What is a REST API?

APIs are essential in modern software for data transfer, workflow initiation, and service integration, with REST being the most widely used architectural style due to its direct use of HTTP. REST APIs model data as addressable resources and manipulate them with standard HTTP verbs, offering a scalable, familiar interface for developers. Introduced by Roy Fielding, REST is defined by six constraints: client-server architecture, statelessness, cacheability, a uniform interface, a layered system, and optionally, code-on-demand. These constraints address scalability, fault tolerance, and loose coupling, enabling web systems to evolve efficiently. REST's principles, such as stateless communication and cacheable data, facilitate horizontal scaling and improve reliability, as seen in high-traffic APIs like those of Amazon S3. The uniform interface, characterized by predictable URIs and standardized formats, simplifies client-server interactions, although some implementations may skip hypermedia, leading to "REST-ish" APIs. A layered system enhances flexibility and security by allowing multiple intermediary layers, while code-on-demand provides optional client-side extensions, though often avoided due to security concerns.