Delayed Consistency Model for Distributed Interactive Systems with Real-Time Continuous Media

The advanced multimedia and high-speed networks make distributed interactive systems more promising and practical. These systems are distributed systems, which allow many clients located in different locations to concurrently explore and interact with each other. The systems can be built either in the local area network (LAN), or the wide area network (WAN), such as the Internet. Operations issued at one site are immediately executed at the local sites for a good response time, and are propagated to other sites. One of the challenging issues raised in the systems is consistency maintenance. Such issue in the discrete interactive media has been studied in many literatures. However, the consistency maintenance scheme for discrete interactive media is not suitable for continuous media domain. This paper illustrates a consistency problem in continuous interactive media by a simple example. The absolute consistency model, a strong requirement, is suitable for LAN and results in a bad responsiveness in WAN. To make the model more practical for WAN, a new consistency model, named delayed consistency model (DCM), is proposed. In this model, if an operation on an object x is issued at site i, every site is required to execute the operation at a specified time. The essential idea behind the proposed model is that other sites are enforced to update the state at a certain amount of time later than site i does. Thus, other sites will finally view the same state of x as that of site i. The DCM model is flexible, since it is unnecessary for all sites to have the identical delayed time. In case that the system is based on a real-time network, another advantage of the model is providing the real-time network scheduling with important timing parameters.

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