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What Is API Testing and How to Approach It?

woman sitting in front of the computer in the office

Today, APIs are used in many software projects. Especially since the microservice architecture has become more widespread, software development teams make synchronized communication between these services via APIs. Even in many monolithic applications, the backend application communicates with the frontend application and APIs and exchanges data. The popularity of using APIs is increasing, as they have many advantages, such as data provision and transfer. Testing these developed APIs before other systems use them is as important as producing APIs. So what is API testing?

Testing web services is a concept we hear frequently in software development projects. In fact, there are test teams affiliated with almost every software team that test the produced software. In this article, we will take a closer look at what is API testing question. Then, we will also touch on the importance of API testing, API testing approaches, and many topics related to API testing.

What Is API Testing?

Firstly, API testing is all of the tests done to check if an API is working properly. It covers testing all API functions.

API tests are done to make sure the API is working properly. They are usually applied as automated tests but can also be applied to manual testing. With API tests, test teams test almost every feature of APIs, such as whether the input parameters of the API are correct, whether the error messages are appropriate, whether the response times are within the desired time, and whether the performance of the API is appropriate.

What Is the Importance of API Testing?

API tests are the processes performed to ensure that the APIs developed or intended to be used are working correctly. It is very important for businesses and software development teams. The following are some of the reasons why API testing is important:

Reduces the error rate: API tests are a factor that directly increases the quality of the developed application and reduces the error rate. With API testing, developments are tested before they work on the production environment, and bugs are noticed beforehand. In this way, any error situation in the live environment is prevented.

Increases security: APIs are the doors that applications open to the outside. And testing these doors is the first thing to do to ensure security throughout the application. Ensuring the APIs are working properly is essential to detecting and fixing potential security vulnerabilities.

Increases customer satisfaction: API tests help increase customer satisfaction by affecting the correct operation of the software. Correct operation of APIs in the application allows customers to use the application more easily and reliably. So, it increases customer satisfaction.

What Is the API Testing Approach?

people standing in front of the whiteboard

It is important to follow a certain approach in order to perform API tests accurately and effectively. Following an approach for API testing means setting up the end-to-end testing process and determining a roadmap. Below is an approach to be followed and questions to be asked when conducting API tests:

Analysis of API Requirements

First of all, it is necessary to understand what requirements the API must meet. For this, test teams analyze API documentation and requirements. Then, the test team may ask these questions at this stage:

  • What requirements should the API meet?
  • What kind of data should the API get?
  • Which data formats should the API support?

Creation of Test Scenarios

Creating scenarios to cover certain business rules is important when testing APIs. Designing scenarios to test the full functionality of the API is necessary. Then, the test team may ask these questions at this stage:

  • Do the constructed API test cases cover all cases?
  • Which scenarios will we use to test the API?

  • What input data will each scenario work with?

Preparation of Test Data

It is important to use correct data for API tests to work as intended and to give accurate results. Test data refers to the input data that use while requesting API. Then, the test team may ask these questions at this stage:

  • What data will we use in test cases?
  • How will we prepare and manage the test data?
  • Where to get test data?

Performing the Tests

This is the step of testing the prepared test cases on APIs with the correct test data. Then, the test team may ask these questions at this stage:

  • By whom and how will we conduct the tests?
  • Which API testing tools will we utilize?
  • When and where will we save the test results and analyze them?

Analysis of Test Results

The results of the tests performed provide important information about the performance, accuracy, and reliability of the API. Then, the test team may ask these questions at this stage:

  • What do the test results mean?
  • Does it meet the tested API requirements?
  • How are the reliability and performance of the API?

What Are the Types of API Testing?

api testing types diagram

There are many different types and methods of API testing. In this section, we will examine some of the types of API testing.

Validation Testing

It is a data validation test. Validation testing is a test type that validates API inputs and outputs. So, it tests that the API processes the requests correctly and produces the correct response.

Functional Testing

It is a type of test that tests a specific feature of the API.

UI Testing

It is user interface testing. It is a test applied to ensure that the web or mobile interface of the API is working properly.

Security Testing

It is the basic security test. Test teams test how effective the API is at protecting data and whether it has vulnerabilities.

Load Testing

It is API load testing. That is, test teams make heavy requests to the API and test how the API performs under heavy usage in this test type.

Runtime and Error Detection

A test type that tests how the API behaves and handles errors at runtime.

Penetration Testing

It is the type of test where penetration tests are performed to ensure security. So, tests are performed to find vulnerabilities in the security of the API.

Fuzz Testing

It is a phase test. Test teams test the API to verify that it functions correctly on unexpected input.


To summarize, API testing is very important to ensure that the APIs produced or used are working correctly. Having and performing a good API testing approach directly increases the customer experience of businesses and takes them one step ahead of their competitors.

Get free​ a lot of APIs with high security, so easily test these APIs with the poweful interface provided by APILayer securely.


Q: How to Test API?

A: There can be more than one method and step for API testing. But nowadays, a testing tool is used to create API test automation and even to perform integration testing in API tests. With an API testing tool, an API tester can also perform API load tests.

Q: Why is Application Programming Interface Testing (API Testing) Important?

A: Developers or test teams perform API testing to ensure that a developed application works correctly and produces the desired results. Also, API tests help validate the functionality of the application. Additionally, it enables early detection of faulty situations in the testing process.

Q: What are the Steps to Take for a Good API Test?

A: It is very important to perform good API testing and make sure that the API has been thoroughly tested. There can be many steps followed to perform a good API test, and this may also differ from business to business. The following steps are the most popular steps businesses take to perform API testing.

  1. Define test objectives and requirements
  2. Identify test scenarios
  3. Set up the test environment
  4. Create test cases
  5. Execute test cases
  6. Analyze test results
  7. Report and track defects
  8. Regression testing

Q: Is Every API Listed Under APILayer Testable?

A: Yes, every API listed under APILayer is testable. Every API listed on APILayer has free subscription plans, so developers and test teams can test this API. Besides, APILayer provides a ‘Live Demo’ service to its users. With this service, developers and test teams can quickly and easily test the APIs they want through an interface provided by APILayer.

Q: Are the API Products Under APILayer Reliable?

A: Yes, they are. Every API listed under APILayer is highly secure. These APIs are thoroughly tested before being listed in APILayer. In addition, APILayer guarantees the security of these APIs. In this way, the connection with these APIs is provided with high security.

Q: What Is the Difference Between API Tests and GUI Tests?

A: API tests and GUI tests are different types of tests that testers and developers use for different purposes in the software testing process. Firstly, API tests represent testing the APIs behind the software. On the other hand, GUI tests represent the testing of the user interface (GUI). API tests are used to ensure that the developed APIs work correctly between software components and are suitable for data exchange, while GUI tests are used to ensure that the software is presented to users correctly. Finally, API tests and GUI tests are two different types of tests that complement each other in the software testing process.

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