'What's the proper file format for local data to implement a sync feature with a SQL database server?

This question might be quite dumb, but I couldn't find anything related.

Let's say a user can create and modify a shopping list in an application, that data would be synchronized to the SQL server whenever network is available and vice-versa.

What file format should the local data stored in? Using a local SQL database server won't make much sense for a mobile platform.

Should I store everything into a JSON file locally, or there is a better approach?



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