  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 capable of making
concurrent operations natively commutative, 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, CRDT solutions are
rarely found in working co-editors, while OT solutions remain the choice for
building the vast majority of co-editors. Contradictions between this reality
and CRDT's purported advantages have been the source of much debate and
confusion in co-editing research and developer communities. What is CRDT really
to co-editing? What are the real differences between OT and CRDT for
co-editors? What are the key factors that may have affected the adoption of and
choice between OT and CRDT for co-editors in the real world? In this paper, we
report our discoveries, in relation to these questions and beyond, from a
comprehensive review and comparison study on representative OT and CRDT
solutions and working co-editors based on them. Moreover, this work reveals
facts and presents evidences that refute CRDT claimed advantages over OT. We
hope the results reported in this paper will help clear up common myths,
misconceptions, and confusions surrounding alternative co-editing techniques,
and accelerate progress in co-editing technology for real world applications.
