WhatsMonitor is an Android App that lets you monitor the activity of other persons on WhatsApp. The app tracks public information about phone numbers and persists it so that the users can check it at any time. The app is useful for concerned parents and it can help managers get a feel of how much employees spend chatting. It’s also a great tool to get a clear view of who’s chatting with who by comparing availability intervals.
The App has personalized notifications for contacts that come on-line, can track multiple numbers and it’s in real time. For optimization purposes, the app has smart local caching to get the best user experience and scale properly.

The app was removed from PlayStore but you can access their website.

For this application, the most important part is the Backend that was written using Ruby on Rails. The glue between data mining and App clients is a versioned RESTful application with a Postgres database that saves millions of results each day. The whole system is hosted by Amazon Web Services and it is safe, secure, robust, performant and scalable.