WhatsApp like applications require the following
1) User management
2) A mechanism to send receive messages to another user
You can save all the messages that are being sent as JSON on App42 NoSQL service. Then you can query for documents that have you as the receiver.
You should also send push notification for cases where the receiver is not connected to the App. For this you should use Push Notification services.
3) Ability to send media (photos n videos)
For this, messaging should remain the same as Text, however instead of the message you should send a link to the file. You can upload the file on to App42 file storage service.