Globally consistent event ordering in one-directional distributed environments

We consider communication structures for event ordering algorithms in distributed environments where information ows only in one direction. Example applications are multilevel security and hierarchical databases. Although the most general one-directional communication structure is a partial order, partial orders do not enjoy the property of being consistently-ordered, a formalisation of the notion that globally consistent event ordering is ensured. Our main result is that the crown-free property is necessary and su cient for a communication structure to be consistently-ordered. We discuss the computational complexity of detecting crowns and sketch typical applications