Requirements
1. We need a Server push that can support Android, iOS and Web.
2. User can communicate with word or picture by this system, like WhatsApp or Facebook chat
3. Reference : [login to view URL]
User story :
1. Register a new account only need user name and password.
2. When users log in the system by Android, iOS or Web, they can see their friend groups in a list ;.
3. Users can choose one or several friends, and make a chat.
4. Users can watch their chat history in the chat window.
We need a functional-orientation back-end, emphasizing on Server and Database. It doesn't matter how it performs on such UI layout.