When do you need an app server for Firebase Cloud Messaging?

Question

I am new to using FCM notifications for Android Application at https://firebase.google.com/docs/cloud-messaging/server. I was reading up on it and found that in the About FCM Server page requirements, it says the following:

An app server that you must implement in your environment. This app server sends data to a client app via the chosen FCM connection server, using appropriate XMPP or HTTP protocol

However, I am sorely confused about this. As I read more into the article, I see that there is an API that looks like this:

POST http://fcm.googleapis.com/fcm/send

If I invoke this API using something like OkHttpClient and build my request like so, (provided that I have authentication headers and a POST body included)

private void sendRegistrationToServer(String token) {
    OkHttpClient client = new OkHttpClient();
    RequestBody body = new FormBody.Builder().add(“Body", "").build();
    //Assuming I have authentication and body put in
    Request request = new Request.Builder().url("http://fcm.googleapis.com/fcm/send”).post(body).build();
    try {
        client.newCall(request).execute();
    } catch (IOException e) {
        e.printStackTrace();
    }

}

Would I in theory, be able to send a notification with whatever information I want to that device? I can receive the message through the following class:

public class NotificationService extends FirebaseMessagingService {
    ...
    // [START receive_message]
    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {

        // TODO(developer): Handle FCM messages here.
        Log.d(TAG, "From: " + remoteMessage.getFrom());

        // Check if message contains a data payload.
        if (remoteMessage.getData().size() > 0) {
            Log.d(TAG, "Message data payload: " + remoteMessage.getData());
        }

        // Check if message contains a notification payload.
        if (remoteMessage.getNotification() != null) {
            Log.d(TAG, "Message Notification Body: " + remoteMessage.getNotification().getBody());
        }
    }

I’m sure my understanding is incorrect somewhere as the documentation does say we need an application server, but if someone could please point out where I am misunderstanding how to implement FCM notifications, that would be great. If someone could give an example of where or when we would need an app server, or how it should ACTUALLY be implemented, that would also be much appreciated. Thanks!

Answer 1

When the documentation says that you need an app server is mainly because you need an application that store the tokens of the devices to which you would like to send the notifications and this application should update the tokens if any of your client devices change its token. However, you could use the OkHttpClient to send request to the FCM service and therefore send notification to other devices if you have, off course, the token ID of those devices. It depends on what you want to do and it depends on how you want to manage the notifications.

If you want an example on how to implement the server app in java here is a good example example 1 that was posted or here is another post with an implementation on PHP. If you want an example on how to implement the client application and how to test it from the firebase console here is another good example.