Error while calling FCM backend service from Google AppEngine - google-app-engine

Question

I am setting up my first app with FCM (we used GCM) and following the tutorials. I have an Android app and a servlet-based app on GAE Standard. To test it a have a servlet that sends a message to the app.
This was working a few hours ago, I could call the servlet and received the test message on the app, but now I am only getting exceptions in the FirebaseMessaging.getInstance().sendAsync(message).get() method
com.google.firebase.messaging.FirebaseMessagingException: Error while calling FCM backend service
and
java.net.UnknownHostException: accounts.google.com
Sometimes one and sometimes the other.
This is the method the GAE servlet calls. I have checked if the token I was using was correct and it is the one currently active in the Android device.
final static String urlFCM = "https://fcm.googleapis.com/fcm/send";
private static void initFCM() {
FileInputStream serviceAccount;
try {
serviceAccount = new FileInputStream("WEB-INF/Orchestram-e2e1ceeb2481.json");
FirebaseOptions options = new FirebaseOptions.Builder()
.setCredentials(GoogleCredentials.fromStream(serviceAccount))
.setDatabaseUrl("https://orchestram-cerqana.firebaseio.com/")
.build();
FirebaseApp.initializeApp(options);
} catch (Exception e) {
e.printStackTrace();
}
/// TODO:
}
public static String sendFCM(final JSONObject value, final String token, final boolean back) {
initFCM();
Message message = Message.builder()
.putData("score", "850")
.putData("time", "2:45")
.setToken(token)
.build();
try {
String response = FirebaseMessaging.getInstance().sendAsync(message).get();
} catch (Exception e) {
e.printStackTrace();
return "ERROR";
}
return "HECHO";
}

Answer 1

Turns out the problem was billing on this specific project on GAE was disabled. Once enabled it worked again (and I'm still bellow the free tier, so no real cost added)

Answer 2

There might be a case where you are subscribing to topics multiple topics at once. There is a exponential back off at firebase. I derived this from the link here.
https://firebase.google.com/docs/cloud-messaging/android/topic-messaging
Topic messaging supports unlimited subscriptions for each topic. However, FCM enforces limits in these areas:
One app instance can be subscribed to no more than 2000 topics.
If you are using batch import to subscribe app instances, each request is limited to 1000 app instances.
The frequency of new subscriptions is rate-limited per project. If you send too many subscription requests in a short period of time, FCM servers will respond with a 429 RESOURCE_EXHAUSTED ("quota exceeded") response. Retry with exponential backoff.