Comment by josephg | Hacker News Reader

Comment by josephg | Hacker News Reader

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> It seems to me the burden of proof is on you. You were the one who claimed that “CRDTs perform better than OT-based systems.”

Ah, I assumed we were talking about Jupiter based OT systems - which are outperformed by their newer cousins (like eg-walker). Like you say, these use a different data structure to transform changes and that's why they're faster.

> My reasoning is that CRDTs require idempotence and commutativity, while OTs do not.

The only property not required by a centralized OT system is the OT TP2 property. Ie, T(op3, op1 + T(op2, op1) == T(op3, op2 + T(op1, op2)). Central servers also give you a single global ordering.

If you discard TP2 and add global ordering, does that open the door to new optimisations? I don't know, and I certainly can't prove the absence of any such optimisations. So I think the burden of proof is on you.

The root of our misunderstanding or debate is clear: although CRDT is fairly well defined, I don’t think the same is true for OT.

What I have in mind is what I mentioned earlier:

> OT can be id-based, in which case operations are transformed directly on the document, not on other operations.

This is exactly what I do in my library DocNode[1], which I describe as “id-based OT”.

With this model, it’s not even necessary to satisfy TP1. In fact, the concept T(o1, o2) doesn’t exist, because operations aren’t “transformed” against other operations, but against the document. Maybe the word “transform” is a bit misleading, and “apply” would be more appropriate. The problem is that there is still a slight transformation. For example, if a client inserts a node between nodes A and B, but by the time it reaches the server B has been deleted, the effective operation might become “insert between A and C”.

The server is append-only. The client has several options to synchronize with the server: rebasing, undo-do-redo, or overwriting the document.

Maybe I’m the one who shouldn’t describe DocNode as “[id-based] OT” and should instead coin a new term. Operational Application (OA)? Operations Without Transformation (OWT)? Operations Directly Transformed (ODT)? Operational Rebasing (OR)? Not sure. What would you recommend?

[1] https://www.docukit.dev/docnode

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