Which database to choose for a fitness tracker?

Question

Database

The task is to write a small fitness tracker.
It is necessary to ensure data synchronization between devices.
Synchronization can be bi-directional or uni-directional, depending on the type of device.
For example, data flows from a pedometer only in one direction, and there can be a two-way exchange between a phone and a PC.
How to implement serverless synchronization?
How then to exchange data with friends?

Answer 1

SQLite

Answer 2

Serverless synchronization will not work completely without a server, store the ip of clients and their types on the server so that they can find each other. Next, sync. The simplest option, completely server-based, send data to the server, and from the server to other devices.
The pedometer is first connected to the phone via some kind of bluetooth, and from there to the server.
DB - consider the option with radishes, it depends heavily on the user.