Diamond types author here. I’m working on another blog post - I had a big breakthrough getting collaborative text editing many times faster again using some new ideas. I’m in the process of writing up our new work now.
And there’s another hard problem I’m stuck on: so, CRDTs like mine and Yjs use (random agent id, sequence number) as unique IDs for operations. But a troublesome user might reuse an operation ID, sending different operations to different peers and giving them the same (agent, seq) pair. This results in really awful, weird bugs. Martin Kleppman wrote a paper about BFT in CRDTs. His suggestion is we do what git does and use SHA hashes for IDs instead.
But there’s two problems:
1. We don’t want to store a hash for every keystroke a user makes. That would get big, fast. We want a DAG of hashes like git, except a hash is generated with each keystroke. How do we build an efficient system that can query by hash, without storing all of the hashes?
2. I want users to be able to permanently delete inserted text. If I accidentally paste an AWS key into a document, I don’t want that key in the document history forever. How do I enable that while still getting all the benefits of the hashing system above? (Note if your hash includes the typed character, even if you delete the character itself from disk, you can trivially brute force the hash to recover any typed characters).
Each of these problems in isolation has a solution. But the real prize is how do you solve both at the same time? I think this problem has a solution but I don’t think anyone has solved it yet. Want to name an algorithm and get my eternal respect? Here’s your chance.
> 1. We don’t want to store a hash for every keystroke a user makes.
So, why does it have to be every keystroke? We can batch them locally. An approximate rule of thumb to use to guide the batching is typing speed. Using 200 keystrokes/minute, we get an average of 3.33 keystrokes/second. If we use 3 seconds latency for the collaborative editing to resolve concurrent edits, we get 10 keystrokes per batch (3 secs). You can also have an override where smaller batches are replicated to nodes if sufficient time has elapsed since last local edit (eg: 10 secs have elapsed and we have a partial batch of 3 keystrokes only).
> 2. If I accidentally paste an AWS key into a document, I don’t want that key in the document history forever. How do I enable that while still getting all the benefits of the hashing system above?
In order for this to work, we should assume that every replica's current state is always obtained by starting with the empty document and then playing the operations log. We should also permit the author of an operation to rollback the operation. So, I suppose if the collaborative editor shows an interface of all operations that originated locally and the author can select the operations that inserted the AWS key into the replica, they can issue a rollback operations op and have it propagated to all replicas.
Since the point-in-time state of the replicas are regenerated by replaying the operations log, we would have deleted the accidentally inserted key.
In order to address the issue of the AWS key still being in the operations log, we can define the semantics of the rollback op to also include secure erasure of the previous operation from the operations log (i.e., the rollback op would update the original operation into a no-op.
So, when all replicas apply the rollback op initiated by the author, eventually all replicas will converge to having the accidentally inserted AWS key removed from the replica and the operations log.
I think keystrokes was just a metaphor for whatever operation the application has to deal with.
> This repository contains a high performance rust CRDT for text editing. This is a special data type which supports concurrent editing of lists or strings (text documents) by multiple users in a P2P network without needing a centralized server.
> This version of diamond types only supports plain text editing. Work is underway to add support for other JSON-style data types. See the more_types branch for details.
In any case, I agree with the metaphor and batching granular operations can always be done.