Which REST constraint allows progressive enhancement through intermediary servers such as proxies and gateways?

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

Answer: (B)

The concept tested here is how REST lets systems grow by inserting intermediary layers between the client and the server. The Layered System constraint says you can compose the architecture from hierarchical layers, with proxies, gateways, and other intermediaries sitting in between. These layers can handle tasks like caching, authentication, rate limiting, or protocol translation, improving performance and security without the client needing to know what’s happening behind each layer. Because of this, you can progressively enhance the system by adding or replacing layers, and requests flow through the stack transparently to the client.

For example, a client hits a reverse proxy that caches responses and enforces security checks; if the cache misses, the proxy forwards the request to the backend service, and the response travels back through the proxy to the client. This demonstrates how intermediary servers enable progressive enhancement while keeping the client’s interaction simple.

Other constraints govern different aspects, such as Code on Demand (executing code on the client), Uniform Interface (how resources are identified and manipulated), or Cacheable (caching semantics), but they don’t define the ability to layer the architecture with intermediaries in this way.