Capturing causality by compressed vector clock in real-time group editors

Real-time group editors allow a group of users to view and edit the same document at the same time over the Internet. They are a special class of distributed applications in the area of groupware. A vector logical clock is a powerful technique to capture causality in distributed computing systems. In general, the minimum size of a vector clock is the number of communicating processes in a distributed system (N). In this paper, we propose a novel technique to compress the vector size from N to a constant (2) by means of operational transformation - an innovative technique invented by groupware research for consistency maintenance in real-time group editors. We show how compressed vector clocks can be used as an effective and efficient means for operation time-stamping and concurrency detection in group editors. The proposed technique has been implemented in a Web-based real-time group editor which allows an arbitrary number of users to participate a collaborative editing session. The basic ideas and techniques of this work may be generalized and are potentially applicable to other distributed computing systems and applications.