Architectures and Mechanisms to Maintain efficiently Consistency in Collaborative Virtual Environments

Collaborative virtual environments (CVE) enable users to collaborate and interact with each other by sharing a common virtual environment. Consistency of the virtual environment is very important to provide an efficient collaboration between users. However, users sharing a CVE may be scattered over different physical locations, so CVE systems have to guarantee the consistency of the virtual environment despite network issues such as low bandwidth or network latency. Absolute consistency is nearly impossible to achieve because it would prejudice the responsiveness of the system during users' interactions. So, CVE systems have to deal with a trade-off between consistency and responsiveness of the system. This paper presents a detailed survey of architectures and mechanisms used to improve the consistency of a shared virtual environment. Architectures of CVE systems are studied according to their impact on the consistency. Contrary to previous state of the art reports which classify CVE systems according to the network connections or to the data distributions, we choose to examine these two properties separately. This classification enables us to deal with the increasing number of hybrid architectures which mix different architectural choices to meet their requirements. Consistency maintenance mechanisms are also examined. First, a time synchronization has to be achieved in order to enable users to have the same state of the virtual environment at the same time. Second, the virtual environment can be seen as a database shared by several users, so CVE systems have to manage users' concurrent access to the objects of the virtual environment. Finally, we discuss the need to enable CVE systems to adapt themselves to performance constraints induced by the use of mainstream network such as DSL Internet access or by security requirements of industrial users.