Design replication of spring boot API consumer app

As title said I'm trying to design replication of API consumer app. Imagine that in system exists server with some sort of subscribe API. So client have to register and after that client will receive data. In one moment only 1 APP is registered and receive data from that server. So if that APP is down I want second APP to subscribe and receive data until first APP is back up and running.

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If first APP is back up and running, second app will unsubscribe.

I hope I explained it clearly.