Choosing a push notification server with ruby ​​support?

Question

Android

Tell me which server is better to choose for push notifications for android, how do they differ mainly?
As I understand it, there are GCM (google), SNS (amazon) these are paid solutions.
Are there reliable free solutions, which ones?
As I understand it, you can still write your own, how problematic is it? (there is a resource for server specialists, ruby)

Answer 1

GCM is completely free! Although now FCM has appeared and it really works better