Research On Concurrency Control In Cooperative Graphics Editing Systems

The Object-based Cooperative graphics Editing System (OCES) in distributed environment is now a very active research area of CSCW (Computer Supported Cooperative Work). To support free and natural interaction, this kind of system should have such characteristics as collaborative distribution, interactive in real-time and operation's non-restriction. In order to satisfy these features, the fully replicated architecture should be adopted. One of the key problems in studying and designing the replicated collaborative editing system is the consistency of the multi-duplicated data view, and which is also the core discussed in this thesis.Technologies supporting consistency preservation based on a consistency model, which includes three consistency properties, are first surveyed. Our focus is how these properties are maintained. Several representative consistency schemes explored by other researchers are also introduced.Then a multi-versioning scheme based on object replication is discussed. Based on analysis of the performance limitation of the distributed algorithm for multiple versions incremental creation, a new algorithm using conflict set is proposesd which not only can decrease the comparison-number of compatible operations, but also can avoid the operation of removing the redundant object versions. The theory certification and simulation experiment results shows that performance of the new algorithm is more efficient than the original algorithm.Then a distributed collaborative multi-versioning scheme based on version replication, which can resolve some complex problems that the object-replication can not deal with, is discussed in detail. An improved multi-versioning using hierarchical storage strategy for operations is proposed to avoid same operation to be stored in different versions to the largest degree and reduce the number of operation stored in system.At last, this paper analyses the conflict operations between multi-versions, and proposes an idea of subtree construction to preserve the intention of these conflict operations. This paper first proposes a conception of generalized max conflict set, and uses it to realize the confict operation between muilti-versions.A prototype system CoWork is designed and implemented as a test-bed for the correctness of proposed strategies and algorithms. Other related cooperative techniques used in this prototype, as well as their runtime performance, are also discussed briefly in the end of this thesis.