Handling and Resolving Conflicts in Real Time Mobile Collaboration

Real time group editors allow two or more users at different locations to work on a shared document at the same time. In a mobile network environment with nondeterministic communication latency, a replicated architecture is usually adopted for the storage of the shared document in order to provide high responsiveness. A conflict occurs when two or more users have different intentions for editing the same part of the replicated document. Conflict can be categorised into two types: exclusive and non-exclusive conflicts. An exclusive conflict occurs when the conflicting operations cannot be realised at the same time, and if serially executed, the effect of the later operation will override the earlier operation. In contrast, a non-exclusive conflict occurs when the conflicting operations can be realised at the same time and both operations can be applied to the target without one overriding the other. The approaches adopted by the existing algorithms to resolve conflicts can be categorised into: (1) locking approaches [1], (2) operational transformation approaches [2], and (3) multi-versioning approaches [3]. The locking approach is a conflict prevention approach rather than a conflict resolution approach, and it does not promote concurrency as only one person can modify an object at one time. The operational transformation approach is optimistic and is able to handle non-exclusive conflicts, but it cannot handle exclusive conflicts. The multi-versioning approach handles exclusive conflicts by creating different object versions with each version realising each conflicting intention. Xue et al. [4] combine multi-versioning, operational transformation, and post-locking to handle both types of conflict and to restrict the number of object versions. However, while post-locking simplifies the conflict resolution process by locking versions to protect them from further modification, it suffers from a partial intention problem insofar as conflicts could be better resolved if the conflicting intentions have been completely realised and thus every user can see the full intention of all other users.