Offline to Online Sync DB

In android , support for offline use SQLite database as a local database.

Save data in SQLite when app in offline mode.

You need to write API's to send data to server.

When Internet Connectivity available you send local data to server.

Make one status column in which updated your status when data has been send to server then make status true. next time send only those data has status false.

I suggest Firebase Database (Real-time database or Firestore)!!!

Using Firebase DB, when clients go offline, they write new data in their local cache so that users can enjoy your app just like it's online.And when returning to online, they automatically sync with a host DB and all devices, so users can enjoy a literally "real-time" experience.

Additionally, it provides a No-SQL scalable data structure that you really can save your time.

Plus, there is a free plan of Firebase, so you can freely try and build some mini-apps with Firebase DB.

First you need to decide database for offline storage. Like Realm, SQLite.

First API Will call and respond data will be dumped in Local Database. In next time you have to get data from Local DB and show up. Parallely you have to call API in background as soon as API return the data you need to check whether Repond data and Local data in same or different if its same then ignore other wise replace Local data with API Respond data.

Same thing you have to apply when you have to send data to server. Need to store in Local DB if user Offline, Once he online send data to server.