Skip to content(if available)orjump to list(if available)

CRDT: Text Buffer

CRDT: Text Buffer

2 comments

·August 19, 2025
Visit

josephg

This is a pretty good description of RGA (Replicated Growable Array). Which is a list & text CRDT that works pretty well in practice. Automerge used to use this algorithm for text editing, before moving across to FugueMax.

This algorithm has another obscure downside: It has interleaving problems if you insert items backwards. If two users do a series of inserts in reverse order, their inserts will get interleaved in a weird, unpredictable way. Eg, if I type "aaaaa" (as a series of prepended inserts) and you type "bbbb" in the same way, we can end up with "ababababa" or "aabbabbaa" some combination like that. We generally want CRDTs to be non-interleaving - so, "aaaaabbbb" or "bbbbaaaaa" should be the only possible results.

This problem is fixed by FugueMax, described in "The Art of the Fugue" paper[1]. If you're thinking of implementing a text CRDT, I recommend starting there. Fuguemax is a tiny change from RGA. We swap out the sequence numbers for a "right parent" pointer and the problem goes away. Coincidentally, the algorithm is also a 1 line change away from Yjs's CRDT algorithm.

And its really not that complicated. Most of the complexity in the fuguemax paper comes about because - like with RGA - they describe the algorithm in terms of inserts into a tree. If you ask me, this is a mistake. The algorithm is simpler if you primarily think of it as inserts into a list. (Thanks Kevin Jahns for this insight!) I programmed Fuguemax up live on camera a few months ago like this. You can fit a simple reference implementation of fuguemax in ~200 lines of code[2]. (The video is linked from the readme in that repository. In the video I explain the algorithm and all the code along the way).

[1] https://arxiv.org/abs/2305.00583

[2] https://github.com/josephg/crdt-from-scratch/blob/master/crd...

j0e1

(2024)