Android Firebase Device To Device Messaging

I want to create an app that can simply using firebase allow users to add friends and be able to send one to one messages like whatsapp but with usernames. I have seen and used the

Solution 1:

Firebase does not support direct communication between two devices. You need an additional app server that communicates with gcm. A nice article that helped me to understand the structure can be found here. http://www.grokkingandroid.com/xmpp-server-google-cloud-messaging/. A nice project that worked for me is the friendlyPing example https://github.com/googlesamples/friendlyping.