In an ideal scenario, every message produced by the Producer is passed to the Subscriber as and when the message is produced without any delay. There is a chance that the Subscriber is unable to handle the messages at the same rate as they are produced and this can cramp its resources.

Backpressure is a method by which the Subscriber can tell the Producer to send messages at a slower rate to give the Subscriber time to handle these messages properly without putting too much pressure on its resources.

Since this is the first chapter, we are just introducing you to these important reactive concepts. Code examples will be covered in subsequent chapters.

Now that we have a brief idea of Reactive Streams and Reactive Streams Specification, we will go into next important reactive concept in Java, namely Reactive Extensions.