Collaborative Text Editing Without CRDTs or OT
85 comments
·May 21, 2025Animats
jahewson
This is literally a degenerate CRDT. Central server for tie-breaking goes back to Google Wave.
bikeshaving
Any algorithm that uses a central server as a tie-breaker could easily be replaced by one where client ids are used for the tie-breaker.
jonstokes
If you used UUIDv7 you get time-ordered UUID and could use that for a tie breaker.
eiginn
I miss Wave a lot, very quirky in a good way imo. We ran a few D&D games over it. RIP
yubblegum
Is this really that novel? I mean using a central process for serializing a distributed system is like a no brainer -- didn't we start off from here originally? -- until you have to worry about network partitions, and CAP and all that jazz. You also now have a single point of failure. Also I skimmed the thing but was performance discussed?
motorest
> Is this really that novel? I mean using a central process for serializing a distributed system is like a no brainer -- didn't we start off from here originally?
Yes. This article reads like the adage "a month in a lab saves you an hour in a library".
bongodongobob
Yeah I have the same question. I'm not familiar with the problem space but this seems like my naive first idea so I'm wondering what the catch is.
mweidner
As the author, same.
My best guess is:
- Central-server collaborative editing work focuses on Operational Transformation (OT), likely due to inertia (studied since 1989) and the perception that storing an ID per character is inefficient. In fairness, it is, absent the optimizations introduced by RGASplit and Yjs (~2015).
- For decentralized editing, OT is very complicated, and CRDTs took over as the solution of interest (studied since 2005). Storing every operation permanently in a log - needed to use the linked approach without a server - feels inefficient, as does server reconciliation's undo/re-apply process. So CRDT research has focused on avoiding those inefficiencies, sacrificing simplicity along the way, instead of just embracing them as the easy way out.
To me, the "inefficiencies" seem quite manageable. Storage is cheap, text is small, and you probably want a complete op log anyway for auditing and document histories (cf. git). Server reconciliation's undo/re-apply process can be batched aggressively, e.g., only do it a few times per second; that just makes remote ops take a little longer to show up.
Granted, I have not built a complete app around server reconciliation or the linked approach, so perhaps there is a hidden catch. But I am encouraged by the success of Replicache (https://doc.replicache.dev/concepts/how-it-works), which is where I learned of server reconciliation.
thebeardisred
I really want to believe you were trying to make a reference to cap'n jazz - https://en.m.wikipedia.org/wiki/Cap'n_Jazz
:)
yubblegum
Never heard of them
:(
sdeframond
What you describe is a CRDT, isn't it ?
ori_b
No; there is no single consistent final state that the system must converge to if the parts go offline. If you have this document:
a{uuid=1}
b{uuid=2} insert-after{uuid=1}
c{uuid=3} insert-after{uuid=1}
abc
acb
YoumuChan
Like Raft is a "special case" of Paxos, this feels like a "special case" of CRDT.
It has all the flavor of CRDT, but adds a leader and a different way for the total ordering (basically using leader's local lamport clock to break tie).
Throw in leader reelection and some ledger syncing and then give everything some other names, I bet you can have "collaborative text editing on one page".
motorest
> No; there is no single consistent final state that the system must converge to if the parts go offline.
Sounds like a naive implementation of delta state CRDT. I mean, what if the author has this major epiphany that it's nice to sync states to get convergence?
sdeframond
What about ordering concurrent operations by id? Then "abc" is the only consistent final state.
I get what you mean though, having a central authority greatly relaxes the requirement.
anon291
Yeah, so this is an operational transform where the transform operation is just identity.
Yoric
If you have the additional semantics that says "operation with lowest uuid gets applied first", don't you essentially get a CRDT?
I mean, a uuid is kind of a poor man's Lamport clock, isn't it?
pcthrowaway
ctrl+a
ctrl+x
ctrl+v
Good luck
apgwoz
Isn’t this a NOP?
I think you could batch these things though. You just need a slightly more advanced protocol. If B is the first id, and L is the last, create range(B, L) as R and insert R after L (assuming Ctrl-v a second time).
motorest
> Isn’t this a NOP?
I think that's the point: conceptually it's a co-op, but the implementation forces each character to be individually removed following a ctrl-x, and then individually inserted back (after issuing a new UUID) following a ctrl-v.
Now, the outcome of neither a Ctrl+x nor a Ctrl+v is atomic, which means communicating each character operation through the wire takes time. In turn this means that this is a scenario where conflicts will take place (someone is editing text midway through another user does ctrl-a,ctrl-x) and where the choice of operation will result in important user impacts.
This is a basic test scenario of CRDTs, and why research focuses on higher-level operations instead of atomic character-level operations.
It should be noted that ctrl-a,ctrl-x,ctrl-v is an extreme case of a very frequent operation: copying and pasting text. Editor-level undo/redo can manifest the same way, too.
hem777
Cool to see a write up on this! Discovered the same method years ago and also wondered why it doesn’t show up in academic literature.
I implemented this in a decentralized context and as a CRDT though, so that the properties hold (commutative, idempotent and associative).
k__
If the idea is to have an alternative to CRDT, what did you gain from building it as CRDT?
hem777
It wasn’t really that we wanted to have a CRDT per se, but as it was implemented on top of an op-based append-only log CRDT, it turned out to hold those properties, which makes it a CRDT. We wanted to have the edits to be able to arrive in any order or after delays due to network partitions (this was for a p2p network).
Footkerchief
Surprised to see no discussion of other data structures like dicts/maps, or arrays of arbitrary type. Hopefully they'd be a straightforward extension. IME, apps need collaborative data structures more often than they need pure collaborative text editing.
The motivating examples (update validation, partial loading, higher-level operations) are interesting, but I don't see a strong case that the reason Yjs etc. lack these features is the underlying CRDT implementation, as opposed to these features being intrinsically difficult to build.
filleokus
> Surprised to see no discussion of other data structures like dicts/maps, or arrays of arbitrary type. Hopefully they'd be a straightforward extension. IME, apps need collaborative data structures more often than they need pure collaborative text editing.
Totally agree. I guess an array of "atomic" objects, where the properties of the objects can't be changed can be done just by replacing the string with your own type. Changes inside of the object is probably trickier, but maybe it's just about efficiently storing/traversing the tree?
I've also always thoguth it should be possible to create something where the consumer of the helper library (per OP terminology) can hook in their own lightweight "semantic model" logic, to prevent/manage invalid states. A todo item can't both have isDone: true and state: inProgress at the same time. Similar to rich text formatting semantics mentioned in the linked article.
SkiFire13
CRDTs essentially work by deterministically picking one side when a conflict arises. The issue is that in general this does not guarantee the lack of data loss nor data being valid (you can resolve the conflict between two pieces of valid data and get invalid data as a result).
Imagine if every git merge conflict you got was resolved automatically by picking one side. Most of the time it would do the wrong thing, sometimes even leading to code that fails to compile. Imagine then you were not there ready to fix the issue, it would lead to even more chaotic results!
This is why CRDTs are not more widespread, because they only fix the problem you think you have, not the problem you actually have, which is to fix conflicts in a way that preserves data, its validity and meaning.
And arguably they make this issue even worse because they restrict the ways you can solve these conflicts to only those that can be replicated deterministically.
josephg
> This is why CRDTs are not more widespread, because they only fix the problem you think you have, not the problem you actually have, which is to fix conflicts in a way that preserves data, its validity and meaning.
I’ve been saying this for years, but there’s no reason you couldn’t make a crdt which emitted conflict ranges like git does. CRDTs have strictly more information than git when merging branches. It should be pretty easy to make a crdt which has a “merge and emit conflicts” mode for merging branches. It’s just nobody has implemented it yet.
(At this point I’ve been saying this for about 5 years. Maybe I need to finally code this up if only to demonstrate it)
hansworst
Automerge has this: https://automerge.org/automerge/api-docs/js/functions/getCon...
motorest
> I’ve been saying this for years, but there’s no reason you couldn’t make a crdt which emitted conflict ranges like git does.
I don't get your point. The C in CRDT stands for "conflict-free". Why would a CRDT have conflict ranges?
SkiFire13
That might be an improvement over git, but not automatically fixing the conflicts will be a dealbreaker for most people.
Fundamentally people want something that automatically fixes conflicts, and do so the way they expect it to, but this just doesn't exist yet.
practal
Ok, so the main point that makes it different from CRDTs seems to be: if you have a central server, let the server do the synchronization (fixing an order among concurrent events), and not the data structure itself via an a-priori order.
Because all communication is between client and server, and never between clients, when the client connects to the server, the server can make sure that it first processes all of the client's local operations before sending it new remote updates.
math_dandy
Is the take-home message of the post that the full complexity of CRDTs/OT is necessary only in the absence of a central server?
mweidner
Even in the absence of a central server, you can still avoid CRDT/OT complexity if you have a decentralized way to eventually total order operations & apply them in that order: https://mattweidner.com/2025/05/21/text-without-crdts.html#d...
As others in the comments argue, this is technically a CRDT (though a fully general one); also, undoing/replaying ops is itself non-trivial to implement. However, I hope this is still simpler than using a traditional CRDT/OT for each data type.
math_dandy
Did you perhaps mean to write, “though not a fully general one”?
mweidner
A decentralized, eventually consistent total order on operations is a fully general CRDT, in the sense that you can put whatever (deterministic) operations you want in the total order and clients will end up in eventually consistent states.
Whether the converged result is at all reasonable is a different question.
motorest
> Is the take-home message of the post that the full complexity of CRDTs/OT is necessary only in the absence of a central server?
That's the whole point of CRDTs: multiple replicas of the same data structure are managed throughout many nodes, each replica is updated independently, and they all eventually converge.
sampton
OT requires centralized server.
n_u
Some OTs do, some don't. OTs with the TP2 property do not require a central authority to order edits I believe.
In my experience if you are persisting your edits or document state, you have something that creates an ordering anyways. That thing is commonly an OLTP database. OLTPs are optimized for this kind of write-heavy workload and there's a lot of existing work on how to optimize them further.
But now even S3 has PUT-IF, so you could use that to create an ordering. https://docs.aws.amazon.com/AmazonS3/latest/userguide/condit...
hencq
I'm not an expert on this, but the main difference with a CRDT like Automerge seems to be the server reconciliation. See for example this article [1]. Automerge handles concurrent insertions by using a sequence number and relying on an agreed ordering of agent ids when insertions are concurrent, while this scheme relies on the server to handle them in the order they come in.
The article mentions this:
> This contrasts with text-editing CRDTs, in which IDs are ordered for you by a fancy algorithm. That ordering algorithm is what differs between the numerous text-editing CRDTs, and it’s the complicated part of any CRDT paper; we get to avoid it entirely.
I can buy the idea that many apps have a central server anyway, so you can avoid the "fancy algorithm", though the server reconciliation requires undoing and replaying of local edits, so it's not 100% clear to me if that's much simpler.
hem777
Agreed, the undoing and replying isn’t exactly non-fancy. Peristent B+Tree is also not very non-fancy.
mweidner
I believe that Automerge internally stores all operations in an eventually consistent total order, which you can use as a substitute for the server in server reconciliation (cf. https://mattweidner.com/2025/05/21/text-without-crdts.html#d...). However, Automerge does not actually do that - instead it processes text operations using a traditional CRDT, RGA, probably because (as you point out) undoing and replaying ops is not trivial to implement.
asdffdasy
so, an unoptimized crdt? just set max set size to 1 and yolo?
th0ma5
It's very appealing as a kind of irreducible complexity. It feels close to what is actually happening and is simple hahah if like you say not optimized.
pshc
Use of server reconciliation makes me think client-side reconciliation would be tricky… how do you preserve smooth editor UX while applying server updates as they arrive?
For example, if your client-sent request to insert a character fails, do you just retry the request? What if an update arrived in the intervening time? (Edit: they acknowledge this case in the “Client-Side” section, the proposal is to rewind and replay, and a simpler proposal to block until the pending queue is flushed)
From a frontend vantage I feel like there may be a long tail of underspecified UI/UX edge cases, such that CRDT would be simpler overall. And how does the editor feel to use while riding the NYC subway where coverage is spotty?
straws
Both ProseMirror and the newer version of CodeMirror have a pretty elegant solution to this: each modification to the document is modeled as a step that keeps track of indices instead of node/text identities, and uses a data structure called a "position map" that the buffered steps can be mapped through to create steps with updated positions, which are then applied to your document.
In practice, it works quite well. Here's more info:
https://marijnhaverbeke.nl/blog/collaborative-editing.html https://marijnhaverbeke.nl/blog/collaborative-editing-cm.htm...
null
null
hbbio
Someone should try to use a local LLM (maybe a 4b) to merge the diffs in case of conflict beyond straightforward cases...
Not energy efficient but should work surprisingly well without CRDT, OT, or anything else.
gregoriol
How is an LLM supposed to merge the example from the article "My name is", insert "Charlie" after "is", insert "Dave" after "is"?
coolcase
Is collaborative text editing with offline sync a nerd snipe [0]? I work for a big tech and write a lot and usually worse case someone else edits at the same time and the server can figure it out. yes it needs some kind of algo but most concurrent edits are on different parts of a huge doc.
Compare this to Git workflows. Git already handles merging most changes seamlessly.
That is very neat. The algorithm:
- Label each text character with a globally unique ID (e.g., a UUID), so that we can refer to it in a consistent way across time - instead of using an array index that changes constantly.
- Clients send the server “insert after” operations that reference an existing ID. The server looks up the target ID and inserts the new characters immediately after it.
- Deletion hides a character for display purposes, but it is still kept for "insert after" position purposes.
This might have potential outside text editing. Game world synchronization, maybe.