In order to model dependent asynchronous service calls, we'll take advantage of two features in Java 8. Streams are useful for processing data, so we'll use them in our example to extract usernames from a list of followings and map a function to each element. Java 8's CompletableFuture can be composed, which allows us to naturally express dependencies between futures.
In this recipe, we'll create a simple client application that calls the social service for a list of users that the current user follows. For each user returned, the application will get user details from the users service. We'll build this example as a command-line application for easy demonstration purposes, but it could just as well be another microservice, ...
