API or application programming interface is a set of rules that enables applications and software to communicate and connect. An API that adheres to the design tenets of the REST, or representational state transfer, architectural style is known as a REST API. Because of this, REST APIs are also known as RESTful APIs.

REST offers developers a fair amount of flexibility and freedom and was first introduced in 2000 by computer scientist Dr. Roy Fielding in his Ph.D. dissertation. REST APIs have become a popular way to connect apps and components in a microservices architecture, in part due to their flexibility.

REST design principles

An API's most fundamental function is to provide a way for one application or service to access a resource contained within another application or service. The application or service requesting to access the resource is referred to as the client, while the application or service hosting the resource is referred to as the server.

Developers are required to adhere to a rigid architecture by some APIs, such as SOAP or XML-RPC. Yet, REST APIs enable a wide range of data formats and may be created in almost any programming language. The following six REST design principles, often known as architectural constraints, must all be adhered to as their single requirement:

1. Uniform interface. No matter where the request originates, all API calls for the same resource should have the same appearance. The REST API should make sure that a given piece of information, like a user's name or email address, only applies to one particular uniform resource identifier (URI). Resources shouldn't be excessively huge but should include all the data the client would want.

2. Client-server decoupling. Client and server apps must be independent of one another when designing a REST API. The client application should only be aware of the URI of the requested resource; it should not be able to communicate with the server application in any other way. Similarly to this, a server application should only transmit the client application to the desired one.

3. Statelessness. Because REST APIs are stateless, each request must contain all the data required to process it. In other words, server-side sessions are not necessary for REST APIs. No client request-related data may be stored by server programs.

4. Cacheability. Resources should be client- or server-side cacheable whenever possible. Information regarding whether caching is permitted for the provided resource must also be included in server answers. The objective is to increase server scalability while enhancing client performance.

5. Layered system architecture. The calls and responses move via various tiers in REST APIs. Always presume that the client and server programs are not directly connected. The communication loop may include a variety of different middlemen. REST APIs must be created in a way that neither the client nor the server can determine whether they are communicating directly with the end application or through a middleman.

6. Code on demand. Typically, REST APIs transmit static resources, but in some circumstances, answers may also include executable code (such as Java applets). The code should only execute in these circumstances.

How REST APIs work

To execute common database operations like creating, reading, updating, and deleting records (also known as CRUD) within a resource, REST APIs interact via HTTP requests. A POST request is used to create a record, a DELETE request is used to delete one, and a GET request is used to obtain a record from a REST API. Calls to APIs can utilize any HTTP method. A well-designed REST API resembles a website that is accessible by HTTP and is shown in a web browser.

The resource representation, often known as a timestamp, is the status of a resource at any given moment. Almost any format, including HTML, XLT, Python, PHP, or JavaScript Object Notation (JSON), can be used to communicate this information to a client.

With REST API calls, request headers and parameters are especially crucial since they contain crucial identification data including metadata, authorizations, uniform resource identifiers (URIs), caching, cookies, and more. Throughout properly-designed REST APIs, request and response headers as well as standard HTTP status codes are used.

REST API best practices

Although REST API design freedom is a major benefit, it also makes it simple to create an API that is broken or performs poorly. Because of this, qualified developers collaborate on REST API specifications to exchange best practices.

The OpenAPI Specification (OAS) creates a framework for describing APIs so that any developer or application can find it and fully understand its parameters and capabilities - including available endpoints, permitted operations on each endpoint, operation parameters, authentication methods, and other information. The most recent version, OAS3, offers practical tools for creating API clients and server stubs in several programming languages, like the OpenAPI Generator.

Industry-recommended practices, such as the use of hashing algorithms for password security and HTTPS for secure data transmission, are another place to start when securing a REST API. The permissions granted to third-party applications can be restricted with the aid of an authorization framework like OAuth 2.0. An API can also reject any request that comes in after a particular amount of time using a timestamp in the HTTP header. Further safeguards to guarantee that only authorized clients can access the API include parameter checking and JSON Web Tokens.