How to send push notifications to multiple devices

Question

I was findind a way to deliver push messages from my expressJS server to my ionic app and I found GCM. With GCM I could deliver the message passing a list of tokens, like this :

 sender.send(message, {
        registrationTokens: deviceTokens
    }, function (err, response) {
        if (err) console.error(err);
        else console.log('response' + JSON.stringify(response));
    });

But as I found that GCM became FCM I was trying to do the same using FCM, but no luck until now. I've heard about sending topics but I couldn't find an example.

Can anyone give an example on how send topic messages using FCM ?

my FCM code: (working with just 1 token)

 var FCM = require('fcm-node');

var serverKey = 'xxx';
var fcm = new FCM(serverKey);

var message = {

    to: 'device-token',

    notification: {
        title: event.title,
        body: event.information
    }

};

fcm.send(message, function (err, response) {
    if (err) {
        console.log("Something has gone wrong! \n" + err);
    } else {
        console.log("Successfully sent with response: \n ", JSON.stringify(response));
    }
});

Answer 1

I reckon you are using fcm push library for your push notification, if you want to send same notification to multiple users then use "registration_ids" parameter instead of "to". this tag accepts an array of strings.

ex: registration_ids:["registrationkey1","registrationkey2"].

note: limit is 100 key at a time.

Answer 2

I think it is documented pretty well by Google. Basically, there are two ways to send notifications to multiple groups:

1. Topic Messaging : You have the client subscribe to specific topics and then while sending notifications you just modify the request to target a specific topic. All the clients subscribed to that topic would receive the message.

   POST request to this end point.
   
   https://fcm.googleapis.com/fcm/send
   Content-Type:application/json
   Authorization:key=SERVER_AUTHORIZATION_KEY
   
   {
      "to": "/topics/foo-bar",
      "data": {
         "message": "This is a Firebase Cloud Messaging Topic Message!"
      }
   }
   

   How you subscribe to a specific topic depends on the device context. Documentation for Android and IOS are mentioned in the link I provide.

2. Device Groups : This is basically building on the approach you have provided you have the registration tokens of the devices you want to target. You can form a device group like so:

   POST request
   
   https://android.googleapis.com/gcm/notification
   Content-Type:application/json
   Authorization:key=API_KEY
   project_id:SENDER_ID
   
   {
     "operation": "create",
     "notification_key_name": "appUser-Chris",
     "registration_ids": ["4", "8", "15", "16", "23", "42"]
   }
   

The following request returns a notification_key which you can use in the to field to send notifications. Yes, You will have to save this notification_key somewhere and use it simply like:

POST request

https://fcm.googleapis.com/fcm/send
Content-Type:application/json
Authorization:key=SERVER_AUTHORIZATION_KEY

{
  "to": "aUniqueKey", //This is your notification_key
  "data": {
    "hello": "This is a Firebase Cloud Messaging Device Group Message!",
   }
}

Ofcourse, you can add and remove devices from the group and all the other fine control. Like I mentioned, it is all documented very well and should get you started without a hiccup.