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Is there a way for server application that sends topic messages over FCM to find out which topics actually have subscribers?
We have potentially large number of topics and would like to avoid having to always push messages to all of them since only a subset would be active at any given time. I know that server can query about particular InstanceID's subscribed topics, but iterating over thousands of them sounds like a bad idea.
We can roll our own solution, but since that information already exists in Firebase itself, it would be preferred to just get it from there.
No. There is no current way to query the number of subscribers to a topic, you would have to maintain the relationship between token and topics on you app server.