Sudden increasing of "unregistered" errors on the Android platform

On June 21, the legacy FCM protocol was completely shut down, and the new HTTP V1 (Firebase Cloud Messaging API (v1)) protocol came into full effect. As a part of the new protocol, FCM introduced new rules for managing registration tokens. Specifically, they define two types of inactive tokens — stale and expired: https://firebase.google.com/docs/cloud-messaging/manage-tokens#stale-and-expired-tokens

According to these definitions, if the app was not opened (there was no connection with FCM) on a mobile device during 270 days, FCM marks its token as invalid (unregistered) and rejects send-outs to it. Therefore, it is most likely that these "unregistered" errors were returned to Pushwoosh according to this new tokens management policy. An important point here is that FCM may return these errors not in just one shot, but gradually and (or) repeatedly. They don't disclose the details about timing accuracy and recurrence of this process.

Please note that the Unregistered error is a direct response we receive from FCM, not an error caused by our systems or the result of incorrect Pushwoosh SDK integration. Moreover, our SDK forcibly refreshes push tokens once in two months according to FCM guidelines, and additionally track the token changes occurred upon the FCM initiative. This means that your core active audience on Android is not affected (and it won't be).

On a separate note, it is technically possible to restore this unregistered segment of devices from our backup. However, there is no point in this action since FCM already marked them as expired so that they will be marked as unregistered again (and removed from our database) upon your next send-outs.