The core microservice presented in the parts from 1 to 3 was assuming that everything happens as expected and all the events and conditions are on the happy path. This is not always the case. For example, our ActiveMQ broker may experience issues and be only partially available. Or it may simply be stopped. Or the network failure might prevent the connections to get established, etc.
In all these cases the consumer calls the service which will finally timeout but, before doing that, might waste resources without eventually being able to fulfill its role. Entry Netflix Hystrix service.