Comment by aboodman | Hacker News Reader

Comment by aboodman | Hacker News Reader

Parent Full thread

Hi! Thanks for the thoughtful question.

>List operations just work:

Fair point. Will change. What I really meant here was "(Many) list operations just work" (just like above I said "all kinds of things just work".

> All updates reach the server at the same time. What is the solution to that? Timestamps? Fallback to LLW?

There are a number of issues you might be pointing out, and I'm not sure which one you mean.

In general, you can't use list indexes as identifiers for edit/delete in Reflect because they aren't stable. That's why in this example I commented to use the item itself (if atomic) or more likely a stable ID: https://i.imgur.com/IKzmf0q.png

There is also the potential issue of User C's inserts getting deleted. This seems like a problem at first, but in a realtime collaborative context, nothing can fix this. User C's insert can land just before User A or B's delete. In this case, sync would be correct to delete user C's insert, no matter the protocol. But User C might still be sad. From her perspective, she just wrote something and it got deleted. This is the nature of two humans working in the same place at the same time and potentially having different intentions. For these problems, undo and presence indicators go a long way to avoid these problems through human/social mechanisms.

>nothing can fix this

Exactly.

>For these problems, undo and presence indicators go a long way to avoid these problems through human/social mechanisms.

x1,000 to this. This guy builds. That's exactly my takeaway as well from years working on this; provide a solid (by that I mean clearly defined) deterministic algo, and solve the rest w/ UX.

If you actually look at how people use most multiplayer editing tools they tend to avoid conflicts anyway. It’s extremely painful to have two people editing the same sentence in Google Docs let alone more than that even if it sort of works. Collaboration tends to be negotiated so you end up with I-go-you-go, reviews and other patterns forming around the use of the tools. People can also see and resolve conflicts in real-time.

That said this sort of reconciliation can be extremely painful if people are working separately offline for any length of time as arbitrarily resolving many conflicts doesn’t necessarily result in coherent results. But as I understand it that’s also an issue for other ways of solving the issue as well.

Another missing piece is signalling failure. For example two people trying to fill the last slot of a limited resource. Having to infer from the state that comes back whether your operation succeeded or not is not a fun way to write multiplayer code.

This points towards a big challenge that I think gets overlooked way too often. A lot of these sync libraries/frameworks claim to solve for both real-time interaction, and local-first, sync when you reconnect. But the UX needs are wildly different.

For the former you want (as mentioned) good presence info and undo. For the latter you (sometimes) want explicitly flagged conflicts and manual resolution, like we have in git etc.

With the Replicache/Reflect model, you could actually handle both approaches. Might not be easy but the model can support it.

If a client is offline for a while and makes a bunch of edits, would rebasing many thousands of mutations not be a major performance issue? (For example: if complexity is O(n) per edit, as is often the case with string insertions, the process might hang for a long time.)

It seems to me that maybe saying that CRDTs are "good for one particular problem," whereas Reflect can do anything, is a bit misleading? Reflect is eventually consistent, but if you want e.g. string mutations to make sense, you still need to de facto model your text as a CRDT. (Unique IDs per letter, tombstones, etc.) Otherwise, you'll end up with garbled text.

I didn't mean to say that Reflect can do _anything_. We do think it's a general model that ends up working well for very many things in practice.

Reply on news.ycombinator.com