FCM Android - null message ID

Question

I've implemented my subclass of FirebaseMessagingService and I'm successfully receiving downstream messages in FirebaseMessagingService.onMessageReceived(RemoteMessage). My problem is that RemoteMessage.getMessageId() always returns null. From what I understood, message ID is mandatory and should be automatically generated by the FCM server. In fact, calling https://fcm.googleapis.com/fcm/send returns a message ID, I just cannot access it on the app side.

Is there something I am missing?

Here's my FCM messaging service class:

public class FcmMessagingService extends FirebaseMessagingService {
    private static final String TAG = "FcmMessagingService";

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        Logg.d(TAG, "Received message; id: " + remoteMessage.getMessageId());
        Map<String, String> data = remoteMessage.getData();
        Logg.d(TAG, "Data: " + data);

        String message = data.get("message");
        /* ... */
    }
}

Answer 1

As of Firebase Android SDK 9.4 this issue has been resolved

From the release notes:

FIXED RemoteMessage#getMessageId() now returns the correct message-id for received messages. Previously, it returned null.

Updating the com.google.firebase:firebase-messaging dependency to version 9.4.0 has indeed fixed the problem.

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Update 2.07.2016: I was informed that this is indeed a bug and a fix will be included in the nearest release.

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Turns out we cannot get message ID on the app side. I've asked the Firebase support for help and this is what they told me:

When FCM has successfully receives the request, it will attempt to deliver to all subscribed devices, FCM will return a response which includes messageID to your server. You cannot directly get MessageID on your client side (Android device). MessageID is a parameter in response payload.

As a workaround I am generating custom message identifiers on the server and passing them in the payload.

What RemoteMessage.getMessageId() actually does remains a mystery.