Let's take a real-world example of an e-commerce website integrating itself with a third-party payment gateway. Here, the website uses an API from the payment gateway to pop up the payment screen and enters security credentials. At the same time, the website may call another API called analytics to record the attempt of payment. This process of forking a single request into multiple is called fan-out. In the real world, there can be many fan-out services involved in a single client request.
Another example is MapReduce. Map is a fan-in operation, while Reduce is a fan-out operation. A server can fan out a piece of information to the next set of services (API) and ignore the result or can wait until all the responses from those servers are returned. As shown in the following diagram, an incoming request is being multiplexed by the server into two...