Modern distributed applications must deliver near-realtime performance while at the same time managing big data and high user loads spread across environments ranging from cloud systems to mobile devices. Unlike traditional enterprise applications that enforce highly-structured pathways for data and procedures, Reactive applications are built around loosely-coupled, asynchronous components that "react" efficiently to events like competition for shared resources, system failures, and user demands. These systems are highly-concurrent and fault-tolerant, with minimal dependencies among individual system components. Because they're designed to use fresh tools like Scala and Akka, they can combine familiar ideas, such as message passing, with new ideas like actor-based concurrency and CQRS, the Command-Query-Responsibility-Segregation data pattern.
