[CalebJohn]

I should have been more clear in my original comment.

I don't think that the conflict detection/resolution needs to live inside the CRDT data structure. Ultimately you might want to bake it in out of convenience, but it should be possible to handle separately (of course the resolution will ultimately need to be written to the CRDT, but this can be a regular edit).

Keeping the conflict resolution in the application layer allows for CRDT libraries that don't need to be aware of human-in-the-loop conflicts, and can serve a wider range of downstream needs. For example, a note app and a version control system might both be plain text, but conflict resolution needs to be handled completely differently. Another example would be collaborative offline vs. online use cases, as noted above, they are very different use cases.

[josephg]

I’m not sure I agree that that approach would work. There’s two reasons:

1. The crdt has an awful lot of information at its disposal while merging branches. I think “branch merging” algorithms should ideally be able to make use of that information.

2. There’s a potential problem in your approach where two users concurrently merge branches together. In that case, you don’t want the merges themselves to also conflict with one another. What you actually want is for the standard crdt convergence properties to hold for the merge operation itself - and if two people concurrently merge branches together (in any order) then it all behaves correctly.

For that to happen, I think you need some coordination between the merging and the crdt’s internal data structures. And why not? They’re right there.

A sketch of a solution would be for the merge operation to perform the normal optimistic merge - but also annotate the document with a set of locations which need human review. If all peers use the same algorithm to figure out where those conflict annotations go, then the merge itself should be idempotent. The conflict annotations can be part of the shared data model.

Another, maybe simpler approach would be for the crdt library itself to just return the list of conflicting locations out of band when you do a merge operation. (Ie, we don’t change the crdt data structure itself - we just also return a list of conflicting lines as a return value from the merge function). The editor takes the list of conflicting locations and builds a ui around the list so the user can do manual review.