What are the principles of a unified interface?

Principles of a uniform interface (also known as RESTful API design principles, although applicable to other distributed systems):

1. Resource Identification: Each addressable piece of information must have a unique identifier. Clients interact with resources using these identifiers.
2. Manipulation of resources through representations: Clients receive a representation of a resource (e.g., in JSON or XML format) and can modify the resource's state by sending the modified representation back to the server.
3. Self-descriptive messages: Each message between client and server should contain enough information for its complete interpretation. The client should not rely on external information or session state on the server.
4. Hypermedia as the engine of application state (HATEOAS): The server should provide hyperlinks (links) in responses that indicate the next possible actions or related resources. The client navigates through the application by following these links, rather than relying on fixed URLs or logic.
5. Client-server architecture: Clear separation of responsibilities between the client (responsible for the user interface and initiating requests) and the server (responsible for data storage and request processing). This separation improves scalability and portability of the client side.
6. Statelessness: Each client request to the server must contain all the information needed for its understanding and processing by the server. The server does not store session state information between requests. This enhances server scalability and fault tolerance.

Adhering to these principles helps create scalable, flexible, and easily maintainable distributed systems.