The Essential Guide To API Testing | 2022 Updated


API testing, or Application Programming Interfaces testing is a kind of software testing that involves verifying and validating APIs and Web services. It’s also a part of integration testing which determines whether the developed APIs meet the tester’s pre-established expectations, such as functionality, reliability, performance, or security. API testing is entirely different from testing. While GUI testing mainly focuses on the functions of an application that are visible to the users, API testing primarily centers on the logic layer of the software architecture.


Learn more: Introduction to API Testing with Real-life Examples

Key advantages of API testing

  • Test for core functionalities: The core advantage of API testing is providing access to the application with a user interface. In API test cases, the core functionalities are tested to expose minor bugs that can fester and become larger issues during GUI testing. Also, testing the API-level functionalities of the app gives an early evaluation of its overall build strength before running GUI tests.
  • Language-independent: In API testing, all data is converted to JSON or XML for transfer over the internet. This also means that any language can be used for automation, independent from the languages used to develop the application.
  • Time effectiveness: API testing is less time-consuming than GUI testing. With API testing, executing a regression test suite takes only 1 to 2 hours; while the same scenario under GUI testing takes 8 to10 hours. Furthermore, API testing provides faster test coverage and requires less code than automated GUI tests.
  • Easy GUI integration: You can easily integrate GUI testing with API testing. This is extremely helpful when you want to perform GUI tests after API testing.

Types of API testing

  • Functional testing: The main purpose of functional testing is to check the correctness of functionality. This includes testing of particular functions in the codebase, such as validation of response, error codes in case an API returns any error, test cases to verify HTTP response codes, etc.
  • End-to-end testing (or UI testing): This type of testing involves testing the user interface of the API and other integral parts. Its main purpose is to check the UI end-to-end functionality.
  • Load testing: This testing type checks whether an API can handle load. It is basically performed to ensure that the performance under both normal and at peak conditions is normal for an API.
  • Runtime/Error testing: this testing type is mainly concerned with the actual running of the API, especially with the universal results of utilizing the API codebase. It is done to expose problems such as race conditions, exceptions, and resource leaks.
  • Security testing: Security testing, penetration testing, and fuzz testing are the three separate elements of the security auditing process. The common point for all these three elements is to ensure that the API implementation is secure from external threats.


SOAP (Simple Object Access Protocol) and REST (REpresentational State Transfer) are two broad classes of web service for Web API.

SOAP is a protocol specification for exchanging structured information in the implementation of web services. Its purpose is to provide extensibility, neutrality, and independence. SOAP uses XML for exchanging information.

REST is the web standards-based architecture that defines a set of constraints to be used for creating Web services.

Learn more: 16 Common REST API Testing Interview Questions

Getting started with API testing

Before testing your API, you have to be clear about what you need to do and the necessary step to do it. Below are the five essential steps to help you perform API testing.

  1. Understand your API requirements

The first thing you need to do is write down all the API testing requirements. The purpose of this step is to help you plan API tests throughout the testing process. Ask yourself these questions to make sure that you thoroughly understand your API requirements.

  • What is the purpose of the API?
  • What is the workflow of the application; and where does the API fall in that flow?
  • What are the integrations supported by the API?
  • What are the features and functions of the API?
  1. Set up test environment

After having a plan for your API tests, the next step you need to do is set up your test environment according to your plan. This involves configuring the database and server for the application’s requirements. To make sure nothing is broken before you proceed to start more thorough testing,  you should make an API call right after the test environment has been set up.

  1.  Combine application data

To ensure that the API performs as expected against possible known input configurations, you need to combine your application data with your API test.

  1.  Organize yourself around the API test

Forming your API test strategy by asking yourself these questions:

  • Who is the target audience of your API testing?
  • Who is your API customer?
  • What is the expected result in normal circumstances?
  • What aspects are you testing?
  • What problems are you testing for?
  • What are your priorities to test?
  • What will happen in abnormal circumstances?
  • What are your pass and fail scenario?
  • What kind of APU tests do you want to execute?
  1. Test execution and reporting

Execute API test cases and that you have created and document the results for further use.

API testing tools

Selecting the right API testing tool plays an essential role in the success of a testing project. A suitable API solution can help save plenty of time and budget for the team. Without further ado, below are the top 5 API testing tools for 2019:

  1.      SoapUI
  2.      Katalon Studio
  3.      Postman
  4.      JMeter
  5.      REST Assured


The demand for API testing has been tremendously increasing along with Agile and CI/CD adoption trends in recent years. Amid the rise of API testing, it’s a good time for you to start learning API testing if you haven’t done so.

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