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What is API testing?

Read our guide to find out what API tests are, how they work, why they’re important, best practices & some commonly asked questions.

api testing

Application Programming Interfaces (APIs) are the invisible connectors in our interconnected world between organizations offering services and clients consuming them. And they require consistent and thorough API testing if your IT infrastructure is to remain robust and impervious to threats.

In this article, we’ll look at:

  • What is API testing?
  • How does API testing work?
  • Different types of API testing
  • Why do APIs need testing?
  • Benefits of API testing
  • Challenges related to API testing
  • How can organizations address these challenges?
  • FAQs
  • Conclusion

What is API testing?

Put simply, it’s a form of software testing that scrutinizes the integrity, reliability, performance, and security of APIs, ensuring they perform their tasks accurately and protecting overall system integrity.

It’s a process that goes right to the heart of a system, checking for flaws, performance issues, and potential vulnerabilities. API testing may be a stand alone task, or included within other testing activities such as during web, mobile, or thick client tests.

In addition, an unprotected API could be the entry door to an organization during a red-teaming engagement.

How does API testing work?

API testing focuses on the API, and may be applicable when no interface exists, or for when public APIs are exposed that may have many different interfaces that only use a subset of the functionality.

Different types of API testing

There are various methods to test APIs, each focusing on different aspects of its functionality, performance, or security.

Validation testing

The aim of validation testing is to confirm that the behavior of the API aligns with its defined specifications. This process ensures that the API provides the expected responses.

Functional testing

The main goal here is to assess the performance of the API across different scenarios. It’s all about ensuring that the API can correctly perform its expected tasks under various conditions and inputs.

Load testing

The central focus of load testing is to evaluate the capacity of the API when handling a high volume of requests. It ensures the API maintains its performance and functionality under these demanding conditions.

Reliability testing

A vital part of the testing process, reliability testing involves checking the stability of the API over time. The aim of this method is to ensure consistent connections and dependable outcomes.

Fuzz testing

Fuzz testing involves sending invalid, unexpected, or random data to the API. This method tests how the API handles these abnormal inputs, looking for any weaknesses or potential points of failure.

Unit testing

An approach where individual components of the API are tested in isolation. By focusing on each part of the API separately, it ensures that all pieces function correctly independently.

Integration testing

When multiple components are linked together and tested as a group, it’s called integration testing. The goal of this testing type is to verify that data flows correctly between various parts of the system.

End-to-end testing

An exhaustive testing method where the complete application environment is tested. This method is used to replicate real-world scenarios to ensure the system works as expected from start to finish.

Performance testing

Performance testing is carried out to check how the API behaves under heavy loads and high stress. The idea is to understand the capacity and limitations of the API to improve its performance.

UI testing

This type of testing is not always applicable to APIs, but when it is, it focuses on the user interface. The main objective is to verify that the UI interacts correctly with the API and provides expected, user-friendly outcomes.

Runtime and error detection

This testing method focuses on identifying any errors or issues that occur while the API is running. It aids in catching any problems that might cause the API to crash or produce unexpected results.

Why do APIs need testing?

APIs are complex, often integrating numerous functions, features, and services. With this complexity comes the risk of errors, performance issues, security vulnerabilities, and compatibility concerns.

These issues can hinder the API’s ability to operate as intended, potentially disrupting the applications that depend on it. Let’s look at the main reasons API testing is needed.

Find security issues

API testing is a critical component of overall application security testing. It helps identify vulnerabilities and security weaknesses in the way an application exposes and handles data and functionality through APIs, ultimately contributing to a more secure and robust system.


APIs must undergo testing to verify their correct functionality, performance, and security, and to ascertain that they return expected results.

Identifying and fixing bugs 

APIs may contain bugs, leading to unexpected results or errors. Testing aids in identifying and rectifying these before they cause user problems.

Compatibility with other applications

APIs must interact seamlessly with the applications for which they are designed. Testing helps ensure the API’s compatibility, facilitating accurate data exchange.


Certain industries, like healthcare and finance, mandate API testing to meet compliance requirements. Testing APIs ensures these are met, protecting organizations from potential legal issues.

Benefits of API testing

The process of API testing offers a multitude of benefits that extend beyond the basic function of identifying and rectifying issues. When integrated into the development cycle, API testing can significantly optimize the product quality, development efficiency, and ultimately, the user experience. 

Quality assurance

API testing assures that the API functions as intended, bolstering overall software quality.

Early issue validation, detection, and resolution

It helps identify and rectify issues early, reducing the overall cost and time of the software development process.

Resource conservation

API tests require fewer resources than GUI tests, making them more cost-efficient.

Rapid iteration

API tests can be executed rapidly, allowing for quicker iterations during the development phase.

Easier test maintenance

Due to their independence from the user interface, API tests are more stable, making them easier to maintain.

Speed and coverage

API tests are faster and cover a broader scope of your system compared to GUI tests.

Language independent

APIs use a standard messaging system, allowing tests to be language independent.

GUI independent

API testing is performed at the message layer, making it independent of the user interface.

Faster releases

With the swift execution of API tests, software can be released faster, leading to shorter go-to-market times.

Challenges related to API testing

API testing, despite its many benefits, is not without its complexities. These stem from the intricate nature of APIs, their varying structures, diverse protocols, and the dynamic environments in which they operate.

Additionally, the evolving nature of technology adds layers of challenges, making the testing process even more nuanced. Below are some of the major challenges related to API testing.  

Poor documentation

This can limit test coverage, hinder test case development, and create challenges in understanding data requirements, authentication, and error handling. It can also affect integration testing, security testing, and collaboration.

Parameter selection

Choosing the right parameters for testing is critical and can be complex.

Parameter combination

Determining the best combinations of parameters to get accurate test results can be a challenge.

Call sequencing

Ensuring calls are made in the correct sequence can be difficult, especially when dealing with multiple APIs.

Choosing the right tool

With many testing tools available, choosing the one that fits your specific needs is essential.

Missing test specs

Lack of detailed test specifications can lead to missing critical test scenarios.

Learning curve

API testing has a steep learning curve, requiring a deep understanding of both the system and the API.

How can organizations address these challenges?

Navigating the complexities of API testing requires a structured approach. Organizations can address these challenges by Incorporating the following measures.

API testing best practices

Adopting best practices can streamline the API testing process and mitigate potential risks.

Use a variety of testing methods

Leveraging different testing methods can help ensure comprehensive coverage.

Create a dedicated testing environment

Having a dedicated environment for testing ensures that tests don’t impact the live system.

Test the API under different conditions and data

Testing the API under varying conditions, and with different data sets, can help identify potential issues.

Automate API tests

Automating API tests can make the process more efficient and reliable.

Run tests throughout the API lifecycle

Running tests at different stages of the API lifecycle ensures that any changes don’t affect its performance or functionality.

Write reusable subtests

Creating reusable subtests can save time and make the testing process more efficient.


What are some common bugs that API testing can detect?

API testing can detect a range of bugs, including missing or duplicate functionality, reliability issues, security vulnerabilities, and performance problems.

What’s the difference between API and unit testing?

API testing verifies the functionality, reliability, and security of APIs, while unit testing focuses on the smallest testable parts of the software.

Can API testing be automated?

Yes, API testing can be automated, which makes it far more efficient and consistent. Automated API testing involves using specialized tools and frameworks to automate the process of sending requests to APIs, verifying responses, and checking for the expected behavior.

What is the relationship between API testing and API monitoring?

API testing verifies that the APIs function as expected, while API monitoring continuously checks the API’s performance and availability in production.

What do you need to begin API testing?

To start API testing, you need a detailed understanding of the API’s functionality, a well-defined testing strategy, the right testing tools, and an efficient team of testers.

What tools can you use for API testing?

Numerous tools are available for API testing, including the following:

  • Burp Suite is a versatile and powerful tool that plays a crucial role in the security assessment of web applications, helping to identify and remediate vulnerabilities before they can be exploited by malicious actors.
  • Postman is a popular API testing tool known for its user-friendly interface, which supports both manual and automated tests and is highly effective for exploratory testing. With features like the ability to save requests for future use and share them with a team, it promotes a collaborative environment.
  • SoapUI is a comprehensive tool for testing Simple Object Access Protocol (SOAP) and REpresentational State Transfer (REST) APIs and it allows users to create and execute automated functional, regression, compliance, and load tests on different web APIs. It also offers a user-friendly interface for developing complex scripts.
  • Rest-Assured is a Java library that simplifies the testing and validation process of RESTful APIs. It integrates seamlessly with the Java ecosystem and is particularly suited to those familiar with Java development.
  • Katalon Studio is a robust tool that supports both SOAP and REST requests, designed to handle API, web, mobile, and desktop testing. With its built-in keywords and dual scripting interfaces for beginners and experts, it offers a comprehensive environment for testers of all levels.

How do you introduce API testing to your organization?

To introduce API testing, you need to train your team, choose the right tools, create a testing strategy, and foster a culture of ongoing learning and improvement.


API testing is a crucial component in maintaining the strength and integrity of digital architecture. By understanding what API testing is, how it works, and how to effectively implement it, organizations can ensure their APIs function as expected, are secure, and continue to deliver value.

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