Testing
Your local-first app feels instant. Writes land immediately, the UI updates without a spinner, and sync happens invisibly in the background. Until it doesn't. A conflict resolver fires incorrectly, a shape subscription misses rows, or the client state diverges from Postgres after a partition. Nobody sees it
Local-first
Replicache is the local-first sync framework that made optimistic UI updates on top of server-authoritative state mainstream. Its core bet: every mutation applies optimistically on the client, gets rebased when the server responds, and conflict resolution happens through a deterministic rebase algorithm. Testing this correctly requires understanding the
Testing
ElectricSQL takes a fundamentally different approach to application data.
Local-first
PowerSync takes a different approach to local-first sync: it works with your existing Postgres (or MongoDB) backend and SQLite on the client, replicating rows automatically based on sync rules you define. Testing PowerSync means verifying that sync rules route the right data to the right users, upload queues handle