In 2019 I built an offline-first datastore called debe. It used CRDTs and multi-master replication and never became something I could confidently ship. It did teach me that convergence is only a small part of sync. The difficult parts are durable writes, authorization changes, lost acknowledgements, schema upgrades, bounded bootstrap, and explaining what happened after something fails.
I eventually came back to the problem and built Syncular over the last year.
Every client has a real SQLite database: sqlite-wasm on OPFS in the browser and native SQLite elsewhere. Writes apply locally and enter a durable outbox in the same transaction. A server-authoritative commit log validates and orders them, using explicit scopes resolved inside the application backend.
The protocol is written down independently of the implementations. SSP2 is currently a draft, with canonical binary encoding and golden byte vectors. There are independent TypeScript and Rust client cores, both run against the same 95-scenario conformance catalog. The Rust core is exposed through Swift, Kotlin, Flutter, React Native, Tauri, Rust, and a small C API.
It also includes realtime WebSocket sync, resumable bootstrap segments, explicit conflict evidence, authorization revocation and local purge, windowed replicas, blobs, optional CRDT columns, per-column encryption, and typesafe generated queries for five languages. Important boundaries: it is pre-1.0, self-hosted, and there is no managed service or peer-to-peer mode. Native packaging maturity still varies by ecosystem.
Let me know what you think, I'm happy about feedback!
Demo: https://demo.syncular.dev Protocol: https://github.com/syncular/syncular/blob/main/docs/SPEC.md Background: https://syncular.dev/blog/offline-first-writes/ Docs: https://syncular.dev/