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You don't need a CRDT to build a collaborative experience

zknill.io

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Re: You don't need a CRDT to build a collaborative experience

#11

Post by btown »

jitl wrote: post #10

I agree broadly with the article’s position but I think locks are more harmful than helpful. When I was a Quip user (2018) it was super frustrating to get locked out of a paragraph because someone’s cursor idled there. Instead just allow LWW overwrites. If users have contention and your sync & presence is fast, they’ll figure it out pretty quick, and at most lose 1-2 keystrokes, or one drag gesture, or one color pick…

Are you considering having a CRDT for each text block individually, or moving to a CRDT for the entire data model for a document? Really curious about the design approach here, especially insofar as there's now an external API that the data models need to service!

Re: You don't need a CRDT to build a collaborative experience

#12

Post by lewisjoe »

> everyone’s gonna say “but hey, google docs uses operational transform not CRDTs”.. OK yes, but you are not google.

Well, google docs works not because they somehow figured out how to converge OT edits with as much precision as CRDTs do, but simply because they have a central server which orders edits anyway and don't need true leader-less convergence.

In fact, I agree not many things don't need a CRDT. CRDTs help with mathematical rigidity of convergence when you want true peer-2-peer collaboration which works without any central authority.

However, most apps anyway work on top of a central authority (example SaaS apps) so there is no real reason to accomodate all the compexity that comes with CRDT. They might get far with a simpler OT based model + central server based ordering.

For example even Figma doesn't call its model a 100% pure CRDT. It's a partial, simpler CRDT implemented with an assumption that there's going to be a server that understands ordering.

It's the same with Google Docs. They don't need a CRDT because it's a cloud app after all, which means OT is more convenient with major heavylifting (ordering and conflict handlings) outsourced to the server.

Re: You don't need a CRDT to build a collaborative experience

#13

Post by jitl »

dsmmcken wrote: post #3
MontagFTB wrote: post #2

One of the main points of this article is to "just use locks", which glosses over a lot of technical complications about locking elements within a document. How long is the lock held? Can it be stolen from a user who has gone offline, or is still online but off to lunch, and we _really_ need to make this change before the presentation in an hour? What if the user comes back online and they have changes, but the lock…

> How long is the lock held? For however long the user has a browser focus state on the element seems like a reasonable answer, and submit changes as they are made. However, I don't know how you resolve conflicts of two users simultaneously attempting to acquire a lock.

Having used an app with locks like this (Quip) I can tell you it really sucks to have a lock lease >10s after the last edit. The “lock” should be a UI level concept anyways - clients should broadcast “hey I’m taking field X”; if you have two users submit a write+lock request concurrently you’ll need to pick a winner anyways.

Re: You don't need a CRDT to build a collaborative experience

#14

Post by [deleted] »

MontagFTB wrote: post #2

One of the main points of this article is to "just use locks", which glosses over a lot of technical complications about locking elements within a document. How long is the lock held? Can it be stolen from a user who has gone offline, or is still online but off to lunch, and we _really_ need to make this change before the presentation in an hour? What if the user comes back online and they have changes, but the lock…

[deleted]

Re: You don't need a CRDT to build a collaborative experience

#15

Post by jitl »

btown wrote: post #11
jitl wrote: post #10

I agree broadly with the article’s position but I think locks are more harmful than helpful. When I was a Quip user (2018) it was super frustrating to get locked out of a paragraph because someone’s cursor idled there. Instead just allow LWW overwrites. If users have contention and your sync & presence is fast, they’ll figure it out pretty quick, and at most lose 1-2 keystrokes, or one drag gesture, or one color pick…

Are you considering having a CRDT for each text block individually, or moving to a CRDT for the entire data model for a document? Really curious about the design approach here, especially insofar as there's now an external API that the data models need to service!

It’s Complicated (tm), a hybrid of the two. Text content across multiple CRDT enabled blocks may be connected in a single logical CRDT, but the segment visible in any given block is stored in that block object, which maintains our permission system invariants. We’ll still support moving blocks to different pages, so one page may have a few different CRDTs visually interleaved.

All the edge cases are very interesting, like what happens if I use backspace to join a block of CRDT B into block of CRDT A.

API consumers are unaffected. Likely we’ll support API updates of text as best-effort diff-match-patch of the text on our side applied as CRDT ops.

Re: You don't need a CRDT to build a collaborative experience

#16

Post by zknill »

MontagFTB wrote: post #2

One of the main points of this article is to "just use locks", which glosses over a lot of technical complications about locking elements within a document. How long is the lock held? Can it be stolen from a user who has gone offline, or is still online but off to lunch, and we _really_ need to make this change before the presentation in an hour? What if the user comes back online and they have changes, but the lock…

You're absolutely right, locking is a hard problem. Especially when you get into the edge cases of clients not disconnecting cleanly.

If you've ever used one of those awful early-2000s Microsoft word collaboration systems where you have to check-out the doc, and remember to check it back in, and no one can use it until you've checked it back in... it's awful!

I'm not directly in this team, but one of the teams at my company have been working on this problem. They call it "Spaces", and one of the features solves this component locking problem.

https://ably.com/examples/component-locking

Re: You don't need a CRDT to build a collaborative experience

#17

Post by antidnan »

I don't think you need a pure CRDT either but I think locking and presence is a bit of an oversimplification.

LWW is a good place to start, and updating the smallest piece of information possible is the right idea in general but there is a lot more nuance to handling complex applications like a spreadsheet (I'm working on one) and whiteboard apps.

Things like reparenting or grouping shapes [1], or updating elements that aren't at the lowest scale like deleting a row or column in a spreadsheet make locking challenging to implement. Do you lock the entire row while I'm moving it? Do you lock the entire group of shapes?

With the exception of text editing, the popular libraries like Yjs don't just give you a perfect CRDT out of the box. You still have to construct your data model in a way that enables small scale updates [2], and CRDT libraries and literature are the best source of thinking for these problems that I've found.

[1] https://www.figma.com/blog/how-figmas-multiplayer-technology...

[2] https://mattweidner.com/2022/02/10/collaborative-data-design...

Re: You don't need a CRDT to build a collaborative experience

#18

Post by zknill »

jitl wrote: post #10

I agree broadly with the article’s position but I think locks are more harmful than helpful. When I was a Quip user (2018) it was super frustrating to get locked out of a paragraph because someone’s cursor idled there. Instead just allow LWW overwrites. If users have contention and your sync & presence is fast, they’ll figure it out pretty quick, and at most lose 1-2 keystrokes, or one drag gesture, or one color pick…

So Last-Write-Wins (LWW) basically _is_ a CRDT, but not in the sense that anyone really expects, because they aren't that useful or intention preserving. Especially if the two writes happen in very quick succession / concurrently.

LWW becomes useful if you can:

a) help humans to see who is doing what on a doc

b) reduce the size of the change that is LWW

As you've said:

> However our LWW texts are individually small - one block/paragraph in size

This is really important, because by reducing the size of the individual element that any single user is updating you can also reduce the chance of conflicts. With a small amount of contextual feedback (like the notion icon next to the block) a lot of the problem cases are just avoided.

Clearly locking and updating the entire document would be terrible, but if you can do it on a small enough scope that others can change other elements, it can work really well.

(If you've worked on the notion things you're describing then I'm sure you know this better than I do, but just spelling it out really clearly.)

Re: You don't need a CRDT to build a collaborative experience

#19

Post by jitl »

zknill wrote: post #18
jitl wrote: post #10

I agree broadly with the article’s position but I think locks are more harmful than helpful. When I was a Quip user (2018) it was super frustrating to get locked out of a paragraph because someone’s cursor idled there. Instead just allow LWW overwrites. If users have contention and your sync & presence is fast, they’ll figure it out pretty quick, and at most lose 1-2 keystrokes, or one drag gesture, or one color pick…

So Last-Write-Wins (LWW) basically _is_ a CRDT, but not in the sense that anyone really expects, because they aren't that useful or intention preserving. Especially if the two writes happen in very quick succession / concurrently. LWW becomes useful if you can: a) help humans to see who is doing what on a doc b) reduce the size of the change that is LWW As you've said: > However our LWW texts are individually small -…

You can have a LWW CRDT, but not every LWW is a CRDT. LWW CRDTs generally pick a winner based on causal order which is convergent, the C in CRDT, because every peer receiving the same ops in any order would pick the same winner.

Picking a winner based on wall clock time order (as suggested in the article, and implemented by Notion) is not convergent; if peers used that algorithm to apply ops I they would not converge to the same state. Instead you need an authority (your server) to essentially pick an arbitrary order of ops and clients just have to deal with it.

A practical example is to consider three users (A, B, C) working on a document.

1. A, B, C start online working together.

2. C goes offline

3. A, B stay online, and make 100s of changes.

4. C makes a few changes while offline (not sent to other peers yet)

5. C comes back online and sends their changes to (the server / other peers).

In wall-clock LWW, C's changes will overwrite A & B's changes, even though A & B have done a lot more work.

In a causal ordering CRDT implementing LWW, C's changes will "lose" to A & B's changes, since they were actually made "earlier" in causal ordering.

> Clearly locking and updating the entire document would be terrible, but if you can do it on a small enough scope that others can change other elements, it can work really well.

I'm sure good UX is possible with locks, but I haven't used one yet for document editing. I'm still traumatized from Quip which did per-paragraph (per block?) locking and was really annoying. Eventually they added an unlock button next to the block so anyone could "steal" the lock at will due to all the user complaints, but at that point, why put it in at all?

Re: You don't need a CRDT to build a collaborative experience

#20

Post by earthboundkid »

I've seen locks used at the CMSes of large news organizations. It's fine, but they all need a mechanism to kick out an editor who has an idle tab left open. For my own small scale CMS, I just wrapped Google Docs and let them handle all the syncing headaches.
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