Ensuring content and intention consistency in real-time group editors

Real-time group editors allow distributed users to work on local replicas of a shared document simultaneously to achieve high responsiveness and free interaction. Operational transformation (OT) is the standard method for consistency maintenance in state-of-the-art group editors. It is potentially able to achieve content consistency (convergence) as well as intention consistency (so that the converged content is what the users intend), while traditional concurrency control methods such as locking and serialization often cannot. However, existing OT algorithms are often not able to really guarantee consistency due to important algorithmic flaws that have been there for fourteen years. We present a novel state difference based transformation (SDT) algorithm to solve the problem. Our result also reveals that the standard priority schemes to break ties in distributed systems should be used with more caution.