Realtime applications are becoming more and more popular, streams of data travel from one service to another continuously. We want our entire microservices architecture behaving like closed electronic circuits, always reacting on any input changes, triggers, or interruptions. Everything working together like a symphonic orchestra. And if this is not complex enough, we have to continuously grow our businesses, more and more data has to go through our reactive circuit. Services have to process more without appearing slower to the end consumer. Frontend applications are becoming realtime mirrors that reflect any change that our orchestrated backend services do. We can achieve this by streaming data from backend services to frontend applications in realtime, keeping the end user updated with all changes the system is going through. It is an impressive evolution of cloud applications. But growth always comes with challenges, one of which is to make an API service scalable, a service that exposes a reactive API that opens an event stream on each HTTP request and keeps it open for a long period of time, updating the customer with any state change, continuously for many minutes or even hours. At JustEat Takeaway.com, the Tracker application uses this reactive API every day to inform millions of customers about the state of their orders in realtime.