  Distributed Real-Time (DRT) systems are among the most complex software
systems to design, test, maintain and evolve. The existence of components
distributed over a network often conflicts with real-time requirements, leading
to design strategies that depend on domain- and even application-specific
knowledge. Distributed Virtual Environment (DVE) systems are DRT systems that
connect multiple users instantly with each other and with a shared virtual
space over a network. DVE systems deviate from traditional DRT systems in the
importance of the quality of the end user experience. We present an analysis of
important, but challenging, issues in the design, testing and evaluation of DVE
systems through the lens of experiments with a concrete DVE, OpenSimulator. We
frame our observations within six dimensions of well-known design concerns:
correctness, fault tolerance/prevention, scalability, time sensitivity,
consistency, and overhead of distribution. Furthermore, we place our
experimental work in a broader historical context, showing that these
challenges are intrinsic to DVEs and suggesting lines of future research.
