REST API – what is it?

A REST API is a way for one system to communicate with another over the Internet using standard HTTP requests—the same principles on which websites operate. An API (Application Programming Interface) is an interface that allows programs to exchange data and call each other’s functions. REST (Representational State Transfer) describes the style in which such an interface is built: clear, consistent, and independent of the technology used to develop the system on the other end.

For companies, this primarily means the ability to integrate applications with one another. Thanks to the REST API, a document workflow system can retrieve data from an HR system or forward an approved invoice to an accounting system without having to manually re-enter the information.

How does REST API work?

In REST, everything the system operates on is called a resource: it could be an employee, an invoice, an order, or a business partner. Each resource has its own address, e.g., /employees/123 for employee number 123. An application that wants to perform an action on such a resource sends a request to the server, specifying the address and the type of operation. The server executes the operation and returns a response along with a status code indicating the result, e.g., that the operation was successful or that the resource was not found.

The type of operation is specified using standard HTTP methods:

  • GET – retrieving data, e.g., invoice details,
  • POST – creating a new resource, e.g., adding an order,
  • PUT or PATCH – modifying an existing resource, e.g., updating the status of a request,
  • DELETE – deleting a resource.

Data is typically sent in JSON format, a lightweight and human-readable format that both people and programs can understand. XML is used less frequently.

An important feature of REST is that the server does not remember previous requests: each request contains all the information needed to process it, including authentication data, such as an API key or token. This makes the system easier to scale, and communication can be secured with an encrypted HTTPS connection and access control.

What is the REST API used for?

Today, the REST API is the most popular way to make system functions available on the Internet. Companies use it primarily for integration: connecting ERP, CRM, document management, HR, banking, and accounting systems so that data flows automatically between them. It’s also used to create custom applications that leverage data from other systems, as well as to automate processes where an event in one tool triggers an action in another.

To use an API, a system must provide it and describe it in its documentation: what resources are available, what methods can be used, and how to authenticate. Some platforms, including low-code and no-code solutions, allow you to use APIs from external systems via pre-built connectors, without any programming.

REST API vs. Other Approaches

SOAP is an older XML-based protocol with strictly defined message rules that is still used in some enterprise systems. REST is simpler and lighter, which is why it has become the dominant choice in newer solutions.

GraphQL is an approach in which the client specifies exactly what data it needs and receives it in a single response. In REST, the scope of data returned by a given endpoint is typically determined by the server. GraphQL works well for complex queries, while REST is simpler to implement and widely supported.