nomination item; modifying the nomination item; storing the modified nomination item in the database; and deleting the lock object.