Designing Chat App like WhatsApp
The only requirement for Whatsapp: both sender and receiver must have installed the app. The service behind the app is based on a client-server structure. For instant messaging to work, the clients, your smartphone, log on to a server that handles all communications. The server manages all users and contacts and transmits status information and file attachments. Instead of calling or texting, you can see at a glance who's online. Contacts who have switched off their mobile phone or who do not reveal their presence status will receive their messages as soon as the device is logged back into the network.