If you are a junior looking to enhance your skills or a more experienced tester looking to revise your knowledge, you came to the right place. 

This article will present what API testing is, what tools you can use, and how to get started with it.

What Is API?

Firstly, API is an acronym for Application Programming Interface. Commonly, applications have three layers: a presentation layer (UI), a service layer (API), and a data layer. 

API is the middle layer between the presentation and database layers. It enables communication and data exchange between two separate software systems. 

What Is API Testing?

API testing is a type of testing that validates the API. Its purpose is to check the functionality, reliability, performance, and security of the programming interface. As part of integration testing, API testing validates the logic of the build architecture within a short period of time. 

Benefits of API Testing

  • Language independent 

Data is exchanged using XML and JSON formats, therefore any language can be used for automation.  

  • Easier test maintenance 

User interfaces are always changing and moving around based on how they are accessed. API changes are much more controlled and infrequent. 

  • Improved test coverage

Most  API or web services have specifications, allowing you to develop automated tests with high coverage. 

  • Time efficiency 

If you are working on an automation heavy project, you should consider API testing. In comparison with GUI testing, API takes less time.

For example, a UI level test takes seven minutes to run, while an API test takes twelve seconds. 

Types of API Testing 

API testing can cover functional testing, reliability testing, load testing, security testing, and validation testing. 

1. Review the API specification

Before starting, you should first understand how the API functions and what to expect when using API. 

2. Determine API testing requirements

To determine the API testing requirements, you need to understand the target consumer, its features and functions: as well as the workflow of the application, the aspects, priorities, and the problems you are testing for. 

3. Define input parameters

Next on, you have to define the parameters. This means you have to configure the database and server for the application’s requirements.

4. Create positive and negative tests

To evaluate whether the API works as expected, you should run a series of positive and negative tests. The positive tests are to check the functionality of the API using the required parameters, while the negative tests are to check how the API responds to prohibited operations using valid and invalid user input. 

5. Select an API testing tool

Now that you are familiar with API testing, the next step is to choose a tool that works best with your project. 

The Top 5 Tools for API Testing

Deciding what tool to use is a hassle on its own. To make it easier, we’ve prepared a list of 5 tools that can be used for API testing. However, when working on a project, you should choose a tool with a programming language the developers are already using: 

  1. Postman

Postman is an API platform for building and using APIs. It serves as an HTTP client capable of both automated and manual testing, allowing you to autopilot routine tests while you try specific use cases. 

  1. SoapUI

SoapUI is a functional testing tool for SOAP and REST testing. It has an easy-to-use graphical interface that allows you to create automated functional, regression, and load tests. 

The tool comes in two variants, SoapUI Open Source, a free tool, and ReadyAPI, a paid tool. ReadyAPI provides more options to construct, deploy and monitor your tests than SoapUI Open Source.

  1. Katalon Studio

Katalon Studio appeals to users looking for a more comprehensive solution for deployments across multiple platforms. It combines manual and automated testing for web, desktop, and mobile applications. 

  1. Apache JMeter

JMeter was initially intended for API load testing, but it can now handle performance testing. You can test performance both on static and dynamic resources. 

  1. REST Assured 

Rest Assured is a free, open-source testing tool. It is a Java library that streamlines HTTP requests to REST APIs. 

API is considered the engine of the application and should work as expected without errors. To ensure they meet functionality, performance, security, and reliability expectations, APIs must undergo API testing.