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Mitrais, Software Development

What is API Testing?

An Application Programming Interface (API) is a certain amount of code that helps two software programs communicate with each other. API testing is a black box test that aims to verify if the API can deliver the required functionality. Tests can be executed directly in the API or as integration tests.

In general, API testing is done by sending a request to the API endpoint and comparing the response data to the expected results as specified in the API contract while focusing on analyzing the business logic and security of the application.

Benefits of API Testing

API testing provides several benefits for software development and quality assurance teams. Here are the five main advantages to keep in mind.

The Ability to Find Bugs During the Early Stages of Software Development

During API testing, we can access the application without a UI. We can also run tests without testing the entire software, making it possible to find bugs early to fix them before they affect the UI. This approach saves time and money for quality assurance.

Effective Time Management

API testing takes less time than Graphical User Interface (GUI) testing. A UI-level test takes 7 minutes, while API tests only require 12 seconds. The time savings are apparent if we need to run 1000 tests in this case.

During UI-driven tests, most of the time is spent loading and rendering web pages and UI elements that might not be needed. For example, a tester must assess that a website’s form can work effectively by filling it out, submitting it, and ensuring that the user is moved to the next screen.

Reduced Testing Expenses

API testing provides quicker results with early defect diagnosis, which reduces overall testing costs.

Non-Attachment to Programming Languages

An API test exchanges data using special XML or JSON formats. This way of transferring information is entirely independent of the application language. So, if we automate API testing, engineers can efficiently utilize many programming languages that support technologies, including Ruby, JavaScript, Java, PHP, Python, etc.

Improved Test Analysis

Unlike unit tests that check for a separate function, source code component, or method, automated API tests are usually broader in range. Often, problems appear at the intersection of modules, where one module’s scope ends and another begins. API level tests check to see if all system components work correctly. In addition, it helps to identify potential defects in interfaces, servers, and databases. In turn, it improves the overall quality of the software and its interaction with users. 

Types of API Testing

There are several types of API testing, including:

  • Unit Testing. This type of testing checks the minor units of the API’s code to ensure they function correctly. It helps in identifying and fixing issues early on in development.
  • Functional Testing. This testing aims to confirm that the API works as intended and satisfies the requirements. It includes checking the API’s functionality, error handling, and response to invalid inputs.
  • Integration Testing. This testing looks at how the API interacts with other systems, like databases and other APIs, to ensure that the data is transferred correctly and that the API is integrated smoothly.
  • End-to-End Testing. This testing checks the overall flow of the system, including the API, to see if it operates properly from start to finish. It often involves simulating real-world scenarios and testing from a user’s point of view.
  • Performance Testing. This type of testing assesses the API’s performance, stability, and scalability. It can include testing the API’s response time, ability to handle multiple requests, and capacity to manage large volumes of data.
  • Security Testing. This testing aims to uncover any security risks and vulnerabilities in the API. It can involve testing the API’s authentication, authorization, data encryption, and protection against common attacks.
  • Load Testing. This testing looks at the API’s performance and behavior under heavy loads. The goal is to identify limitations and determine the maximum capacity.
  • Stress Testing. This testing evaluates the API’s behavior and performance when under extreme stress. It helps in determining the API’s breaking point and potential performance problems.

API Testing Tools

Many API testing tools are available, each with its strengths and weaknesses. Choosing the best one will depend on the specific use case and requirements we are trying to fulfill. 

Here are a few of the most used tools:

Swagger UI

Swagger UI is an open-source tool for visualizing and interacting with REST APIs. It is an excellent choice for quickly envisioning and assessing an API documented with the Swagger specification.


Postman is an API development and testing tool that offers a feature-rich GUI for sending HTTP requests and viewing responses. In addition, Postman supports collections for grouping related API requests. It is a popular choice among API developers and testers due to its rich feature set, user-friendly interface, and support for collections.


Insomnia is a free, cross-platform REST client that offers a modern interface for sending HTTP requests and viewing responses. It supports request templates and environment variables. 


SoapUI is an open-source tool for testing SOAP and REST APIs. It provides a GUI for creating and executing tests and has functional and performance testing capabilities. Additionally, it integrates with other tools such as JUnit and Jenkins. For those looking for a tool that can effectively test SOAP and REST APIs, SoapUI is a great option that offers both operational and performance testing capabilities.

Apache JMeter

Apache JMeter is an open-source performance testing tool for web applications that can simulate a heavy load on a server and measure its response time. It offers a GUI for creating and executing test plans and advanced features like distributed testing and continuous integration. Apache JMeter is a good choice for performance testing, especially for those who need to simulate high server usage.


Katalon is an automated web, mobile, and API testing platform that offers a GUI for creating and executing test cases and integrations with popular tools like JIRA and Selenium. In addition, it supports multiple scripting languages, such as Groovy and Java, making it a good choice for those who need an all-in-one solution for automated web, mobile, and API testing.

Ultimately, the choice will depend on several project factors, like specific needs and requirements, cost, ease of use, and compatibility with other tools.


With the many uses of API in today’s applications, each element in the application may have different data storage and commands when interacting with the database. When one application feature is updated, the goal is that other components can still function without interruption; therefore, API testing is used to ensure all parts work properly according to their function, even when a feature gets an update. API testing can also improve the test coverage, and save time and other resources required to release an application. Overall, API testing is essential in ensuring the reliability and performance of applications that rely on APIs, leading to high-quality software and a better user experience.




Author: Yudhi Pratomo, Software Tester

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