Client-Server Mandate
This mandate underscores the fact that REST is a distributed approach via the nature of the separation between client and server. Each service has multiple capabilities and listens for requests. Requests are made by a consumer and accepted or rejected by the server.
Statelessness
Due to the nature of statelessness, it is a guiding principle of RESTful architecture. It mandates what kind of commands can be offered between client and server. Implementing stateless requests means the communication between consumer and service is initiated by the request, and the request contains all the information necessary for the server to respond.
Cache
Cache mandates that server responses be labeled as either cacheable or not. Caching helps to mitigate some of the constraints of statelessness. For example, a request that is cached by the consumer in an attempt to avoid re-submitting the same request twice.
Interface / Uniform Contract
RESTful architecture follows the principles that define a Uniform Contract. This prohibits the use of multiple, self-contained interfaces within an API. Instead, one interface is distributed by hypermedia connections.
Layered System
This principle is the one that makes RESTful architecture so scalable. In a Layered System, multiple layers are used to grow and expand the interface. None of the layers can see into the other.
This allows for new commands and middleware to be added without impacting the original commands and functioning between client and server.
Optional: Code-On-Demand
RESTful applications don’t have to include Code-On-Demand, but they must have Client-Server, Statelessness, Caching, Uniform Contract, and Layered Systems. Code-on-Demand allows logic within clients to be separate from that within servers. This allows them to be updated independently of server logic.
Credit: bmc.com