Controller -> Service -> Use of MyCallable to make an async call to -> Router
Controller -> Service -> AsyncRouterMethod
Reference
1. https://www.journaldev.com/20457/spring-async-annotation
2. https://www.callicoder.com/java-8-completablefuture-tutorial/
3. https://docs.spring.io/spring/docs/current/spring-framework-reference/integration.html#scheduling-annotation-support-async
4. https://www.callicoder.com/java-callable-and-future-tutorial/