Before we move to API Automation Testing, we need to understand what an API is and why it is becoming the backbone for any software application.

API (Application Programming Interface) allows the communication between two applications or components or computer hardware using a set of protocols. We can use API for different purposes:

  • Web based applications
  • Computer Operating System
  • Database System
  • Computer Hardware
  • Software Libraries

Application programming interfaces or APIs are vital for most data-driven applications. Due to the adoption of API frameworks like Swagger and RESTful API design patterns, all over the world is growing, connected with each other faster, and accessed so much data in minutes. APIs are always overlooked during the QA phase as customers or users cannot submit their complaints or a bug issue of the actual API, all they analyze is the loading screen that takes so much time to be filled with data. Despite these challenges, APIs and web services testing become both critical factors internally and externally for producing the quality in software, which is the priority of end-users.

Software applications often have three layers: a data layer, a logic layer, and the GUI (Graphical User Interface) or presentation layer. APIs also work on the messaging or logic layer. It maintains the bridge between the databases around the world and your front-end software that a user communicates with.

Web services v/s API

The web services are the implementation of APIs to communicate between two applications over a network.

  • SOAP – Simple Object Access Protocol
  • XML
  • JSON
  • REST – Representational State Transfer [70% of all web services are implemented using this]


  • GET            Retrieve information about the REST API resource
  • POST          Create a REST API resource
  • PUT            Update a REST API resource
  • DELETE      Deletes a REST API resource or related component

What is API Testing?

API testing is a kind of software testing that aims to validate the application programming interfaces (APIs) and checks the programming interfaces' reliability, functionality, security, and performance. API tests vary from GUI tests and don't focus on the look and feel of an application because the main focus while testing the APIs is given to the business logic layer of the software architecture. Instead of using standard user inputs like keyboard and outputs, we use software for API testing to send calls to the API, obtain the outcome, and understand the system’s response.

Types of Bugs that One Can Easy to Discover with API Testing:

  • Security issues and multi-threading issues.
  • Unused flags.
  • Duplicate or missing functionality.
  • Reliability issues & performance issues.
  • Response data is not structured accurately (XML or JSON).
  • Improper warnings/errors to a caller.
  • Faulty management of valid argument values.

Testing Approach for APIs

The testing approach for APIs is a predefined method or strategy that the testing or QA teams follow for the API testing when the build is ready. This type of testing doesn’t involve any source code. With an API testing approach, you can easily understand the testing techniques, functionalities, input parameters, and the execution of test cases. Below are some points that help users to follow the API testing approach:

  • Understanding the API program’s functionality and clearly describing the scope of the program.
  • Applying test methods such as boundary value analysis, equivalence classes, error guessing, and writing effective test cases for the API.
  • Planning and defining of input parameters of the API accurately.
  • Implement the test cases and compare the actual results with expected ones.

How Should You Perform API Testing?

Various things one should cover for the API automation testing other than the common SDLC process.

 # Usability Testing

The purpose of running usability testing is to verify whether the API is user-friendly and fully-functional. It also helps integrate the API with other programs.

 # Discovery Testing

If you are the test group, you need to execute the set of calls manually that are documented in the API. For example, verify that a particular resource exposed by the API can be created, listed, and deleted appropriately or not.

 # Security Testing

When it comes to checking what type of authentication is required and whether the extremely sensitive data is encrypted over HTTP, security testing must be carried out.

 # Automated Testing

API testing can result in a collection of a method or scripts that one needs to use every day for running the APIs.

 # Documentation

Testers should involve documentation in the final deliverables. To help people interact with the API, the testing team must ensure that the documentation is sufficient and provide well-required information.

Some Test Cases to Consider for API Testing

  • Return value as per the input condition: It is easy to evaluate because the input can be specified, and results can be authenticated.
  • Update data structure: When updating the data structure, some effect or result is produced on the system. It is necessary to authenticate during API testing.
  • Doesn’t return anything: A behavior of the API on the system needs to be checked when you don’t get any return value.
  • Adjust certain resources: Whenever specific resources adjust the API call, it should be checked by the respective resources.
  • Trigger some other API/interrupt/event: If any event or interrupt is triggered by an API output, such events and interrupt viewers should be tracked.

Why is API Testing important?

As Agile development is becoming the standard in all the organizations and the way the softwares is developed.

  • Earlier Testing – Once we create the logic, we can build the test cases to validate the correctness in the response and the data and the front-end build is not required at that point.
  • Easier Test Maintenance – As UI needs to change constantly, as it is being accessed from different browsers, devices and screen orientation, which is unstable but in API has no such challenges.
  • Faster Time to Resolution – When an API tests fail, it is easier to find the defect, as we will already know where the system broke and helps us to quickly fix the bug.
  • Speed and Coverage of Testing – UI tests may take longer time but API tests give results in less time while also being about to fix them immediately.

API Testing Types

  • Functionality and Behavior
  • Performance and Reliability
  • Security

Prerequisites required performing an API Testing

  • API DOCUMENTATION – This documentation contains all the information related to the API like headers, request and response format, error codes, parameters.
  • TEST SCOPE – This helps to understand how much functionality needs to be covered in the while testing the API that includes Integration Testing of the API with the front-end application.
  • TEST Tools

API Automation Testing Tools

  • POSTMAN – we can use POSTMAN to develop, test APIs as it is more efficient, and it has all the capabilities to work with APIs.
  • REST-ASSURED – It uses the RESTful Web Service library that can be included in the existing framework and we can call its methods directly for fetching responses and perform the required actions.
  • SOAP-UI – It is a popular tool for API testing. Functional, Performance, Compliance testing can be performed using this.
  • JMETER – It is mostly used for performance and load testing but we can use it for testing the APIs.
  • Katalon Studio – It is a free automated testing tool built on the top of Selenium and Appium, which widen the scope to Mobile Testing including Web testing and API Testing.
  • CloudQA TruAPI


API refers to a set of functions or classes or procedures that define the business logic layer. In other words, the API (application program interface) is a set of protocols, processes, routines, and tools that help to build critical software applications. APIs help make our phones “smart”. It gives value to an application and helps people streamline their business processes.

Testing of API is necessary because if it doesn't work effectively and efficiently, it would be hard to convince your customer to adopt an application. Traditional strategies rely on paperwork, large sales forces, outdated, time-consuming, and expensive measures to run the business. When you understand the scope of the internet of things, you can quickly know how important APIs are for tech-based companies and transform businesses. To acquire the quality, instant ROI, time & cost savings, API testing becomes the challenging concept of the chain of software & QA testing to ensure that the digital lives will run seamlessly.

Also Read: Impact of Virtualization on API Testing


Kanika Vatsyayan

Kanika Vatsyayan is Vice-President – Delivery and Operations at BugRaptors who oversees all the quality control and assurance strategies for client engagements. She loves to share her knowledge with others through blogging. Being a voracious blogger, she published countless informative blogs to educate audience about automation and manual testing.


Add a comment

BugRaptors is one of the best software testing companies headquartered in India and the US, which is committed to catering to the diverse QA needs of any business. We are one of the fastest-growing QA companies; striving to deliver technology-oriented QA services, worldwide. BugRaptors is a team of 200+ ISTQB-certified testers, along with ISO 9001:2018 and ISO 27001 certifications.

USA Flag

Corporate Office - USA

5858 Horton Street, Suite 101, Emeryville, CA 94608, United States

Phone Icon +1 (510) 371-9104
USA Flag

Test Labs - India

2nd Floor, C-136, Industrial Area, Phase - 8, Mohali -160071, Punjab, India

Phone Icon +91 77173-00289
USA Flag

Corporate Office - India

52, First Floor, Sec-71, Mohali, PB 160071,India

USA Flag

United Kingdom

97 Hackney Rd London E2 8ET

USA Flag


Suite 4004, 11 Hassal St Parramatta NSW 2150

USA Flag


Meydan Grandstand, 6th floor, Meydan Road, Nad Al Sheba, Dubai, U.A.E