What is a RESTful API

A RESTful API (Representational State Transfer API) is a type of web API that adheres to the architectural principles of REST, which was defined by Dr. Roy Fielding in his 2000 doctoral dissertation. REST is a set of guidelines that, when followed, enable scalable, simple, and flexible interactions between clients (like web browsers or mobile apps) and servers. Here are some key characteristics and principles of RESTful APIs:

1. Client-Server Architecture: RESTful APIs follow a client-server architecture where the client and server are independent of each other, allowing each to be developed and scaled separately.
2. Statelessness: Each request from a client to a server must contain all of the information the server needs to understand and fulfill the request. The server should not need to retain any session state about the client.
3. Cacheability: Responses must define themselves as cacheable or not to avoid clients reusing stale or inappropriate data, which improves the efficiency and performance of applications.
4. Uniform Interface: This principle simplifies and decouples the architecture, which enables each part to evolve independently. The four guiding principles of the uniform interface are:
   • Resource-Based: Resources (specific pieces of information) are identified in requests using URIs (Uniform Resource Identifiers).
   • Manipulation of Resources Through Representations: When a client holds a representation of a resource, including any metadata attached, it has enough information to modify or delete the resource on the server, provided it has permission to do so.
   • Self-descriptive Messages: Each message includes enough information to describe how to process it, which typically is achieved through the use of media types and other HTTP headers.
   • Hypermedia as the Engine of Application State (HATEOAS): Clients interact with a RESTful API via hypermedia provided dynamically by application servers – a concept known as HATEOAS. Clients make state transitions only through actions that are dynamically identified within hypermedia.
5. Layered System: A client cannot ordinarily tell whether it is connected directly to the end server, or to an intermediary along the way. Intermediary servers can improve system scalability by enabling load balancing and by providing shared caches.
6. Code on Demand (optional): Servers can temporarily extend or customize the functionality of a client by transferring executable code.

RESTful APIs are commonly used to build interactive applications that use web services. They enable different clients like browsers, mobile devices, or other applications to communicate with a server via the HTTP protocol. The outputs of these APIs are often formatted in JSON or XML. This architectural style is popular because it leverages existing web technologies and protocols, is easy to use, and can handle various types of calls, return different data formats, and even manage changes in the structure of the application effectively.