Application Programming Interface (API)

An application programming interface (API) is a way for two or more computer programs to communicate with each other.

Concepts related to API

Electronic Data Interchange (EDI) is a is a process between two computer systems that enables two companies to exchange business information.

An example of EDI is EPCIS, a GS1 standard that helps to share information about the physical movement


A web service is a service offered over the web.

A web API is an API that allows communication through the internet. Because of their popularity, web API are often referred simply as API. A web API is in fact a type of web service.

An open API or public API is an API that can be accessed by anyone. This concept usually applies to web APIs.

HTTP methods are used within the HTTP protocol to exchange information. Some of these mothods are POST and GET.

API Architectures

API architectures:

  • SOAP
  • REST
  • GraphQL


SOAP is a protocol.

Web Services Description Language (WSDL) is SOAP is a standarized XML language that defines the interface for web services.

SOAP may be complex and cumbersome, but robust against errors when compared to REST.

Other characteristics:

  • Reliant on XML format (not JSON or others)
  • Used for stateful communications.


Rest API, also known as RESTful API, is an application programming interface (API or web API) architecture that conforms to the constraints of REST architectural style and allows for interaction with RESTful web services.

REST calls web resources by using uniform resource identifiers (URIs).

OpenAPI, an evolution of Swagger, is a specification for describing RESTful APIs in a standardized way.

RESTful are lighter than SOAP, but less robust against errors.

Other characterists:

  • Based on URLs
  • Offers outputs in many formats, such as XML or JSON
  • Used for stateless communications.


GraphQL is a data query and manipulation language for APIs.

There are different GraphSQL servers.

Express GraphSQL is FOSS.

Mercurius is FOSS under a MIT license.

Apollo is a suite of tools for working with GraphQL, which includes the Apollo Server and Apollo Client libraries. Apollo Server is a server-side JavaScript library that helps you build GraphQL APIs, while Apollo Client is a client-side JavaScript library that allows you to consume those APIs from your front-end application.

Apollo is source-viewable software, under an Elastic license.

API Formats

Some formats used for API communication are JSON or XML, among others.

SOAP works only with XML format, while REST is compatible with different formats like JSON and XML.

API Platforms

An API platform allows to share, build and test APIs

API platforms:

  • Insomnia
  • hoppscotch
  • Postman

The most popular is probably Postman.


Insomnia is free and open source software (FOSS).

Insomnia repository


hoppscoth is FOSS under a MIT license.

hoppscotch repository


Postman is an API platform that allows to share, build and test APIs, among other actions. It was the largest hub in 2023.

It is closed source and proprietary, owned by company Postman Inc.

Newman tool to manage Postman collection is FOSS.

API Models

API models, according to CCSP certification:

  • Public API
  • Partner API
  • Private or internal API

Declarative API

Declarativeness is a programming paradigm where the programmer indicates the desired state. It opposes other programming paradigms like imperative, where the programmer indicates the action to take.

A declarative API is an API that is centered around the desired state.

API Security

Common API security issues are:

  • Authorization for object access
  • Authentication weaknesses
  • Rate limiting

OWASP top 10 API on this external link.

OWASP API Security Project on this external link.

An API gateway is set before an API to aggregate API access, provide authentication for API use, rate-limit, and gather statistics and data about API usage.

An API gateway can be installed in the perimeter of a network. There are cloud API gateway like Amazon API Gateway.

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External References

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