API Functional Testing – Why Is It Important And How to Test

API Functional Testing - Why Is It Important And How to Test

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Imagine you’re planning a road trip across the country. You’ve got your car, a map, and plenty of snacks for the journey. But there’s one crucial thing missing – a functional engine that will get you from point A to point B without any issues. That’s exactly why API functional testing is so important.

APIs are the engines that power our software applications. They allow different systems to communicate with each other and transfer data seamlessly. But just like any other engine, APIs can break down and malfunction if not properly tested.

That’s where API functional testing comes in. It involves testing the behavior and functionality of an API to ensure that it performs as expected, meets the specified requirements, and returns the correct response under different scenarios.

This article will discuss why functional API testing is important and how you can automate it for better test results.

What is API Functional Testing?

API functional testing is a type of software testing that validates the functionality of Application Programming Interfaces (APIs). It helps ensure that the application and the API are working as expected under a specific workload it can handle. Here, the developers and QA team have to analyze what the application is supposed to do and ensure that it satisfies the purpose.

Non-functional API testing is where the testers check the non-functional aspects of an application, like its performance, security, usability, and reliability. Simply put, the functional test focuses on whether a product works, whereas non-functional tests focus on how well a product works.

Read here – all about API Testing.

Why is API Functional Testing Important?

API functional testing is vital to validate if the API is functioning as expected and can handle different data types, requests, and responses. Using the test results, developers can easily identify and fix bugs, security vulnerabilities, or issues in the API.

It also helps check if the APIs are secure and data exchange between the application and the API is accurate and up to date.

What happens when you don’t test your products properly?

Ensuring a quality interaction between users and your product is vital to a product’s success. It is crucial to test all the aspects of your application and not just the aesthetic part. That is why API functional testing is more important than UI testing or any other testing.

Customers will keep coming back only when the application satisfies their needs without any issues in terms of security, functionality, and reliability.

API Functional Testing Example

Let’s use online travel booking and banking applications as examples for API functional testing

1. Testing the API of an Online Travel Application

Let’s take the world’s leading online travel agency, Expedia, as an example. When travelers search for all the available flight options for a certain date range, the application will display all the available flights with price, travel time, and other details.

The Expedia app must communicate with the different airlines via APIs to show all these data. Hence, testers must perform API functional testing to ensure optimal communication and data exchange between various applications.

Let’s understand this example,

Test Case – Verify that the API can retrieve flight information based on the given parameters.

  • Origin: Mumbai
  • Destination: Manchester
  • Departure Date: 01-05-2023
  • Return Date: 05-05-2023
  • Number of Passengers: 2

Expected Output

The Expected output would be checked via Status and Response body as mentioned below:

Status – If the response status code is 200, then the API has executed successfully.

Response Body – The response body will contain the list of all flights that will meet the above criteria. In this case, the result will be Air Arabia, Air India, IndiGo, Air France, and so on.

Furthermore, when customers confirm their bookings and decide to pay the fare using any online payment platform like Stripe or Paypal, testers have to test if the travel app can communicate with the payment service to ensure a successful and secure payment for users.

2. Testing the API of an Online Shopping Website

Take Amazon, for instance. It makes online shopping easy for customers globally. Users can easily search and find their required product from a pool of products available on Amazon.

To make this possible, Amazon has to interact with different companies to list product details correctly and up to date. Also, it provides various online payment options, enabling customers to pay on the go securely. This is where API comes in. To interact with the various online payment platforms, amazon needs to interact with the APIs provided by them. Here is an example API provided by google pay to facilitate transactions:

Here, testers need to perform API testing to make sure that payments via this API go through as needed. If there is an error, like the user does not have enough money in their account or there is a network issue, they should be handled gracefully.

How to Get Started With API Functional Testing

Read the steps below to learn how to start functional API testing.

1. Understand the API test requirements

Before starting API functional testing, you need to understand the requirements for the testing and the API that needs to be tested. The requirements include the following:

  • The expected behavior of the API under various scenarios.
  • Understand the purpose of the API
  • What is the workflow of the software product, and where is API involved in that flow?
  • What aspects of the API should we test?
  • The input and output values.
  • Any error conditions.

Let’s use the example quoted above to understand this. In the case of the travel app example, you need to identify the endpoints, parameters, and expected response formats for each endpoint. That is, the API may have endpoints for flights, hotels, and car rentals. You should also understand what parameters each endpoint accepts and what type of response it returns.

2. Select an appropriate automation tool

Once you understand the test requirements, the next step is to select an appropriate test automation tool that supports API testing. Many automation testing tools are available in the market, from open-source to commercial, but you must pick the best automation testing platform based on your project requirements.

Note: It is always ideal to select a testing tool that supports all types of testing, not just API testing, as you will need to perform various types of testing to validate your product.

3. Create a test plan

After choosing the testing platform, you must create a detailed test plan outlining the test cases for API testing. A proper test plan will help you stay organized and ensure all testing aspects are covered.

For example, for the flight’s endpoint, you might test the API’s ability to return flights that match specific search criteria.

4. Implement the API tests

After creating the test plan, it is time to implement the API tests. It includes setting up a test environment and writing the test scripts. API is language-independent, meaning data is exchanged using JSON or XML. You can choose any of the core languages supported.

5. Execute the API tests

Once you have written the test scripts, you can execute them using your chosen testing framework or tool. The testing platform will then run the test scripts and generate a test report which might help you identify and fix bugs quickly.

Benefits of API Functional Testing

Functional API testing is crucial to guarantee your application works. It offers a ton of benefits to your product. Here is a list of some of the benefits of API functional testing,

  • Unlike UI testing, which checks the visual aspects of an application, API testing tests the business logic of the application.
  • API testing offers faster test execution time than UI testing.
  • It helps developers ensure the application works correctly and meets quality standards.
  • API testing helps detect errors in the initial phases of development, saving time and money in the long run. It also provides valuable insights into how the application will behave in production environments.
  • Performing functional API testing will offer a better customer experience.
  • Since API testing works on the business layer of an application, it offers a wide test coverage.
  • Continuous feedback provided via API tests helps improve the code quality.

Why and When to Automate API Functional Testing?

When deciding whether or not to automate API functional testing, it’s essential to consider the use cases for which automation makes sense. Automation is the best option for this type of testing as it offers numerous benefits, such as

  • Faster test execution
  • Improved accuracy
  • Easier maintenance
  • Wide test coverage
  • Time and cost savings
  • Faster feedback cycle

API Functional testing is often automated for continuous testing practices and continuous feedback cycles by DevOps, testing, and development teams. You can prefer manual testing only while performing certain testing like exploratory, unit, and usability testing.

Additionally, automation makes it easier to maintain complex tests over time, as changes to the codebase can be quickly detected and tested. Pick an automation tool that can automate your tests for mobile, web, and APIs in one place to save your testing time and for comprehensive test results.

Automate API tests 10x faster, build and validate API’s using Testsigma. No programming knowledge needed.

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The Best Tool to Automate API Testing – Testsigma

Testsigma allows you to automate your tests for REST APIs. It lets you quickly create, maintain and execute API tests. Its intuitive user interface allows you to create robust and reliable tests with minimal effort.

API Functional Testing

With Testsigma, you can enjoy faster test creation and execution times, increased accuracy in detecting bugs, improved collaboration between teams, and greater visibility into your API testing process. Below are some benefits of using Testsigma to automate API testing:

Simple Setup: It is a cloud automation platform, so you do not have to bother about installing any tools or prerequisites. Simply create, run, and maintain tests on the Testsigma Cloud.

No Coding: A no-code testing tool that helps you avoid writing any code for test scripts. Simply create test cases in plain English using NLPs provided by the tool.

Automate Your API Sequences: Customize and automate your API call sequences to fit the workflow of your application and avoid running into unexpected errors.

Integrated End-To-End Testing: Using Testsigma, you can automate UI testing and API testing in one place.

Test all your API methods: Check the API functionality using various REST API methods like GET, PUT, POST, DELETE, and HEAD via an integrated test or separate tests for each method.

Testsigma for API Testing

Different comparison modes: Compare your API responses using various comparison modes such as Strict, Non-extensible, Lenient, Strict order, and JSON schema to compare actual response values with your expected API response.

You can simply add REST API to your test steps by selecting the REST API from the drop-down box.


For further explanation on creating and executing REST API test cases using Testsigma, refer to the document here.

Apart from API testing, Testsigma automates various other testing types, like,

Testsigma is a one-stop solution for all your testing needs. It supports various application types like mobile, web, API, and mobile web.


As discussed above, API functional testing is a critical aspect of software development, ensuring that APIs function correctly and meet the needs of users. By testing thoroughly and validating against expected outputs, we can ensure the quality and reliability of the APIs that power our digital world.

When you have APIs that you need to test repeatedly, then looking for a solution to automate them makes sense. Testsigma is one such tool that can help you automate your API testing. Testsigma is a no-code test automation tool that provides an easy-to-use interface and a variety of features that allow testers to test API endpoints and validate responses quickly and easily.

It is the perfect solution for any organization looking to automate its API testing. Try our free trial to easily automate your API tests and get detailed test results and reports.

Build and validate API Tests in real time without Code for Web, Mobile and Desktop with Testsigma.

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Frequently Asked Questions

What is an example of a functional API test?

Consider you’re testing a registration API for a website. Here, we need to check if the API is correctly creating the users in the website’s database when a new user registers. We will also use invalid inputs to test how the API handles invalid inputs.

Is API testing functional testing?

Yes, API testing is a part of functional testing, as it checks the functionality of the application. This test aims to validate if the application is working as expected and handles both valid and invalid data.

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