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API Testing Tutorial – Benefits, Types, and Tools

API testing is one of the most demanding segments of the software and quantitative analysis testing. Before we discuss this testing type, we will first understand what exactly the API is. API stands for Application Program Interface. It acts as a mediator between two systems or applications to carry out communication. Moreover, API is a connective tissue between several systems or different layers of the system.

Any application, installed on our mobile devices, consists of three different layers – data layer, service layer (API), and presentation layer. The service layer consists of the application’s business logic. This logic implies rules how users should interact with the application’s functions, services, and data. Now, let us understand what is the role of API with one example.

Consider you have planned a tour and need a hotel room. You will indeed search for a hotel room through an online travel booking site before heading towards the tour. You need to fill all the required fields, like the destination city, check-in and check-out dates, number of people and rooms. After hitting the search button, you will receive the desired information. API is responsible for collecting all data and providing the relevant information. It accumulates data from different hotels. Further, when you hit the search, it communicates with every hotel’s API and delivers the result as per your requirements.

What Is API Testing?

API testing, the software testing type, tests the application program interface to verify whether the application’s performance, reliability, security, and performance meet expectations. Additionally, it also detects bugs and inconsistencies in the system. This testing does not involve manual or keyboard inputs and outputs. Instead, it requires dedicated software to provide calls to API, receive output, and observe the response. Unlike GUI tests, API tests do not focus on the application’s look and feel. Instead, they focus on the system’s business logic or service layer.

The Application Program Interface is one of the most significant elements that provide a value to the system or application. It helps to perform business processes seamlessly. If APIs do not function correctly and efficiently, people will indeed not adopt, despite it is free. An entire chain of business processes may affect due to the breakage or failure of an API.

API Testing Types

There are eight different types of testing that are to be performed on APIs. Let us go through each testing type in detail.

1. Validation Testing

Testers perform validation testing at the end of the software’s development process. This testing plays a crucial and essential role in the software development process. It checks the system’s several aspects, efficiency, and behaviour. In other terms, the validation testing assures that the development of the system is correct and accurate.

2. Functional Testing

Functional testing involves testing particular functions in the codebase. However, these functions are specific conditions or scenarios to assure that the system’s API is functioning accurately within the given parameters.

3. UI Testing

UI stands for User Interface. As its name indicates, the UI testing tests the API’s user interface and its constituent segments. This testing validates the software’s front-end and back-end for their usability, efficiency, and health. Unlike functional and validation testing, UI testing is relatively more specific.

4. Security Testing

Security testing also includes penetration or fuzz testing. It ensures that the API is safe and secure from all external threats and attacks. Moreover, it involves the validation of the API access control’s design and encryption techniques. Security testing also involves the validation of user authorization and rights management.

5. Load Testing

The next testing type is load testing. Testers perform this testing after a specific codebase block or after the completion of the entire codebase. It ensures that all theoretical solutions work accurately as expected and planned. Moreover, it regulates the application’s performance at extreme and normal circumstances.

6. Runtime And Error Detection

Runtime and error detection testing validates the running or execution of the application’s API. In other terms, it focuses on utilizing the API codebase for getting universal results. Additionally, it concentrates on monitoring, resource leaks, execution errors, or error detection.

7. Penetration Testing

Penetration testing is the second step in the auditing process. An individual with sufficient working knowledge of API attacks the API to determine all possible threats and attacks. However, these threats may be related to the application’s functions, processes, resources, API, and its integral parts.

8. Fuzz Testing

Another test in the security audit process is fuzz testing. Here, an extensive amount of data is fed to the system to determine crashes and negative behaviour of the system. Yet, this testing validates the API’s limitations for preparing the worst-case scenarios that could happen when the data is fed.

Benefits Of API Testing

1. Language Independent

In API, the exchange of data takes place using JSON and XML formats. Such data exchange is independent of coding language. Therefore, you can choose any coding language for test automation. XML and JSON format helps in fast and stable verification, as they are structured data formats. Moreover, built-in libraries help to exchange data through XML and JSON formats.

2. GUI Independent

Before executing GUI testing, testers perform API testing. Therefore, performing API testing early will detect many small errors, and the testing team gets feedback soon. With feedback, they can work to overcome errors and enhance the strength of the system’s GUI. API testing requires relatively fewer scripting efforts than GUI testing. Therefore, it is a resource and time-saving.

3. Easier Test Maintenance

API tests are stable and are modified infrequently. They are changed only if the business logic of the system is modified. As a result, the frequency of the changes in API tests is relatively less than UI testing. For any change of event, it requires less work to rewrite tests cases.

4. Improved Test Coverage

API tests cover a wide range of functional as well as non-functional test cases. Moreover, API tests require to consider negative and positive tests to verify the entire scenario. Several combinations of data inputs can be utilized to run function and non-functional test cases since API testing is a data-driven technique.

API Testing Tools

There are several API testing tools available today. Below are the top tools used widely to perform API testing:


With the SoapUI testing tool, testers can test web services and REST and SOAP APIs. This tool comes with Free and Pro packages. The Free package is open-source, whereas Pro requires a yearly subscription. Moreover, users feel more comfortable to create tests using point-and-click and drag-and-drop features. On the other hand, users can fetch data from databases and files with the Pro version.


API testing using Postman is relatively easier and more comfortable. Postman acts as a plugin for Chrome and works well with both Windows and Mac devices. Testers can also perform exploratory and automation testing using Postman. Users do not have to learn any new coding reprogramming language to run API tests using this tool. It is not open-source; instead, it charges $21 per month.


Apigee is another API testing tool that enables users to use other editors, like Swagger. It enables users to develop API and test performance. Using this testing tool, one can track response times, error rates, as well as API traffic. But this tool is quite expensive than the others.


JMeter is a testing tool used for API and load testing. This tool automatically works with CSV files and also integrates with Jenkins to include API tests in the CI pipeline. With JMeter, you can create function API tests and performance tests together.

Citrus Framework

Citrus Frame is one of the most desirable testing tools that automate integration tests. It is compatible with SOAP, HTTP, TCP/IP, JMS, REST, and many other protocols. This tool uses XML or Java to create tests. Moreover, this tool is designed to work well with any headless protocol.

Challenges Of API Testing

  • Some major challenges faced by testers is parameter selection, parameter combination, and call sequencing.
  • Since no GUI is available, testers may feel quite challenging to test application and provide input values.
  • Testers should possess in-depth knowledge of parameter selection and parameter categorization.
  • They need to verify and validate output values in another system, which is problematic.
  • Moreover, testers should possess coding knowledge and should test exception handling function.

After reading this API testing tutorial, one will surely get a clear idea about testing the application’s API, its purpose, and the result. It is one of the requisite testings in software testing, despite challenges the testers encounter while executing it. A weak, breakable, or error-prone API breaks not only a single system but also the entire chain of business processes.


How to do API Testing?

There are several open-source and paid software tools that perform automated API testing. In this article, you come across the top four testing tools.

API Testing falls under which box?

API testing falls under white box testing.

Do You Know?
1. What is Regression Testing?
2. What is White Box testing?
3. What is black box testing?
4. What is Exploratory Testing?
5. What is Functional testing?
6. What is ETL Testing?
7. What is API Testing?

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