Sync sqlite data between iPads in offline mode

I want to make an app which need to Sync sqlite data between iPads in offline mode, and if internet back, sync data to server in background at the same time. Do not know how to make it, is there a valid solution for it? The server is self hosted.

Offline: iPad1 <--> iPad2 <--> iPad3, iPads sync data between each other, share the same database, sqlite.

Online: in background, sync sqlite data with server at the same time, asynchronously.