The futures and promises from Chapter 4, Asynchronous Programming with Futures and Promises, push concurrent programming to a new level. First, they avoid blocking when transferring the result of the computation from the producer to the consumer. Second, they allow you to idiomatically compose simple future objects into more complex ones, resulting in programs that are more concise. Futures encapsulate patterns of asynchronous communication in a way that is clear and easily understandable.

One disadvantage of futures is that they can only deal with a single result. For HTTP requests or asynchronous computations that compute a single value, futures can be adequate, but sometimes we need to react to many different events coming from the same computation. For example, it is cumbersome to track the progress status of a file download with futures. Event streams...