REST APIs must adhere to five requirements :
Client-server decoupling : The client and server can only interact in a series of requests and responses. Only clients can make requests, and only servers can send responses. This simple principle allows both parties to operate independently of each other.
Uniform interface : All communications between the client and server must follow the same protocol. For REST, this protocol is HTTP. A uniform interface simplifies integrations because every application is using the same language to request and send data.
Stateless : In stateless communication, the server does not store any information about past requests/responses. Each request and response contains all information needed to complete the interaction. Stateless communication reduces server load, saves memory, and improves performance. It also eliminates the possibility of a failed request caused by missing data.
Layered system : Layers are servers that sit between the client and API server. These additional servers perform various functions, like identifying spam and improving performance (See also: What Is a CDN?). In REST, layers are modular and can be added and removed without affecting the messages between the client and the API server.
Cacheable : Server responses indicate whether or not the resource is cacheable, so that clients can cache any resources to improve performance.
Additionally, REST includes one optional condition :
Code on demand : An API can send executable computer code to clients in its response. This lets the client application run the code in its own back end.
