How to send device to device messages using Firebase Cloud Messaging?

After searching the docs I could not find any info on how to send device to device messages using FCM without the use of an external server.

For example, if I was creating a chat application I would need to send push notifications to users about unread messages since they won't be online all the time and I can't have a persistent service in the background that would always be connected to the real time database because that would be too resource heavy.

So how would I send a push notification to a user "A" when a certain user "B" sends him/her a chat message? Do I need an external server for this or can it be done with just Firebase servers?

UPDATE: It is now possible to use firebase cloud functions as the server for handling push notifications. Check out their documentation here

============

According to the docs you must implement a server for handling push notifications in device to device communication.

> Before you can write client apps that use Firebase Cloud Messaging, you must have an app server that meets the following criteria: > > ... > > You'll need to decide which FCM connection server protocol(s) you want to use to enable your app server to interact with FCM connection servers. Note that if you want to use upstream messaging from your client applications, you must use XMPP. For a more detailed discussion of this, see Choosing an FCM Connection Server Protocol.

If you only need to send basic notifications to your users from the server. You can use their serverless solution, Firebase Notifications.

See a comparison here between FCM and Firebase Notifications: https://firebase.google.com/support/faq/#messaging-difference

Making a HTTP POST request with the link https://fcm.googleapis.com/fcm/send with required header and data helped me. In the below code snippet Constants.LEGACY_SERVER_KEY is a local class variable, you can find this at your Firebase Project Settings->Cloud Messaging->Legacy Server key. You need to pass device registration token i.e. regToken in below code snippet referenced HERE.

At last you need okhttp library dependency in order to get this snippet work.

public static final MediaType JSON
        = MediaType.parse("application/json; charset=utf-8");
private void sendNotification(final String regToken) {
    new AsyncTask<Void,Void,Void>(){
        @Override
        protected Void doInBackground(Void... params) {
            try {
                OkHttpClient client = new OkHttpClient();
                JSONObject json=new JSONObject();
                JSONObject dataJson=new JSONObject();
                dataJson.put("body","Hi this is sent from device to device");
                dataJson.put("title","dummy title");
                json.put("notification",dataJson);
                json.put("to",regToken);
                RequestBody body = RequestBody.create(JSON, json.toString());
                Request request = new Request.Builder()
                        .header("Authorization","key="+Constants.LEGACY_SERVER_KEY)
                        .url("https://fcm.googleapis.com/fcm/send")
                        .post(body)
                        .build();
                Response response = client.newCall(request).execute();
                String finalResponse = response.body().string();
            }catch (Exception e){
                //Log.d(TAG,e+"");
            }
            return null;
        }
    }.execute();

}

further if you want to send message to a particular topic, replace regToken in json like this

json.put("to","/topics/foo-bar")

and don't forget to add INTERNET permission in your AndroidManifest.xml.

IMPORTANT : - Using above code means your server key resides in the client application. That is dangerous as someone can dig into your application and get the server key to send malicious notifications to your users.

You can do it using Volly Jsonobject request....

follow this Steps first:

1 copy legacy server key and store it as Legacy_SERVER_KEY

> Legacy Server key

you can see in picture how to get

2 You need Volley dependency

> compile 'com.mcxiaoke.volley:library:1.0.19'

Code for send Push:-

private void sendFCMPush() {

    String Legacy_SERVER_KEY = YOUR_Legacy_SERVER_KEY;
    String msg = "this is test message,.,,.,.";
    String title = "my title";
    String token = FCM_RECEIVER_TOKEN;

    JSONObject obj = null;
    JSONObject objData = null;
    JSONObject dataobjData = null;

    try {
        obj = new JSONObject();
        objData = new JSONObject();

        objData.put("body", msg);
        objData.put("title", title);
        objData.put("sound", "default");
        objData.put("icon", "icon_name"); //   icon_name image must be there in drawable
        objData.put("tag", token);
        objData.put("priority", "high");

        dataobjData = new JSONObject();
        dataobjData.put("text", msg);
        dataobjData.put("title", title);

        obj.put("to", token);
        //obj.put("priority", "high");

        obj.put("notification", objData);
        obj.put("data", dataobjData);
        Log.e("!_@rj@_@@_PASS:>", obj.toString());
    } catch (JSONException e) {
        e.printStackTrace();
    }

    JsonObjectRequest jsObjRequest = new JsonObjectRequest(Request.Method.POST, Constants.FCM_PUSH_URL, obj,
            new Response.Listener<JSONObject>() {
                @Override
                public void onResponse(JSONObject response) {
                    Log.e("!_@@_SUCESS", response + "");
                }
            },
            new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    Log.e("!_@@_Errors--", error + "");
                }
            }) {
        @Override
        public Map<String, String> getHeaders() throws AuthFailureError {
            Map<String, String> params = new HashMap<String, String>();
            params.put("Authorization", "key=" + Legacy_SERVER_KEY);
            params.put("Content-Type", "application/json");
            return params;
        }
    };
    RequestQueue requestQueue = Volley.newRequestQueue(this);
    int socketTimeout = 1000 * 60;// 60 seconds
    RetryPolicy policy = new DefaultRetryPolicy(socketTimeout, DefaultRetryPolicy.DEFAULT_MAX_RETRIES, DefaultRetryPolicy.DEFAULT_BACKOFF_MULT);
    jsObjRequest.setRetryPolicy(policy);
    requestQueue.add(jsObjRequest);
}

Just Call sendFCMPush();

1. subscribe an identical topic name, for example:

• ClientA.subcribe("to/topic_users_channel")
• ClientB.subcribe("to/topic_users_channel")

2. send messages inside the application

GoogleFirebase : How-to send topic messages

Yes, it's possible to do it without any server. You can create a device group client side and then you exchange messages in the group. However there are limitations:

1. You have to use the same Google account on the devices
2. You can't send high priority messages

Reference: Firebase doc See the section "Managing device groups on Android client apps"

Google Cloud Functions make it now possible send push notifications from device-to-device without an app server. I have made cloud function which is trigger when new message is added in database

It is node.js code

'use strict';

const functions = require('firebase-functions');
const admin = require('firebase-admin'); admin.initializeApp();
 
exports.sendNotification = functions.database.ref('/conversations/{chatLocation}/{messageLocation}')
  .onCreate((snapshot, context) => {
      // Grab the current value of what was written to the Realtime Database.
      const original = snapshot.val();

       const toIDUser = original.toID;
       const isGroupChat = original.isGroupChat;
       
       if (isGroupChat) {
       const tokenss =  admin.database().ref(`/users/${toIDUser}/tokens`).once('value').then(function(snapshot) {

// Handle Promise
       const tokenOfGroup = snapshot.val()
     
      // get tokens from the database  at particular location get values 
       const valuess = Object.keys(tokenOfGroup).map(k => tokenOfGroup[k]);
     
     //console.log(' ____________ddd((999999ddd_________________ ' +  valuess );
    const payload = {
       notification: {
                 title:   original.senderName + " :- ",
                 body:    original.content
    }
  };

  return admin.messaging().sendToDevice(valuess, payload);
  

  
}, function(error) {
 
  console.error(error);
});

       return ;
          } else {
          // get token from the database  at particular location
                const tokenss =  admin.database().ref(`/users/${toIDUser}/credentials`).once('value').then(function(snapshot) {
                // Handle Promise
  // The Promise was "fulfilled" (it succeeded).
  
     const credentials = snapshot.val()
     
     
     
    // console.log('snapshot ......snapshot.val().name****^^^^^^^^^^^^kensPromise****** :- ', credentials.name);
     //console.log('snapshot.....****snapshot.val().token****^^^^^^^^^^^^kensPromise****** :- ', credentials.token);
    
    
     const deviceToken = credentials.token;
     
    const payload = {
       notification: {
                 title:   original.senderName + " :- ",
                 body:    original.content
    }
  };

  return admin.messaging().sendToDevice(deviceToken, payload);
  
  
}, function(error) {
 
  console.error(error);
});
      
          
          }
    


       
  
  return ;
       

    });
    
    

If you have fcm(gcm) token of the device to whom you want to send notification. It's just a post request to send the notification.

https://github.com/prashanthd/google-services/blob/master/android/gcm/gcmsender/src/main/java/gcm/play/android/samples/com/gcmsender/GcmSender.java

You can use Retrofit. Subscribe devices to topic news. Send notification from one device to other.

public void onClick(View view) {

    HttpLoggingInterceptor logging = new HttpLoggingInterceptor();
    logging.setLevel(HttpLoggingInterceptor.Level.BODY);

    OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
    httpClient.addInterceptor(new Interceptor() {
        @Override
        public okhttp3.Response intercept(Chain chain) throws IOException {
            Request original = chain.request();

            // Request customization: add request headers
            Request.Builder requestBuilder = original.newBuilder()
                    .header("Authorization", "key=legacy server key from FB console"); // <-- this is the important line
            Request request = requestBuilder.build();
            return chain.proceed(request);
        }
    });

    httpClient.addInterceptor(logging);
    OkHttpClient client = httpClient.build();

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("https://fcm.googleapis.com")//url of FCM message server
            .client(client)
            .addConverterFactory(GsonConverterFactory.create())//use for convert JSON file into object
            .build();

    // prepare call in Retrofit 2.0
    FirebaseAPI firebaseAPI = retrofit.create(FirebaseAPI.class);

    //for messaging server
    NotifyData notifydata = new NotifyData("Notification title","Notification body");

    Call<Message> call2 = firebaseAPI.sendMessage(new Message("topic or deviceID", notifydata));

    call2.enqueue(new Callback<Message>() {
        @Override
        public void onResponse(Call<Message> call, Response<Message> response) {

            Log.d("Response ", "onResponse");
            t1.setText("Notification sent");

        }

        @Override
        public void onFailure(Call<Message> call, Throwable t) {
            Log.d("Response ", "onFailure");
            t1.setText("Notification failure");
        }
    });
}

POJOs

public class Message {
    String to;
    NotifyData notification;

    public Message(String to, NotifyData notification) {
        this.to = to;
        this.notification = notification;
    }

}

and

public class NotifyData {
    String title;
    String body;

    public NotifyData(String title, String body ) {

        this.title = title;
        this.body = body;
    }

}

and FirebaseAPI

public interface FirebaseAPI {

    @POST("/fcm/send")
    Call<Message> sendMessage(@Body Message message);

}

Google Cloud Functions make it now possible send push notifications from device-to-device without an app server.

From the relevant page on Google Cloud Functions:

> Developers can use Cloud Functions to keep users engaged and up to > date with relevant information about an app. Consider, for example, an > app that allows users to follow one another's activities in the app. > In such an app, a function triggered by Realtime Database writes to > store new followers could create Firebase Cloud Messaging (FCM) > notifications to let the appropriate users know that they have gained > new followers. > > Example: > > 1. The function triggers on writes to the Realtime Database path where followers are stored. > > 2. The function composes a message to send via FCM. > > 3. FCM sends the notification message to the user's device.

Here is a demo project for sending device-to-device push notifications with Firebase and Google Cloud Functions.

In my case I use retrofit with this class Message:

public class Message {

    private String to;
    private String collapseKey;
    private Notification notification;
    private Data data;

    public Message(String to, String collapseKey, Notification notification, Data data) {
        this.to = to;
        this.collapseKey = collapseKey;
        this.notification = notification;
        this.data = data;
    }
}

Data

public class Data {

    private String body;
    private String title;
    private String key1;
    private String key2;

    public Data(String body, String title, String key1, String key2) {
        this.body = body;
        this.title = title;
        this.key1 = key1;
        this.key2 = key2;
    }
}

Notification

public class Notification {

    private String body;
    private String title;

    public Notification(String body, String title) {
        this.body = body;
        this.title = title;
    }
}

this the call

private void sentToNotification() {
    String to = "YOUR_TOKEN";
    String collapseKey = "";
    Notification notification = new Notification("Hello bro", "title23");
    Data data = new Data("Hello2", "title2", "key1", "key2");
    Message notificationTask = new Message(to, collapseKey, notification, data);

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("https://fcm.googleapis.com/")//url of FCM message server
            .addConverterFactory(GsonConverterFactory.create())//use for convert JSON file into object
            .build();

    ServiceAPI api = new retrofit.create(ServiceAPI.class);

    Call<Message> call = api .sendMessage("key=YOUR_KEY", notificationTask);

    call.enqueue(new Callback<Message>() {
        @Override
        public void onResponse(Call<Message> call, retrofit2.Response<Message> response) {
            Log.d("TAG", response.body().toString());
        }

        @Override
        public void onFailure(Call<Message> call, Throwable t) {

            Log.e("TAG", t.getMessage());
        }
    });
}

our ServiceAPi

public interface ServiceAPI {
    @POST("/fcm/send")
    Call<Message> sendMessage(@Header("Authorization") String token, @Body Message message);
}

You can use firebase realtime database to do so. You can create data structure for storing chats and add observers for the conversation threads for both users. It still does device - server - device architecture, but in this case there is no additional server on the developers' part. This uses the firebase servers. You can check out a tutorial here (ignore the UI part, although, that is also a good starting point for chat UI frameworks).

Firebase Realtime Chat

Here is walk around how to get notifications without second server apart from the Firebase one. So we use Firebase only, without additional server.

1. At the mobile app code, we create its own notifications function by Android libraries like here, not using Firebase libraries like here, without Firebase Cloud messaging. Here is an example with Kotlin:

   private fun notification() { createNotificationChannel()

    val intent = Intent(this, LoginActivity::class.java).apply {
        flags = Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_CLEAR_TASK
    }
    val pendingIntent: PendingIntent = PendingIntent.getActivity(this, 0, intent, 0)
   
    val notificationBuilder = NotificationCompat.Builder(this, "yuh_channel_id")
        .setSmallIcon(R.drawable.ic_send)
        .setContentText("yuh")
        .setContentText("yuh")
        .setAutoCancel(true)
        .setPriority(NotificationCompat.PRIORITY_DEFAULT)
        .setContentIntent(pendingIntent)
    val notificationManager =
        getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
    notificationManager.notify(0, notificationBuilder.build())
   
    with(NotificationManagerCompat.from(this)) {
        // notificationId is a unique int for each notification that you must define
        notify(0, notificationBuilder.build())
    }
   

   }

    private fun createNotificationChannel() {
    // Create the NotificationChannel, but only on API 26+ because
    // the NotificationChannel class is new and not in the support library
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        val name = "yuh_channel"
        val descriptionText = "yuh_description"
        val importance = NotificationManager.IMPORTANCE_DEFAULT
        val CHANNEL_ID = "yuh_channel_id"
        val channel = NotificationChannel(CHANNEL_ID, name, importance).apply {
            description = descriptionText
        }
        // Register the channel with the system
        val notificationManager: NotificationManager =
            getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
        notificationManager.createNotificationChannel(channel)
    }
   

2. In the Firebase database, create collection "pending notifications". Documents should contain user name (to send notification to) and source name (where should user go upon tapping the notification).

3. In the app code, implement option for adding new records to the Pending Notifications collection. E. g. if user A sends message to user B, then the document with the id of user B (who will be notified) is created in the collection.

4. In the app code, set up background (when the app is not visible to the user) service. Like here. In the background service, set up a listener for changes in the "Notifications Pending" collection. When the new record with the user id comes to the collection, call the notification function created in the paragrath 1 supra and delete the consequent record from the collection.

So I had an idea here. See: If the FCM, as well as the GCM, has a endpoit to http request where we can send a post json with our message data, including the token (s) of devices that we want this message to be delivered.

So why not send a post to Firebase server with this notification to be delivered to user B? you understand ?

So, you send the message and chat with a call post to ensure delivery of the notification if the user is with your app in the background. I am also in need of it soon, I will test later. What do you say about?

Simplest way :

void sendFCMPush(String msg,String token) {
    HttpLoggingInterceptor logging = new HttpLoggingInterceptor();
    logging.setLevel(HttpLoggingInterceptor.Level.BODY);

    OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
    httpClient.addInterceptor(new Interceptor() {
        @Override
        public okhttp3.Response intercept(Chain chain) throws IOException {
            Request original = chain.request();

            // Request customization: add request headers
            Request.Builder requestBuilder = original.newBuilder()
                    .header("Authorization", "key="+Const.FIREBASE_LEGACY_SERVER_KEY); // <-- this is the important line
            Request request = requestBuilder.build();
            return chain.proceed(request);
        }
    });

    httpClient.addInterceptor(logging);
    OkHttpClient client = httpClient.build();

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("https://fcm.googleapis.com/")//url of FCM message server
            .client(client)
            .addConverterFactory(GsonConverterFactory.create())//use for convert JSON file into object
            .build();

    // prepare call in Retrofit 2.0
    FirebaseAPI firebaseAPI = retrofit.create(FirebaseAPI.class);

    //for messaging server
    NotifyData notifydata = new NotifyData("Chatting", msg);

    Call<Message> call2 = firebaseAPI.sendMessage(new Message(token, notifydata));

    call2.enqueue(new Callback<Message>() {
        @Override
        public void onResponse(Call<Message> call, retrofit2.Response<Message> response) {
            Log.e("#@ SUCCES #E$#", response.body().toString());
        }

        @Override
        public void onFailure(Call<Message> call, Throwable t) {

            Log.e("E$ FAILURE E$#", t.getMessage());
        }
    });
}

Create Class to make Object:

public class Message {
String to;
NotifyData data;

public Message(String to, NotifyData data) {
    this.to = to;
    this.data = data;
}
}

Create Class to make Object:

public class Notification {
String title;
String message;
enter code here`enter code here`
public Notification(String title, String message) {
    this.title = title;
    this.message = message;
}
}