Fall 2016, Vol. 22, No. 4, Pages 431-450
Traditional digital computing demands perfectly reliable memory and processing, so programs can build structures once then use them forever—but such deterministic execution is becoming ever more costly in large-scale systems. By contrast, living systems, viewed as computations, naturally tolerate fallible hardware by repairing and rebuilding structures even while in use—and suggest ways to compute using massive amounts of unreliable, merely best-effort hardware. However, we currently know little about programming without deterministic execution, in architectures where traditional models of computation—and deterministic ALife models such as the Game of Life—need not apply. This expanded article presents ulam, a language designed to balance concurrency and programmability upon best-effort hardware, using lifelike strategies to achieve robust and scalable computations. The article reviews challenges for traditional architecture, introduces the active-media computational model for which ulam is designed, and then presents the language itself, touching on its nomenclature and surface appearance as well as some broader aspects of robust software engineering. Several ulam examples are presented; then the article concludes with a brief consideration of the couplings between a computational model and its physical implementation.