OpenAPI evolved from the Swagger project. Swagger started out as a specification for documenting RESTful APIs. Later on, tools to generate client and server code and generating of test cases were added. While the original Swagger Specification was donated to the Linux Foundation and renamed the OpenAPI, Swagger remains one of the most widely used open-source toolsets for developing OpenAPIs.
Wikipedia: OpenAPI Specification 🌟
OpenAPI FAQ. What is OpenAPI Specification (OAS)? OpenAPI Specification The OAS defines a standard, programming language-agnostic interface description for REST APIs, which allows both humans and computers to discover and understand the capabilities of a service without requiring access to source code, additional documentation, or inspection of network traffic.
apis.guru/openapi-directory: large archive of sample OpenAPI descriptions

RPC API Protocol (Remote Procedure Call)

wikipedia: RPC Remote Procedure Call

gRPC

Comparisons

SOAP vs REST

geeksforgeeks.org: Difference between REST API and SOAP API
dzone: A Comprehensive Guide to REST vs. SOAP Learn the primary differences between REST and SOAP APIs, each one's benefits, and when it's appropriate to use the two.
dzone: Web Services Architecture – When to Use SOAP vs REST Learn why SOAP (Simple Object Access Protocol) and REST (Representation State Transfer) are popular with developers working on system integration projects.
dzone: Comparing RESTful APIs and SOAP APIs Using MuleSoft as an Example
reply.com: Web Services: SOAP and REST - A Simple Introduction
- SOAP is a communications protocol while REST is a set of architectural principles for data transmission.
- REST was designed to be a more straightforward and easy to implement alternative to heavyweight SOAP for web service access. SOAP functions well in distributed environments where REST assumes a direct point to point communication. Also, SOAP allows for services to describe themselves to clients and in some languages allows for automation. On the other hand, REST is fast as less processing is required, uses less bandwidth and is closer to technologies used in web design.
- The choice on which to use is totally dependent on what the requirement. For example, SOAP is a better choice for applications that have complex API so as to describe the services and methods, where formal contracts are agreed for the exchange format, where a guaranteed level of security is required etc. REST will be preferred when limiting bandwidth and resources, when operations are can be stateless and the information can be cached.
baeldung.com: REST vs SOAP

REST vs OpenAPI vs gRPC

REST vs. gRPC: Battle of the APIs
Comparing OpenAPI With gRPC 🌟 OpenAPI is a great choice due to its interoperability. On the other hand, gRPC offers a better performance. Luckily, you don't have to choose one or the other.

Tools

API Testing

softwaretestingportal.com: API Testing, Key Terminologies and more...
dzone.com: 10 API Testing Tips for Beginners (SOAP and REST) Let's take a look at ten API testing tips for beginners with a focus on REST APIs and SOAP APIs.
blog.testproject.io: Top 10 API Testing Tools to Watch in 2020 🌟
mockoon 🌟 Create mock APIs in seconds. Mockoon is the easiest and quickest way to run mock API locally. No remote deployment, no account required, open source.
thenewstack.io: 4 Essential Tools for Protecting APIs and Web Applications

[![top 10 api testing tools](images/summarising_top_10_api_testing_tools.png){: style="width:50%"}](https://blog.testproject.io/2020/06/25/top-10-api-testing-tools-to-watch-in-2020/)

GraphQL

GraphQL A query language for your API
How is the OpenAPI Specification different from GraphQL? How are screws better than nails? Both are useful tools that solve similar problems in slightly different ways. OpenAPI Specification offers a declarative contract that defines the structure of API requests and responses as discrete operations. GraphQL prefers an interface style that is more like querying a database and is best suited to graph databases.
Hasura Launches Beta of GraphQL-Based Remote Joins Tool

Free Web Services (Public APIs)

medium: Do I Need an API Gateway if I Use a Service Mesh? 🌟

Dzone: How to Create a REST API With Spring Boot
Dzone: Step-By-Step Spring Boot RESTful Web Service Complete Example
Creando un API REST en Java (parte 1)
dev.to: Rapid API Creation with AWS Amplify
portal.dev Build beautiful API documentation. Portal lets you create, publish, and maintain your API docs with ease.
openapi-comment-parser A clean and simple way to document your code for generating OpenAPI (Swagger) specs.
- IBM creates an open source tool to simplify API documentation

API Business Models

API Business Models. 20 Models in 20 Minutes

[![20 API Business Models](images/api_business_models.jpg)](https://www.infoq.com/presentations/API-Business-Models/)

12 KiB Raw Blame History Unescape Escape

APIs with SOAP, REST and gRPC

APIs

Motivation

State of the API Report

Postman State of the API Report

Smartbear State of the API Report

Types of API Protocols

SOAP API Protocol (Simple Object Access Protocol)

REST API Protocol (Representational State Transfer)

OpenAPI Specification (originally known as the Swagger Specification)