[论文解读] Real Differences between OT and CRDT in Building Co-Editing Systems and Real World Applications

David Sun, Chengzheng Sun+2 more|arXiv (Cornell University)|May 2, 2019

Multimedia Communication and Technology参考文献 63被引用 7

一句话总结

本文对实时协同编辑系统中的操作转换（OT）和无冲突复制数据类型（CRDT）进行了批判性评估，揭示尽管CRDT在理论上宣称具有优势，但OT在工业界仍占主导地位，原因在于其在正确性、复杂度和实际部署方面的实际优势。作者分析了真实世界的协同编辑器，驳斥了关于CRDT的误解，表明在实践中OT的成熟度和可靠性超过了CRDT的理论优势。

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? To seek truth from facts, we set out to conduct a comprehensive and critical review of representative OT and CRDT solutions and working co-editors based on them. From this work, we have made important discoveries about OT and CRDT, and revealed facts and evidences that refute CRDT claims over OT on all accounts. We present our discoveries in three related and complementary articles. In prior two articles, we have revealed the similarities of OT and CRDT in following the same general transformation approach in co-editors, and their real differences in correctness and complexity. In this article, we examine the role of building working co-editors in shaping OT and CRDT research and solutions, and consequential differences in the choice between OT and CRDT in real world co-editors and industry products. In particular, we review the evolution of co-editors from research vehicles to real world applications, and discuss representative OT-based co-editors and alternative approaches in industry products and open source projects. Moreover, we evaluate CRDT-based co-editors in relation to published CRDT solutions, and clarify some myths surrounding "peer-to-peer" co-editing.

研究动机与目标

• 调查尽管CRDT在理论上广泛宣称具有优越性，为何OT在现实世界协同编辑系统中仍保持主导地位。
• 从正确性、复杂度和现实应用中的实际可部署性角度，对OT和CRDT进行批判性比较。
• 分析协同编辑系统从研究原型到工业产品的演进过程，并评估现实约束在技术选型中的作用。
• 与已发表的CRDT解决方案和工业标准相比，评估基于CRDT的协同编辑器的实际性能和可靠性。
• 澄清关于“点对点”协同编辑以及CRDT在生产系统中实际可行性的误解。

提出的方法

• 对学术文献和真实世界实现中的代表性OT和CRDT解决方案进行全面、基于事实的综述。
• 分析主流基于OT的协同编辑器（如Google Docs、Microsoft Word协同编辑）以及开源和工业产品中基于CRDT的系统的设计与部署模式。
• 比较OT和CRDT在现实使用场景下的正确性保证、时间与空间复杂度以及实现复杂度。
• 考察现实约束（如网络延迟、冲突解决和系统可靠性）在OT取代CRDT的采用过程中所起的作用。
• 评估理论CRDT属性与其实现在真实协同编辑系统中实现之间的差距，尤其是在点对点环境中的表现。
• 利用已发表的配套论文和真实世界部署的证据，驳回关于CRDT在正确性、简洁性和效率方面优越性的说法。

实验结果

研究问题

• RQ1 尽管CRDT在过去十多年中不断发展，为何OT仍一直是工业级协同编辑系统中的主导范式？
• RQ2 在现实系统中，CRDT在正确性、时间复杂度和空间复杂度方面真正优于OT的程度有多大？
• RQ3 在现实协同编辑应用中，有哪些实际因素使得OT优于CRDT，尽管CRDT在理论上具有优势？
• RQ4 现实世界中的基于CRDT的协同编辑器与它们的理论对应物相比，在正确性和性能方面表现如何？
• RQ5 文献和工业界中关于“点对点”协同编辑和CRDT的哪些误解被持续传播，如何能够加以澄清？

主要发现

• OT因其在现实世界部署中经验证的正确性、稳定性和成熟度，仍是工业级协同编辑系统中的主导范式。
• 尽管CRDT在理论上宣称具有更高的正确性和复杂度优势，但来自真实系统的真实证据表明OT在生产环境约束下仍能可靠运行。
• 理论CRDT属性与实际实现之间存在显著差距，尤其在点对点和高延迟环境中更为明显。
• CRDT很少被用于生产级协同编辑系统，并非因为技术缺陷，而是由于状态管理和冲突解决方面的实际挑战，而这些挑战在OT中得到了更优处理。
• 本文驳倒了CRDT本质上可实现无需协调的真正点对点协同编辑的误解，表明真实系统通常仍需要中心化或混合协调机制。
• 本研究发现，OT与CRDT在底层转换机制上比以往认为的更为相似，其差异主要体现在实际部署方式以及在真实环境条件下的正确性保证方面。