Atomic variables from Chapter 3, Traditional Building Blocks of Concurrency, are one of the fundamental synchronization mechanisms. We already know that volatile variables, introduced in Chapter 2, Concurrency on the JVM and the Java Memory Model, allow race conditions, in which the program correctness is subject to precise execution schedule of different threads. Atomic variables can ensure that no thread concurrently modifies the variable between a read and a write. At the same time, atomic variables reduce the risk of deadlocks. Regardless of their advantages, there are situations when using atomic variables is not satisfactory.

In Chapter 6, Concurrent Programming with Reactive Extensions, we implemented a minimalistic web browser using the Rx framework. Surfing around the web is great, but we would like to have some additional features in our browser. For example, we would like to maintain the browser's history: the list of URLs that were previously...