Simple json schema that describes and documents data sent through FCM downstream messages. Two JSONs: one for sending data to iOS devices, other one for android devices.
It directly corresponds to what's on https://firebase.google.com/docs/cloud-messaging/http-server-ref#downstream
Tested and proven in a real app.
If you find that something is malfunctioning, please let me know :)
Works particulary well with http://www.jsonschema2pojo.org or any other tools that might generate PO*O classes.