> sync engines only really "work" when you remove the offline part

I don't see the gotcha here. I don't care about the offline part. I mean I accept that some people do, but that's not where the value comes from for many major synced products like Linear, Notion, Superhuman.

> complicated/over engineered cache

Sync engines are nothing at all like a cache - that's the point.

They are a replica. Caches are by nature inconsistent. Every entry in the cache is from a different point in time.

Sync engines replicate a consistent subset of your database to the client as an atomic unit. This enables things caches can't do:

  * New, fresh queries can be returned instantly from local state
  * Mutations can apply locally (optimistically) automatically, without custom code
  * The UI updates automatically to reflect server changes

Whether sync engines generalize is an open question. Is it hard? Yes. Is it a distributed systems problem? Yes! Is it worth doing? I think it is. Web applications often suck and sync engines are an important part of many of the ones that don't. I want to enable that experience for more apps without teams having to build it themselves.