As far as protocol composition is concerned, the HORUS system enables to build protocol stacks from existing layers only in a strictly vertical manner. Furthermore, it is based on groups as fundamental addressing and communication facility, and provides no framework and/or pattern for building new protocols layers. HORUS merely provides a finite set of ready-to-use protocol layers, which can only be composed around the group membership protocol.
With BAST, we have tried to model any kind of interaction between distributed objects, not only group communications. This is essential in order to deal with failures in an extensible way, because reliable protocols tend to be much more complex than mere communications. By making protocol objects BAST's basic distributed entities, we can build both the group model and the transaction model [6]. Furthermore, the Strategy pattern provides a powerful scheme for creating new protocols through composition.