We're building a new multiplayer IDE but for docs/tasks [1]. Local-first, real-time collaborative and end-to-end-encrypted sync. Not open source but self-hostable with a single binary and hackable with plugins (custom properties, views, code, etc).
We've built a sync engine from scratch. Our app is a multiplayer "IDE" but for tasks/notes [1], so it's important to have a fast local first/office experience like other editors, and have changes sync in the background.
I definitely believe sync engines are the future as they make it so much easier to enable things like no-spinners browsing your data, optimistic rendering, offline use, real-time collaboration and so on.
I'm also not entirely convinced yet though that it's possible to get away with something that's not custom-built, or at least large parts of it. There were so many micro decisions and trade-offs going into the engine: what is the granularity of updates (characters, rows?) that we need and how does that affect the performance. Do we need a central server for things like permissions and real-time collaboration? If so do we want just deltas or also state snapshots for speedup. How much versioning do we need, what are implications of that? Is there end-to-end-encryption, how does that affect what the server can do. What kind of data structure is being synced, a simple list/map, or a graph with potential cycles? What kind of conflict resolution business logic do we need, where does that live?
It would be cool to have something general purpose so you don’t need to build any of this, but I wonder how much time it will save in practice. Maybe the answer really is to have all kinds of different sync engines to pick from and then you can decide whether it's worth the trade-off not having everything custom-built.