their speed has increased more than 10-million- fold, and they have been interconnected through ever-larger bandwidths. Amazingly, our concept of a program as an implementation of a sequential algorithm has remained the same. While the shift to actor-oriented computing is overdue, we need to start thinking of software systems beyond the composition of actors.

Actors residing on a single node, even if mobile, cannot address the inevitable challenges that will characterize the future of computing. Computers are increasingly ubiquitous and embedded in all sorts of devices, from refrigerators and thermostats to automobiles and wheelchairs. The next logical step is for these embedded computers to be networked, not just in relatively localized sensor networks but through their connectivity to more powerful computers (base stations) to be globally networked. Such networking will result in a cyberspace that parallels physical space and is as pervasive as physical objects in the real world.

Pervasive cyberspace will consist of computers

with heterogeneous architectures, a variety of sensing and actuation capabilities, and security and privacy profiles, many of which turn mechanical things (such as chairs, desks, windows, and walls) into smart devices. The availability of services and resources on these computers will be dynamic.

Explicit programming of such an evolving system is not feasible; elements in the pervasive cyberspace will continue to be added and removed, and architectures and policies will keep changing. Moreover, user requirements will continue to evolve. In such an environment, programmers cannot possibly know where all the resources are or will be or how and when they may be accessed. Thus software in a complex environment cannot consist of preprogrammed sequences of actions.

It is not that programming as such will be obsolete; certain local behaviors of these computers will continue to be programmed, albeit more-or-less automatically from high-level user-friendly interfaces and domain-specific libraries, written in