Spawn async REST calls and collect the results. Use Spring-Reactor and experiment with two types of Dispatchers Threadpool and RingBuffer

Reactor makes it seamless and convenient to handle asynchronous processing. We do not have to think about managing Future and spawning tasks. It's handled by the event-bus

It's easy to implement and easy to maintain and understand.

Additionally, it truly gives the opportunity to operate on individual request as and when it gets completed. This gives us an option to take the process to its completion for one single granular request, rather than having to wait for it.