Reactive

Reactive is an adjective that describes the tendency to respond to stimuli or participate in a process of change. The term is used across disciplines with slightly different emphases.

In chemistry, a substance is described as reactive if it readily participates in chemical reactions. Reactivity depends on factors such as molecular structure, temperature, pressure, catalysts, and the presence of other reactants. Highly reactive substances can pose safety considerations, while inert materials react slowly or not at all under ordinary conditions.

In everyday language, something reactive is capable of rapid response or action, often in contrast to something proactive or passive.

In computer science, reactive refers to approaches that handle asynchronous data streams and events in a way that preserves responsiveness. Reactive programming models computations as data flows and propagation of changes, so when an input changes, dependent values automatically recompute. Reactive systems extend this idea to distributed software architectures, aiming to stay responsive under varying load by using asynchronous messaging, backpressure, and fault tolerance. The Reactive Manifesto, published in 2013, characterizes good reactive systems as responsive, resilient, elastic, and message-driven.

Common implementations include libraries and frameworks based on Reactive Extensions (Rx), which provide composable abstractions for observable sequences and event streams. Variants exist for multiple languages, including RxJava, Rx.NET, and RxJS. Reactive programming is widely used in user interfaces, real-time data processing, and streaming analytics, where complex event timing and dynamic data flows benefit from declarative, flow-based models.