Hey HN, I built SyncKit after shipping two local-first projects (RestBolt and Graft) and realizing there's no simple way to add cross-device sync.
The problem: Existing solutions are either too complex (Automerge/Yjs require learning CRDTs) or too restrictive (Firebase isn't truly local-first, Supabase has no offline support - issue #357 has been open 4+ years with 350+ upvotes).
SyncKit is the middle ground: simple API, works offline-first, self-hostable.
Technical highlights:
- TLA+ formal verification: 118,711 states checked, caught bugs before implementation
- Rust → WASM core (48.9KB gzipped)
- 700+ tests including 80 chaos tests (zero data loss)
- Server: Bun/Hono WebSocket (SDK works in any JS runtime)
- Production-ready: v0.1.0 on npm and Docker Hub
Known limitations (v0.1.0):
- LWW only - advanced CRDTs (Text, Counter, Set) coming in v0.2.0
- React hooks only - Vue/Svelte adapters planned
- Reference server is Bun (Node/Deno coming v0.3.0)
Happy to answer questions about the CRDT implementation, TLA+ modeling, or WebSocket architecture.
See my response to skybrian above. You're both asking the same good question about the LWW messaging. Short version: system-level guarantees, not user-level concurrent edit preservation.
I've built my own sync engine in rust to run against supabase. It doesn't run the browser though, it's for a native app (tauri in my case). It's column level lww.
I have one place where I'm thinking I might need a crdt due to the complexity of the data and how you collaborate with it.
People take my app offline for long periods of time then reconnect with 1000s of records to sync so my sync is backgrounded and built to deal with several mb of data in sqlite files.
Anyway I'm interested in taking a further look at your thing over the holidays to see if it's worth switching or making partial use of it.
Thanks! The TLA+ modeling actually caught 3 bugs even before I wrote any code. Worth the upfront investment. It's way easier to debug a state machine model than distributed sync logic.
Happy to share more about the verification approach if you're interested!
Please do in the repo, and thank you for the wonderful contribution on multiple fronts. This is very well done work and documentation. A few tips to help others discover the value on the Readme: Add the key memory benchmark (currently just above the acknowledgments) to the top of the Readme and link to the rest of the benchmarks on the page that you already have (maybe rename it benchmarks.md in the process): https://github.com/Dancode-188/synckit/blob/main/docs/guides...
That's a great suggestion. The memory benchmark being buried is a fair point. I'll move it up top and create a proper benchmarks.md page. Thanks for the detailed feedback.
I appreciate the enthusiasm! And yeah, I'm planning to start blogging about the build process and some of the technical decisions probably after launch week settles down. I'll post about it on LinkedIn and in the GitHub Discussions when it's live. If you're following either, you'll see it!
But I’m a little confused about why it says “Zero data loss with automatic conflict resolution” on the top level page and “data loss possible” under the disadvantages of last-write-wins conflict resolution. It makes sense that you have to start somewhere, but to my mind, last-write-wins isn’t really conflict resolution? What does using this library solve?
I guess it means edits to different fields don’t conflict. But if it’s the same field, it will get clobbered.
"Zero data loss" means system-level guarantees (no corruption from network failures, crashes, etc), not that concurrent edits to the same field are preserved. LWW definitely clobbers one of the edits.
What v0.1.0 does give you is field-level granularity (edits to different fields don't conflict) and guaranteed convergence (all devices reach the same state). But yeah, same field = last write wins.
The Rust core already has better CRDTs (Text, Counter, Set) that handle concurrent edits properly. They just need to exposed in the TypeScript SDK for v0.2.0.
I should clarify this in the docs. Thanks for pointing it out.
I think I know what this is, but the description is so much in its own context I'm not sure. It's for web-apps that also want an offline local version that works and deals with syncing the data when online again and either local or remote is updated?
It probably markets and explains itself perfectly find for someone in that space and/or looking for this solution, so I'm not sure that's actually a problem, but if you also want to stick in the mind of someone that sees this and doesn't have any current interest, but may stumble into needing a solution like this in the future, a few extra words in your initial description might help it be understood more quickly and be something remembered even if they don't dive into it. Or maybe it's fine and I'm just a bit slow today.
That's a fair point. The description does assume some familiarity with local-first patterns. I'll think about how to make the "why you'd want this" clearer for people outside that space. I appreciate the honest feedback
Interesting/impressive project, and would be doubly interested in the workflow used to develop it. Could stand to have more human-voiced docs though. Aside from all the usual reasons I'd avoid using a <1mo dependency over something like Yjs, the bog-standard claude copy on differentiators/reasons to migrate is fairly off-putting to me.
Also maybe bias, but there are still ennough obvious agent artifacts/byproducts in the code base that it makes me doubt that the details were thoroughly attended to, and that's where the devils are.
This is impressive!
After Realm depreciation I have been trying many different offline sync options, they are especially few on Dart…
Dart-Rust binding generator is pretty great though so it’s nice this is in Rust. Will keep an eye on this.
Thanks! Yeah, Realm's shift in direction left a gap in the offline-first space.
I didn't build Dart bindings yet (Rust → WASM → TypeScript for v0.1.0), but since the core is in Rust, Dart bindings via dart_rust_bridge are definitely feasible. Would actually be a great addition.
Are you actively looking for a Realm replacement for a Dart project? Curious what your offline sync requirements are. It might help me prioritize Dart support
I use Loro CRDT, which is written in Rust as well, on my Dart apps with flutter_rust_bridge and it works pretty well. For syncing I'm using their Loro protocol product although I used iroh before. However, the annoying thing about how the CRDT libraries work is that they're essentially key value NoSQL type storage, not relational. I would love something like SQLite or Postgres on both the server and client that can sync, which companies like ElectricSQL have, but no Dart bindings and also I'd prefer to be in the all Rust ecosystem as well as self host.
You've hit on the core limitation because SyncKit has the same issue. Document/key-value like Loro and Yjs. The relational model problem is fundamental to CRDTs. They guarantee conflict-free convergence because documents are independent, but that breaks with foreign keys and joins. Can't have arbitrary SQL queries and guaranteed convergence without coordination.
ElectricSQL and PowerSync solve it differently (full SQL but weaker offline guarantees). For SyncKit, document-based state is an architectural choice, not something v0.2.0 will change.
I'm curious to know how you're working around this with Loro. Are you building your own query layer or are you keeping everything denormalized?
Yeah I'm essentially writing my own ORM with functions like getFoo which queries the exact substructure of the CRDT to find the info I need. Apparently Postgres is getting a CRDT extension though but we'll see how it goes, typically it's not that fast.
Based on the commits(especially the early/foundational ones) this seems to be built primarily with Claude, but I don't see that mentioned in the README.
I want to create a local first, offline/p2p realtime multiplayer prototype app soon with reactive/signal data model and frontend agnostic design (considering solidjs/svelte). I'm on a tech research stage. How does it compare to rxdb, tinybase and zero sync? For reference right now I'm considering tinydb/rxdb.
Currently I am leaning more into P2P for the zero-effort user side setup, although I was considering if hybrid P2P/client-server approach is feasible: free lite P2P vs paid, managed SaaS for user convenience and improved performance.
Some time ago I was looking for a similar solution –offline first and easy sync– and was happy to find https://jazz.tools/. There are many things in common between these two projects. Just mentioning in case you want to check it out.
I don't want to take away from the work you've done, as you're clearly knowledgeable, but as someone else observed, heavy use of AI assistance can be observed in all your public projects. It's worth explicitly addressing, especially considering the foundational nature of your project: it's not easily replaced if it turns out to have to have subtle bugs.
Though I rarely use it myself, I'd like to know, simply because I'm curious as to how other engineers have incorporated such assistance it into their process.
Thanks! For native Android, not directly yet because v0.1.0 is Rust → WASM → TypeScript (web/Node/Deno/Bun).
But since the core sync engine is pure Rust, native Android bindings are definitely possible. The main paths would be:
1. JNI bindings directly from Rust
2. Or using the existing WASM core with a JVM WASM runtime
Haven't prioritized this yet since v0.1.0 focused on web/JS ecosystem, but if there's demand for Android, I could explore it. Are you working on something that needs offline sync on Android?
That's actually a really cool idea! Notes are just documents, so the sync model would work well. Hadn't thought about Obsidian specifically but I like it.
Not yet, but it's definitely something I'm considering.
Right now v0.1.0 is React (web) only. I'm planning Vue/Svelte for v0.2.0, and React Native could absolutely be v0.3.0 or sooner if there's demand.
The nice thing is the core sync engine is Rust → WASM, so it's framework-agnostic. The main work is just creating the React Native bindings and not rebuilding the sync logic.
Are you working on something with React Native? Curious what your offline sync needs are because it'd help me prioritize what to build next.
Hey HN, I built SyncKit after shipping two local-first projects (RestBolt and Graft) and realizing there's no simple way to add cross-device sync.
The problem: Existing solutions are either too complex (Automerge/Yjs require learning CRDTs) or too restrictive (Firebase isn't truly local-first, Supabase has no offline support - issue #357 has been open 4+ years with 350+ upvotes).
SyncKit is the middle ground: simple API, works offline-first, self-hostable.
Technical highlights: - TLA+ formal verification: 118,711 states checked, caught bugs before implementation - Rust → WASM core (48.9KB gzipped) - 700+ tests including 80 chaos tests (zero data loss) - Server: Bun/Hono WebSocket (SDK works in any JS runtime) - Production-ready: v0.1.0 on npm and Docker Hub
Known limitations (v0.1.0): - LWW only - advanced CRDTs (Text, Counter, Set) coming in v0.2.0 - React hooks only - Vue/Svelte adapters planned - Reference server is Bun (Node/Deno coming v0.3.0)
Happy to answer questions about the CRDT implementation, TLA+ modeling, or WebSocket architecture.
GitHub: https://github.com/Dancode-188/synckit npm: @synckit-js/sdk
The ReadMe says "Zero data loss with automatic conflict resolution (Last-Write-Wins)", bur doesn't LWW guarantee data loss?
See my response to skybrian above. You're both asking the same good question about the LWW messaging. Short version: system-level guarantees, not user-level concurrent edit preservation.
I've built my own sync engine in rust to run against supabase. It doesn't run the browser though, it's for a native app (tauri in my case). It's column level lww.
I have one place where I'm thinking I might need a crdt due to the complexity of the data and how you collaborate with it.
People take my app offline for long periods of time then reconnect with 1000s of records to sync so my sync is backgrounded and built to deal with several mb of data in sqlite files.
Anyway I'm interested in taking a further look at your thing over the holidays to see if it's worth switching or making partial use of it.
> Automerge/Yjs require learning CRDTs
I've been using Automerge for a while and haven't had to look at any CRDTs. To me this looks very similar to Automerge.
Neat project!
TLA+ checked! Whoa!
Thanks! The TLA+ modeling actually caught 3 bugs even before I wrote any code. Worth the upfront investment. It's way easier to debug a state machine model than distributed sync logic.
Happy to share more about the verification approach if you're interested!
Please do in the repo, and thank you for the wonderful contribution on multiple fronts. This is very well done work and documentation. A few tips to help others discover the value on the Readme: Add the key memory benchmark (currently just above the acknowledgments) to the top of the Readme and link to the rest of the benchmarks on the page that you already have (maybe rename it benchmarks.md in the process): https://github.com/Dancode-188/synckit/blob/main/docs/guides...
That's a great suggestion. The memory benchmark being buried is a fair point. I'll move it up top and create a proper benchmarks.md page. Thanks for the detailed feedback.
VERY interested, are you planning on writing a blog? If so I would definitely read that post.
I appreciate the enthusiasm! And yeah, I'm planning to start blogging about the build process and some of the technical decisions probably after launch week settles down. I'll post about it on LinkedIn and in the GitHub Discussions when it's live. If you're following either, you'll see it!
Thanks for the interest!
The conflict resolution guide seems quite clear:
https://github.com/Dancode-188/synckit/blob/main/docs/guides...
But I’m a little confused about why it says “Zero data loss with automatic conflict resolution” on the top level page and “data loss possible” under the disadvantages of last-write-wins conflict resolution. It makes sense that you have to start somewhere, but to my mind, last-write-wins isn’t really conflict resolution? What does using this library solve?
I guess it means edits to different fields don’t conflict. But if it’s the same field, it will get clobbered.
You're right that this is confusing.
"Zero data loss" means system-level guarantees (no corruption from network failures, crashes, etc), not that concurrent edits to the same field are preserved. LWW definitely clobbers one of the edits.
What v0.1.0 does give you is field-level granularity (edits to different fields don't conflict) and guaranteed convergence (all devices reach the same state). But yeah, same field = last write wins.
The Rust core already has better CRDTs (Text, Counter, Set) that handle concurrent edits properly. They just need to exposed in the TypeScript SDK for v0.2.0.
I should clarify this in the docs. Thanks for pointing it out.
I think I know what this is, but the description is so much in its own context I'm not sure. It's for web-apps that also want an offline local version that works and deals with syncing the data when online again and either local or remote is updated?
It probably markets and explains itself perfectly find for someone in that space and/or looking for this solution, so I'm not sure that's actually a problem, but if you also want to stick in the mind of someone that sees this and doesn't have any current interest, but may stumble into needing a solution like this in the future, a few extra words in your initial description might help it be understood more quickly and be something remembered even if they don't dive into it. Or maybe it's fine and I'm just a bit slow today.
That's a fair point. The description does assume some familiarity with local-first patterns. I'll think about how to make the "why you'd want this" clearer for people outside that space. I appreciate the honest feedback
Interesting/impressive project, and would be doubly interested in the workflow used to develop it. Could stand to have more human-voiced docs though. Aside from all the usual reasons I'd avoid using a <1mo dependency over something like Yjs, the bog-standard claude copy on differentiators/reasons to migrate is fairly off-putting to me.
Also maybe bias, but there are still ennough obvious agent artifacts/byproducts in the code base that it makes me doubt that the details were thoroughly attended to, and that's where the devils are.
This is impressive! After Realm depreciation I have been trying many different offline sync options, they are especially few on Dart… Dart-Rust binding generator is pretty great though so it’s nice this is in Rust. Will keep an eye on this.
Thanks! Yeah, Realm's shift in direction left a gap in the offline-first space.
I didn't build Dart bindings yet (Rust → WASM → TypeScript for v0.1.0), but since the core is in Rust, Dart bindings via dart_rust_bridge are definitely feasible. Would actually be a great addition.
Are you actively looking for a Realm replacement for a Dart project? Curious what your offline sync requirements are. It might help me prioritize Dart support
I use Loro CRDT, which is written in Rust as well, on my Dart apps with flutter_rust_bridge and it works pretty well. For syncing I'm using their Loro protocol product although I used iroh before. However, the annoying thing about how the CRDT libraries work is that they're essentially key value NoSQL type storage, not relational. I would love something like SQLite or Postgres on both the server and client that can sync, which companies like ElectricSQL have, but no Dart bindings and also I'd prefer to be in the all Rust ecosystem as well as self host.
You've hit on the core limitation because SyncKit has the same issue. Document/key-value like Loro and Yjs. The relational model problem is fundamental to CRDTs. They guarantee conflict-free convergence because documents are independent, but that breaks with foreign keys and joins. Can't have arbitrary SQL queries and guaranteed convergence without coordination.
ElectricSQL and PowerSync solve it differently (full SQL but weaker offline guarantees). For SyncKit, document-based state is an architectural choice, not something v0.2.0 will change.
I'm curious to know how you're working around this with Loro. Are you building your own query layer or are you keeping everything denormalized?
Yeah I'm essentially writing my own ORM with functions like getFoo which queries the exact substructure of the CRDT to find the info I need. Apparently Postgres is getting a CRDT extension though but we'll see how it goes, typically it's not that fast.
This is a write up that may be helpful for you: https://sqlsync.dev/posts/stop-syncing-everything/
Always nice to see something in here. Would love to hear what is your take on https://www.instantdb.com/?
I haven't tried InstantDB yet, but it looks really interesting. Thanks for the pointer! Will definitely check it out.
Based on the commits(especially the early/foundational ones) this seems to be built primarily with Claude, but I don't see that mentioned in the README.
I want to create a local first, offline/p2p realtime multiplayer prototype app soon with reactive/signal data model and frontend agnostic design (considering solidjs/svelte). I'm on a tech research stage. How does it compare to rxdb, tinybase and zero sync? For reference right now I'm considering tinydb/rxdb.
For P2P multiplayer with Svelte/SolidJS, SyncKit might not be your best fit because:
- It's client-server (not P2P)
- No Svelte adapters yet (coming in v0.2.0)
- Multiplayer games usually need P2P for lower latency
Better options for your use case:
- Jazz (jazz.tools) - Purpose-built for P2P collaborative apps
- TinyBase - Perfect signal model for Svelte/Solid, but you'd add your own sync
- Yjs - Mature CRDT with good P2P support
- RxDB - Heavier but has everything (queries, reactive, plugins)
If you went client-server instead of P2P, SyncKit would work once v0.2.0 adds Svelte support.
What's your preference? P2P or client-server? That'll determine the best fit.
Currently I am leaning more into P2P for the zero-effort user side setup, although I was considering if hybrid P2P/client-server approach is feasible: free lite P2P vs paid, managed SaaS for user convenience and improved performance.
Some time ago I was looking for a similar solution –offline first and easy sync– and was happy to find https://jazz.tools/. There are many things in common between these two projects. Just mentioning in case you want to check it out.
When you mentioned offline sync and graft, I mistook you for the author of this project:
https://github.com/orbitinghail/graft
However, they're clearly two different projects.
I don't want to take away from the work you've done, as you're clearly knowledgeable, but as someone else observed, heavy use of AI assistance can be observed in all your public projects. It's worth explicitly addressing, especially considering the foundational nature of your project: it's not easily replaced if it turns out to have to have subtle bugs.
Though I rarely use it myself, I'd like to know, simply because I'm curious as to how other engineers have incorporated such assistance it into their process.
Good work @danbitengo Is this usable in native Android app development?
Thanks! For native Android, not directly yet because v0.1.0 is Rust → WASM → TypeScript (web/Node/Deno/Bun).
But since the core sync engine is pure Rust, native Android bindings are definitely possible. The main paths would be:
1. JNI bindings directly from Rust
2. Or using the existing WASM core with a JVM WASM runtime
Haven't prioritized this yet since v0.1.0 focused on web/JS ecosystem, but if there's demand for Android, I could explore it. Are you working on something that needs offline sync on Android?
This would make a great backbone for a Obsidian Sync plugin.
That's actually a really cool idea! Notes are just documents, so the sync model would work well. Hadn't thought about Obsidian specifically but I like it.
Is support for react native on the roadmap?
Not yet, but it's definitely something I'm considering.
Right now v0.1.0 is React (web) only. I'm planning Vue/Svelte for v0.2.0, and React Native could absolutely be v0.3.0 or sooner if there's demand.
The nice thing is the core sync engine is Rust → WASM, so it's framework-agnostic. The main work is just creating the React Native bindings and not rebuilding the sync logic.
Are you working on something with React Native? Curious what your offline sync needs are because it'd help me prioritize what to build next.
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