FCM, send to topic and "Not Registered" error

[Massimo Di Cosimo]

Hello there,
I developed an Android app that subscribes to a topic (subscribeToTopic()) and then receives notifications when my server sends a message to that topic.
The problem is that from time to time, someone complains that notifications ceases to arrive.
In this case I usually ask to user to use a function of my app that reads the Firebase Registration Token and sends it to me via email.
Then I try to send a notification to that Token and FCM Server answers with the "Not Registered" error!
How can it be "Not Registered"???
When using the Topic Subscription, shouldn't Firebase manage the whole "token process" and take care of its renewal?

Is there some way to inform app that it is not registered anymore and take appropriate actions?

Thank you,
Max

[Frank van Puffelen]

Please indicate when you cross-post: http://stackoverflow.com/questions/42142061/fcm-send-to-topic-and-not-registered-error

[Massimo Di Cosimo]

No help from StackOverflow, but the "defect" keeps being reported from time to time.
The last today: another user reported the problem. He confirmed that app was installed but when I sent the notification the answer was "Not Registered".
Then he reinstalled app and now it works.
This user seems collaborative: is there anything that I could ask to understand what's the source of the malfunction?
Thank you,
Max

[twinkle...@gmail.com]

Did you try subscribing to the topic again in response to onTokenRefreshed? Seems like a good thing to try, even though the documentation doesn't anywhere suggest this would be important.