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. Reactive Design Patterns is a clearly-written guide for building event-driven distributed systems that are resilient, responsive, and scalable. In it, you'll find patterns for messaging, flow control, resource management, and concurrency, along with practical issues like test-friendly designs. All patterns include concrete examples using Scala and Akka. In some cases, you'll also see examples in Java, JavaScript, and Erlang. Software engineers and architects will learn patterns that address day-to-day distributed development problems in a fault-tolerant and scalable way. Project leaders and CTOs will gain a deeper understanding of the reactive design philosophy.