Say we are implementing a chat application with multiple channels. Presence channels are great for keeping track of who else is in a channel with us, but what if we are outside of a channel and want to see who is in the channel without them being able to see us?
It is possible to do this by using a second pusher instance as a “spy”. This instance identifies as a spy in its auth info, and then when we get the members of the channel, we can filter out all the spies and only report the legitimate members.