The Reactive Manifesto (http://www.reactivemanifesto.org) is a document, defining the four reactive principles, which are as follows:

• Responsive: The system responds in a timely manner. Responsive systems focus on providing rapid and consistent response times, so they deliver a consistent quality of service.
• Resilient: In case the system faces any failure, it stays responsive. Resilience is achieved by replication, containment, isolation, and delegation. Failures are contained within each component, isolating components from each other, so when failure occurs in a component, it will not affect the other components or the system as a whole.
• Elastic: Reactive systems can react to changes and stays responsive under varying workloads. Reactive systems achieve elasticity in a cost-effective way on commodity hardware and software platforms.
• Message-driven: In order to establish the resilient principle, reactive systems need to establish a boundary between components by relying on asynchronous message-passing.

By implementing all the preceding four principles, a system becomes reliable and responsive, thus reactive.