Kinto

A minimalist JSON storage service that speaks plain HTTP, giving an application a small, well-defined API for reading and writing records without a relational database in the middle. It adds record-level permissions for sharing data between users, JSON Schema validation, revision-based change tracking for synchronization, and pluggable authentication and storage backends.

It suits offline-first mobile apps, prototype frontends, configuration delivery and structured data collection from forms, and its synchronization model was designed by the team behind Firefox Sync.

Running it on your own server keeps user data on infrastructure you control, with no vendor bill per user and a database you can back up and restore like any other.