reference-crdts VS gc

> The goog version seems to work well but I have had nothing but frustration with ms word. Bad merges and weird states are typical, particularly from the fat client.

Argh not getting this stuff right is really frustrating. I've been working on collaborative editing for over a decade now, and I still can't implement any of these algorithms correctly without the help of a fuzz testing. But fuzz testing done right finds all of these problems! There's no excuse!

Fuzzers work so well here because all of these algorithms have a clear correctness criteria: After syncing, state should always converge to the same result. So its pretty easy to write code which does this in a loop:

1. Generates some random changes on some fake "peers"

2. Picks 2 peers at random and sync their changes, using your new fancy synchronization algorithm

3. Assert that the state has converged between the peers

I've been working on this stuff for over a decade. I've implemented dozens of these algorithms. And every single time I write a fuzzy boi to check my work I find convergence bugs. Playing whack-a-mole with a fuzzer is a rite of passage for implementing systems like this.

When your fuzzer runs all night, you should never have lingering convergence bugs like you're describing with Word.

As an example, here's a simple fuzzer for a reference list CRDT implementation: https://github.com/josephg/reference-crdts/blob/9f4f9c3a97b4...

The code is so small it almost fits on my laptop screen.

> In this case, the bottleneck at 9 million LoC is not CPU cycles but memory usage. That's where I am considering pushing down into WebAssembly

How often does this come up in practice? I can't think of many files I've opened which were 9 million lines long. And you say "LoC" (lines of code). Are you doing syntax highlighting on 9 million lines of source code in javascript? Thats impressive!

> I guess my point is why do you need balanced trees? Is this a CRDT specific thing? Can you implement CRDT with just an array of lines / gap buffer?

Of course! Its just going to be slower. I made a simple reference implementation of Yjs, Automerge and Sync9's list types in javascript here[1]. This code is not optimized, and it takes 30 seconds to process an editing trace that diamond types (in native rust) takes 0.01 seconds to process. We could speed that up - yjs does the same thing in 1 second. But I don't think javascript will ever run as fast as optimized rust code.

The b-tree in diamond types is used for merging. If you're merging 2 branches, we need to map insert locations from the incoming branch into positions in the target (merged) branch. As items are inserted, the mapping changes dynamically. The benchmark I've been using for this is how long it takes to replay (and re-merge) all the changes in the most edited file in the nodejs git repository. That file has just shy of 1M single character insert / delete operations. If you're curious, the causal graph of changes looks like this[2].

Currently it takes 250ms to re-merge the entire causal graph. This is much slower than I'd like, but we can cache the merged positions in about 4kb on disk or something so we only need to do it once. I also want to replace the b-tree with a skip list. I think that'll make the code faster and smaller.

A gap buffer in javascript might work ok... if you're keen, I'd love to see that benchmark. The code to port is here: [3]

> Undo support -> In which case, you only have to stack / remember the set of commands and not have to store the state on every change. I'm not sure if this overlaps with the data structure choice, other than implementation details.

Yeah, I basically never store a snapshot of the state. Not on every change. Not really at all. Everything involves sending around patches. But you can't just roll back the changes when you undo.

Eg: I type "aaa" at position 0 (the start of the document). You type "bbb" at the start of the document. The document is now "bbbaaa". I hit undo. What should happen? Surely, we delete the "aaa" - now at position 3.

Translating from position 0 to position 3 is essentially the same algorithm we need to run in order to merge.

> I was just looking into TypedArrays.

I tried optimizing a physics library a few years ago by putting everything in typedarrays and it was weirdly slower than using raw javascript arrays. I have no idea why - but maybe thats fixed now.

TypedArrays are useful, but they're no panacea. You could probably write a custom b-tree on top of a typedarray in javascript if you really want to - assuming your data also fits into typedarrays. But at that point you may as well just use wasm. It'll be way faster and more ergonomic.

[1] https://github.com/josephg/reference-crdts

[2] https://home.seph.codes/public/node_graph.svg

[3] https://github.com/josephg/diamond-types/tree/master/src/lis...

It may take some time for WasmGC to be usable by .NET. Based on the discussions the first version of WasmGC does not have a good way to handle a few .NET specific scenarios, and said scenarios are "post-post-mvp". [0]

My concern, of course, is that there is not much incentive for those features to be added if .NET is the only platform that needs them... at that point having a form of 'include' (to where a specific GC version can just be cached and loaded by another WASM assembly) would be more useful, despite the pain it would create.

[0] - https://github.com/WebAssembly/gc/issues/77

Yes, that's definitely true: a single GC will not be optimal for everything, or even possible. Atm interior pointers are not supported at all, for example, but they are on the roadmap for later:

https://github.com/WebAssembly/gc/blob/main/proposals/gc/Pos...

What launched now is enough WasmGC to support a big and useful set of languages (Java, Kotlin, Dart, OCaml, Scheme), but a lot more work will be required here!

GC proposal is from 2018: https://github.com/WebAssembly/proposals/issues/16 and there’s code: https://github.com/WebAssembly/gc/blob/master/proposals/gc/O...

Seems like an awefully long time for progress to be made, given all the possibilities it would unlock.

First is the size. Writing a server-side and client-side program is possible with Rust, and the resulting WASM package will be small enough. At the same time, Microsoft Blazor converts C# code to WASM, but the client delivery has to include the reduced .NET runtime, taking several megabytes for a script. The same is true for GoLang, even with an attempt to reduce the runtime delivery in TinyGo WASM. Developers want to work with their favorite languages, whether it is Java, Kotlin, Dart, C#, F#, Swift, Ruby, Python, C, C++, GoLang, or Rust. These languages produce groups of runtimes. For example, JVM and .NET have many common parts, Ruby and Python are dynamically interpreted at runtime, and all mentioned depend on automatic garbage collection. For smaller WASM packages, browser vendors can include extended runtime implementations, for example, by delivering a general garbage collector as part of WASM. Garbage collection support by WASM is currently in progress: WASM GC, .NET WASM Notes.

My understanding is that the main limitation is technical. WASM doens't do GC or the host system calling conventions and cannot interact directly with object from Javascript because of this. However, this is being worked[0] on and will be solved eventually. Even without this the performance overhead of bridging to JS is low enough that WASM frameworks can beat out React.

0: https://github.com/WebAssembly/gc/blob/main/proposals/gc/Ove...

I’ve just watched a video on YouTube from Google I/O 2023 on Flutter for the web. Kevin Moore explains that Flutter can compile to Wasm, but now that GC support has been added to the standard and WasmGC is supported in Chromium and Firefox, I’m quite intrigued.

I have found the main thread about using WebAssembly GC in C#: https://github.com/WebAssembly/gc/issues/77. If I understand it correctly, it is not possible to use the current prototype version of GC in C#.

The .net team has given their notes on it, the concern is more on the memory layout from what I remember. Though it may be possible still. The runtime would likely still ship some gc code, but only a subset for cases not supported by the wasm gc itself and a few more for interfacing with the gc service, which overall should still result on smaller payloads compared to current sizes.

I assume that's one of the parts of the work done at https://github.com/WebAssembly/gc - not happening any soon yet, but it'll eventually be done.