In early 2026, I started working on Lark, with the goal of creating a realtime database with the same fundamental premise of Firebase RTDB: syncing a JSON tree with persistence to thousands of concurrent users. Today, Lark is releasing as open-source software under an AGPLv3 license.
I've been building on Firebase RTDB since the earliest days, and I still think it's one of the most magical ways to build realtime apps for the web and mobile. However, I've always been bothered by the fact that it's building on top of a closed ecosystem. Lark brings the magic of Firebase RTDB to you with the freedom to run it on your own infrastructure.
Lark is unique in that it is fully drop-in compatible with the existing Firebase client SDKs. That means you can change a few config vars in your project, move your data to Lark, and you're good to go. In addition, you can continue using other parts of Firebase, like Auth[1], Analytics, and Hosting. This makes it very easy to migrate an existing project.
It's written in Rust on top of Glommio, so it uses a thread-per-core architecture to maximize throughput and take advantage of Rust's GC-free memory management to achieve low 99th percentile latency. It also has an edge proxy component written in Go which handles terminating REST, Websocket, and WebTransport connections.
There are a whole host of gotchas, unique behaviours, and technical challenges with making this "just work" with Firebase's SDKs, from the way that Firebase RTDB handles arrays to the rules engine. While it's too soon to say that there are no edge cases left, we actually run the Firebase JS SDK's own integration tests against our server, and pass all of them. There's also been a lot of testing of legacy Firebase apps from various eras to suss out hidden quirks and match them so that migration is as seamless as possible.
In addition, Lark has a number of new features that it brings to the table, including volatile data and WebTransport (for high-frequency data like cursor positions) as well as ephemeral databases with auto-creation and cleanup. It also ships its own JS client so if you just want to start from scratch and build on Lark without any Firebase SDK involved, that's an option, too.
Today is also the public launch of Lark Cloud[2], a hosted version of Lark for those who want to leave the devops to us, at pricing that's significantly cheaper than what you're probably currently paying.
There's a Quickstart guide in the repo if you'd like to take Lark for a spin, as well as examples showing both a lightweight app created using the Lark JS SDK, as well as converting an existing Firebase JS SDK app to use Lark.
Happy to answer questions, and thanks for taking the time to check this out!
[1] https://docs.larksh.com/firebase/auth
[2] https://lark.sh