android - How to send one to one message using Firebase Messaging

Question

I have been trying to read the official docs and guides about how to send message from one device to another. I have saved registration token of both devices in the Real Time Database, thus I have the registration token of another device. I have tried the following way to send the message

RemoteMessage message = new RemoteMessage.Builder(getRegistrationToken())
                    .setMessageId(incrementIdAndGet())
                    .addData("message", "Hello")
                    .build();
FirebaseMessaging.getInstance().send(message);

However this is not working. The other device doesn't receive any message. I am not even sure, if I can use upstream message sending to conduct device to device communication.

PS: I just want to know if device-to-device messaging is possible using FCM? If yes, then is the code I used have some issue? If yes, then what is the correct way.

Update:
My question was to ask whether device to device messaging without using any separate server other than firebase could messaging is possible or not, if yes than how, since there's no documentation about it. I do not understand what is left to explain here? Anyways I got the answer and will update it as an answer once the question gets reopened.

Answer 1

Firebase has two features to send messages to devices:

• the Notifications panel in your Firebase Console allows you to send notifications to specific devices, groups of users, or topics that users subscribed to.
• by calling Firebase Cloud Messaging API, you can send messages with whatever targeting strategy you prefer. Calling the FCM API requires access to your Server key, which you should never expose on client devices. That's why you should always run such code on an app server.

The Firebase documentation shows this visually:

Sending messages from one device directly to another device is not supported through Firebase Cloud Messaging.

Update: I wrote a blog post detailing how to send notifications between Android devices using Firebase Database, Cloud Messaging and Node.js.

Update 2: You can now also use Cloud Functions for Firebase to send messages securely, without spinning up a server. See this sample use-case to get started.