how to make a service in background that trigger a notification? (Android)

I want to create a background service on adroid studio, that listen to changes in the realtime database of firebase and trigger a notification in the app. I know that exist cloud messaging, but that solution can't use data of the RTDB to construct the information that goes in the notification. I know how to make a notification in android, but i dont know how to make a service that trigger that notification even when the application is killed. Can you help me?