Tree-Based Concurrency Control in Distributed Groupware

We present a novel algorithm, called dARB, for solving the concurrency control problem in distributed collaborative applications. The main issue of concurrency control is resolving the conflicts resulting from simultaneous actions of multiple users. The algorithm reduces the need for manual conflict resolution by using a distributed arbitration scheme. The main advantages of our approach are the simplicity of use and good responsiveness, as there are no lock mechanisms. Our algorithm requires the applications to use a tree as the internal data structure. This makes it application independent and suitable for general collaborative applications. The tree requirement is reasonable since many new applications use XML (extensible Markup Language) for data representation and exchange, and parsing XML documents results in tree structures. Example applications of the algorithm, a group text editor and a collaborative 3D virtual environment called cWorld, are implemented and evaluated in the DISCIPLE collaboration framework. We also introduce awareness widgets that users avoid generating the conflicting events and help in manual conflict resolution