Which REST constraint is optional and allows servers to send executable code to clients to extend functionality?

• A. Uniform Interface
• B. Layered System
• C. Cacheable
• D. Code on Demand

Answer: (D)

Code on Demand is an optional REST constraint that lets a server send executable code to a client to extend its functionality. By delivering scripts or other code, the server can add behavior to the client-side application without requiring new server endpoints, enabling dynamic interactions and processing right in the client. Because it’s optional, many RESTful services operate without sending code, simply transferring data in representations like JSON or XML. The other constraints—Uniform Interface, Layered System, and Cacheable—define how resources are discovered and interacted with, how systems are organized, and how responses can be cached, but they don’t enable the server to extend client behavior with executable code.