Passive sessions are opened on only one system, per routing rule definition. All read and write requests are directed to that system. If a request contains write requests to one or more objects, the objects are compared with the objects in the data dictionary. If a write object exist in the data dictionary and the session associated routing rule does not allow write requests to managed objects, the query is rejected with error code 4530, "Query rejected because associated routing rule prohibits writing to managed objects."
For write requests, since there is only one system, Unity does not perform data consistency checks or keep write requests to managed objects in sync. When a read or allowed write request is done against a Unity-managed object (object exists in data dictionary), access to the object is allowed regardless of the state of the objects. This allows passive sessions (sessions associated with a passive routing rule) to access objects that are unrecoverable or in an otherwise inaccessible state in the data dictionary.