Real Differences between OT and CRDT under a General Transformation Framework for Consistency Maintenance in Co-Editors

Abstract

OT (Operational Transformation) was invented for supporting real-time co-editors in the late 1980s and has evolved to become a core technique used in today's working co-editors and adopted in major industrial products. CRDT (Commutative Replicated Data Type) for co-editors was first proposed around 2006, under the name of WOOT (WithOut Operational Transformation). Follow-up CRDT variations are commonly labeled as "post-OT" techniques and have made broad claims of superiority over OT solutions, in terms of correctness, time and space complexity, simplicity, etc. Over one decade later, however, OT remains the choice for building the vast majority of co-editors, whereas CRDT is rarely found in working co-editors. Why? Based on comprehensive review and comparison on representative OT and CRDT solutions and co-editors based on them, we present our discoveries in relation to this question and beyond in a series of three articles.In prior work, we have revealed that CRDT is like OT in following the same general transformation approach and CRDT is not natively commutative for concurrent operations in co-editors, which helps to clarify what CRDT really is and is not for co-editors. In this article, we reveal OT and CRDT differences in correctness and complexity by dissecting and examining representative OT and CRDT solutions. We explore how different basic approaches ̶ the concurrency-centric approach taken by OT and the content-centric approach taken by CRDT ̶ had resulted in different technical challenges, correctness and complexity issues, and solutions. Moreover, we reveal hidden algorithmic flaws with representative CRDT solutions, and discuss common myths and facts related to correctness, time and space complexity, and simplicity of OT and CRDT. We present facts and evidences that refute CRDT claimed advantages over OT. We hope the discoveries from this work help clear up common myths and confusions surrounding OT and CRDT, and accelerate progress in co-editing technology for real world applications. ; URL: https://www.ntu.edu.sg/home/CZSun 20,38,43,54,55,58,61,70,71,77], and support real-time collaboration in rich-text co-editing in word processors [60,65,68,82], HTML/XML Web document co-editing [10], spreadsheet co-editing [69], 3D model co-editing in digital media design tools [1,2], and file synchronization in cloud storage systems [3]. Recent years have seen OT being widely adopted in industry products as the core technique for consistency maintenance, ranging from battle-tested online collaborative richtext editors like Google Docs 1 [11], to emerging start-up products, such as Codox Apps 2 .A variety of alternative techniques for consistency maintenance in co-editors had also been explored in the past decades [14,16,18,41,42,72]. One notable class of techniques is CRDT 3 (Commutative Replicated Data Type) for co-editors [4,5,8,25,32,37,39,40,41,45,47,48,79,80,81]. The first CRDT solution for plain-text co-editing appeared around 2006 [40,41], under the name of WOOT (WithOut Operational Transformation). One mo...

A semi-transparent selective undo algorithm for multi-user collaborative editors

“…In the literature, there are two highly researched concurrency control approaches for co-editing systems: operational transformation (OT) [42][43][44] and commutative replicated data type (CRDT) [45][46][47][48].…”

“…According to the recent research studies [42][43][44], OT and CRDT both adopt the strategy of transforming operations to solve the consistency problems in collaborative applications. They are both a kind of transformation-based concurrency con-trol approaches but different in the way of transforming operations.…”

Self‐compressing object sequence for consistency maintenance in co‐editors

“…However, such simplification is reached at the price of efficiency: the additional object sequences could grow unlimitedly with the increasing number of string‐wise operations in a collaboration session. The unbounded metadata overhead is a serious defect for the object sequence based consistency algorithms, and it is not clear whether the defect could be fixed 15,49,50 …”

“…The unbounded metadata overhead is a serious defect for the object sequence based consistency algorithms, and it is not clear whether the defect could be fixed. 15,49,50 This work contributes a technical solution to the unbounded metadata overhead problem of address space transformation technique. Firstly, we propose a self-compressing object sequence, which can automatically limit its size to the number of concurrent operations.…”