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by nkohari 3596 days ago
There are several CRDT algorithms (LSEQ, Logoot, WOOT, Treedoc) that try to properly solve the merging solution (your outcome #2) while still retaining the intent of the edits of each user. Their implementations differ, but the general idea is that each character (or chunk of characters) is assigned a key that can be ordered. When new text is added, it's given a key that is derived from the key of some adjacent text. As a result, the merge is the best-possible approximation of the intent of the edits.
1 comments
jamesdsadler 3596 days ago
Yes, I think we are in agreement. The algorithm can use heuristics to produce a best-effort pleasing result that captures user intent but if two users want opposite things to happen it has to fall back to a tie breaker.

In an extreme example, if the CRDT state-space was 1-bit and user A wants to make it a 0 and user B wants to make it a 1 a choice must be made by the algorithm.

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nkohari 3596 days ago
Right, exactly. The key question is whether the algorithm's resolution heuristic takes into account the intent of the edit, and I'm not sure the algorithm in the article makes an effort to do that. (Not saying it doesn't, I just don't understand how it does, if it does.)
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vidarh 3595 days ago
It's main consideration appears to be to make changes in a way that makes sense for a tree consisting of maps and lists without ever losing user input. If you look at their diagram with examples you get the gist of their resulting heuristics, which basically boils down to "only delete if a user explicitly deleted, otherwise keep both values if users try to set the same value to two different things", "if two complex values (maps,list) have been updated, recursively merge them", pretty much.

It will cause stuff that users will perceive as conflicts, and that may even appear to be totally illogical (one of their examples leaves an object that appears to be in a broken state, because one side deleted it, and the other side updated a single attribute, leading the map to continue to exist, but with only the one updated attribute) so there's probably room for improvement, though the rules are simple enough that many of these could be resolved at application level by just carefully deciding what operation to provide.

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