Modeling concurrency with dependent futures

We saw in a previous recipe that we can use asynchronous methods to make service calls that are handled in separate threads. This is essential because blocking on network I/O would severely limit the number of incoming requests our service would be able to handle. A service that blocks on the network I/O would only be able to handle a relatively small number of requests per process, requiring us to spend more resources on horizontal scaling. In the example we used, the message service needed to call the social graph service for two users, the sender, and the recipient of a message, and make sure that the two users followed each other before allowing a message to be sent. We modified our request methods to return the CompletableFuture instances that wrapped the response, and then waited on all of the results to finish before verifying...