To synchronize data between devices across wifi.
One or more devices are connected over wifi. Each device will have an app installed with a sqlite database. Each app can have insertion / updations into the db. Whenever an insertion or an updation happens on one device it has to be communicated to the rest of the devices of the changes that has happened. For a start, begin with a dummy app and a sqlite database with two tables. Once you perform an insert, communicate the change to another device using port forwarding. (You will most probably need to setup a master-slave system).
What is required:
Before starting on the project, give an overall flow on how the system will work; libraries if any used and an abstract algorithm on how you plan to implement this in android.