Tombstone Transformation Functions for Ensuring Consistency in Collaborative Editing Systems

In collaborative editing, consistency maintenance of the copies of shared data is a critical issue. In the last decade, operational transformation (OT) approach revealed as a suitable mechanism for maintaining consistency. Unfortunately, none of the published propositions relying on this approach are able to satisfy the mandatory correctness properties TP₁ and TP₂ defined in the Ressel´s framework. This paper addresses this correctness issue by proposing a new way to model shared state by retaining tombstones when elements are removed. An instantiation of the proposed model for a linear data structure and the related transformation functions are provided