The use of multicast in subscription architectures

Subscription architectures have been used or considered for the implementation of distributed systems across the spectrum from hardware, through offline programming and simulation, to very high level agent based approaches. The “publish and subscribe” paradigm is attractive for multiprocessor systems that can be characterised as asynchronous, loosely coupled and coarse grained. As such systems grow in size and complexity however, there is thus a need for efficient implementations of these architectures that avoid the need for messages having to pass through centralised routing processes. The paper explores the proposition that broadcast network transport layers with multicast support can support highly efficient implementations of subscription architectures in a wide variety of distributed systems at all levels of the network hierarchy and abstraction. The article focuses on two active projects: (1) a simulation tool for the design of advanced submersible vehicles that will run on a cluster of workstations and hardware components connected by Ethernet and the Internet; (2) the control architecture for a land vehicle implemented as multiple microprocessors connected by a CAN bus. The paper draws out the common features of these two projects in order to make some generalisations about the benefits that can accrue to implementations of subscription architectures that use multicast supported by hardware. It explores the use of multicast groups to implement subscription “regions”-that is the ability of subscribers to specify dynamically subsets of data to be supplied for a particular subscription