Concurrency Control Mechanisms for Closely Coupled Collaboration in Multithreaded Peer-to-Peer Virtual Environments

As collaboration in virtual environments becomes more object-focused and closely-coupled, the frequency of shared object access conflicts can increase. In addition, two kinds of concurrency control “surprises ” become more disruptive to the collaboration: undo surprises, in which a previously-visible change is undone because of a conflict, and intention surprises, in which a remote structure change to a shared object which occurs just prior to a local request for that shared object leaves the object in a different state than what was expected. A hierarchy of three concurrency control mechanisms is presented in descending order of collaborative “surprises” which allows the concurrency scheme to be chosen tailored to the tolerance for such surprises. One mechanism is optimistic; the other two are pessimistic. Ideal for multithreaded, peer-to-peer virtual environments, these algorithms are easy to understand and implement, using C/C++ and Java, under Windows and Unix on both desktop and immersive systems. In a series of usability experiments, the performance of the most conservative algorithm on a local LAN was found to be quite acceptable