Top 27 API Testing Interview Questions

December 27, 2023Pricilla Bilavendran
Most Commonly Asked API Testing Interview Questions

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If you’re here, you’re probably preparing for an API testing interview. As we can see, the software industry is growing on a daily basis, and every organization is seeking new talent. API testing is becoming increasingly popular with the growing demand for software development and automated testing, but it can be daunting to tackle if you don’t have the right information. 

Most employers prefer to hire employees with prior expertise in API testing. Because of the lack of clarity in API testing, even experienced candidates are unable to answer fundamental questions. That’s why it’s helpful to brush up on your API testing skills with a few API testing interview questions. To help you get ready for your next interview, we’ve rounded up some of the most common and challenging API testing interview questions asked, along with their corresponding answers.

I recommend that you first study the fundamentals and ideas of API testing, and then keep up to date on API trends.

Table Of Contents

Web API Interview Questions for Freshers (Definition & Functions of an API )

Here will discuss the frequently asked Web API Interview questions. Please understand, comprehend, and research further. 

1. How do you Define an API with an Example?

An Application Programming Interface (API) is a set of methods and protocols that allow two applications to interact with each other. It can be used over the Internet or any other network. An example of an API is Google Maps, which allows you to integrate its services into your website or mobile app so that users can search for places, get directions, and view maps without leaving your website or app.

2. What are the Different Types of APIs?

The four types of APIs are:

  • Public APIs: These are APIs that are publicly available to developers, allowing them to utilize a specific set of public functions or data of a service. Popular examples of public APIs include Facebook, Twitter and Google Maps
  • Private APIs: These are APIs that are private to a particular organization or business and are used internally to facilitate the use of certain data and functions. They are not available to the general public and are only available to developers that have been given access to them.
  • Shared APIs: These are APIs that are managed by multiple organizations and provide data and functionality to all of them in a shared manner.
  • Composite APIs: These are made up of multiple types of services and include data from multiple sources. This type of API allows developers to create more complex applications by taking advantage of the public, private, and shared APIs available.

3. What are the Main Differences Between API and Web Service?

  • Every web service is an API but not every API is a web service.
  • Web services have limitations compared to APIs in terms of specifications and capabilities.
  • Web services use specific communication styles like SOAP, REST, and XML-RPC, while APIs can be exposed in various ways.
  • Web services always require a network for operation, whereas APIs can function without a network connection.

4. What is Web API?

API means Application Programming Interface. A Web API is an application programming interface specifically designed for web applications. It allows web applications to interact with each other or exchange data.

5. Who Can Use a Web API?

Web API can be used by any clients that support HTTP verbs like GET, PUT, DELETE, and POST. Web APIs do not require any complex configuration and can be easily accessed by any client. Even portable devices like mobile devices can access web API, making it highly versatile and advantageous.

6. What is the Difference Between Client and Server APIs?

Client APIs are application programming interfaces that an application or service uses to access another system, while Server APIs are methods that serve data or services to other systems. 

Client APIs allow applications to access data, while Server APIs allow applications to provide services.

7. What is a Web Service?

A web service is an interface which provides the functionality of an application over the internet, using a set of protocols like HTTP, XML or JSON. Web services allow programs to communicate with each other over the web without having any prior knowledge of each other’s internal workings. 

8. How do you Test for Performance Issues in an API? 

To test for performance issues in an API, you can use tools like JMeter and LoadRunner that can monitor response times and analyze load tests. Additionally, you can test for server-side performance by checking server logs or using system monitoring tools like New Relic or Datadog.

9. Which is a Read-only HTTP Method?

The GET method is read-only in HTTP protocol.

10. Which Method is Used to Create a Resource in APIs?

Usually, the POST method is used for creating resources in APIs because it sends data from the client side to the server side. And if data sent from the client side changes, then the server-side resource will also change automatically and will notify the client side about changes happening in resources on the server side automatically.

11. Which Protocol is Used by RESTful Web Services as a Medium of Communication Between Client and Server?

HTTP (Hypertext Transfer Protocol) is used by RESTful web services as a medium of communication between client and server. 

12. What is the Category of 4XX HTTP Codes Belonging to?

The Category of 4xx HTTP Codes belongs to Client Error Code. The reason behind these errors could be that the client made an invalid request or failed to send data in its request message. 401 Unauthorized and 403 Forbidden are some examples.

The first step in debugging API difficulties is to enable logging in order to monitor any failures or problems. It is also critical to check all input parameters before sending them through the API, since this can prevent many problems from arising. It is also a good idea to see if there are any known bugs with the API or its dependencies that can be fixed by upgrading or patching them. Finally, in order to correctly determine where any difficulties may be originating from, it is critical to have a clear grasp of the system architecture and how different components interact with one another.

Testing an API – Top Web API Testing Interview Questions & Answers

Here will discuss some Common API Testing Interview questions.

14. What is API Testing?

API testing involves testing the interactions and responses of APIs to ensure they behave as expected and meet the specified requirements. It typically involves sending requests to the API endpoints with different input data and validating the corresponding responses.

It also helps verify the functionality, reliability, performance, and security of APIs.

15. What does the Process of API Testing Involve?

The process of API testing involves verifying that the application is functioning as expected by making sure that there are no errors in the code. It also includes validating that the responses sent by the server comply with pre-defined requirements, as well as ensuring that data sent through the API is secure. It may also include performance testing, which involves measuring the speed and accuracy of the responses sent by the application.

16. What is the Purpose of API Testing?

The purpose of API testing is to validate that the application’s behavior is as expected when it interacts with external systems, such as databases, web services, or third-party applications. It also ensures the integrity of data being transferred and makes sure the application is stable and reliable. 

17. Why is API Testing Important?

API testing is essential for a variety of reasons. API testing ensures an API’s operation and performance, and ensures that an API is made according to the design requirements. API testing also ensures that API follows the required security standards. It also aids in the detection of defects and errors that might otherwise go undetected during traditional testing. API testing may discover API usage rule breaches as well as inappropriate permission and authentication processes. Furthermore, API testing can assist in identifying issues with the version compatibility, performance concerns, and security threats. 

18. What Types of Tests can you Perform on an API?

Some of the tests that can be performed on an API include:

  • Functional tests to make sure that the application works as expected
  • Security tests to ensure that data sent through the API is secured
  • Performance tests to measure the speed and accuracy of responses
  • Compatibility tests to verify that the application will run properly on different operating systems or devices.

API Testing Interview Questions for Experienced

19. What Experience do You have with Designing and Executing Automated API Tests?

We design the API Test automation framework and run the API tests based on the API capabilities and needs. When manually testing the API endpoints is insufficient, we will group the endpoints and test them as end-to-end. This type of setup is really difficult, but it will save us a lot of time and work in the future.

Here, you should add more relevant examples from your own experience.

20. How do You Ensure the Quality and Accuracy of Your API Tests?

By adding assertions to our API tests, we can assure the quality and correctness of our API tests and uncover possible problems that may exist. Monitoring the API tests for a specified period of time to see if any unusual behavior is emerging from the API. Also integration of the tests with CI/CD pipelines and the scheduled tests can convey a lot about the API quality. 

21. What Tools or Techniques do You Prefer to Use When Testing APIs?

Tools such as Testsigma, which is available as open-source and also has a premium cloud-hosted version, make it easier for testers to test APIs. There are also other open-source tools that might be used to automate API testing. 

When it comes to techniques, it’s always best to test all of the API endpoints first, followed by the end-to-end scenarios. Use parameters which make the maintenance easier and more manageable. Continuous testing, Mocking and monitoring are the other techniques which are useful.

22. What Difficulties Have You Encountered While Testing APIs?

API Testing was difficult at first because we had to learn how to use API, but we are now more skilled in employing API and are able to achieve better results with each test.

Initial project setup, skill set necessary for API Testing, test cases for API Testing, test reporting, and tool selection are a few prevalent obstacles.

You can try adding some realistic challenges you faced while testing APIs because these pointers vary from team to team.

23. How do You Perform API Testing?

API testing involves using tools to send calls to an API and analyze the resulting responses. These tools typically have features such as authentication methods (e.g., basic authentication or OAuth2), call management tools (e.g., creating new calls or editing existing calls), response analysis tools (e.g., validating response codes or other data returned by an API call), and reporting capabilities (e.g., creating reports based on test results).

24. What are Some Common Challenges When Performing API Testing?

There are several common challenges when performing API testing: 

Access – APIs may not be publicly accessible and may require special credentials or access rights.

Complexity – APIs can be complex, making it difficult to develop robust test plans.

Security – APIs may be vulnerable to attacks and require special security measures. 

Also, the challenges may vary based on the project architecture and design.

25. What are the Tools Available in the Market for API Testing?

There are several API testing tools available, including Testsigma, Postman, SoapUI, Katalon, Apigee, and others. Each tool has advantages and downsides, therefore it is critical to thoroughly weigh them before selecting one for your project. Always do your PoC and select the right tool.

Automate your tests for web, mobile, desktop applications and APIs, 5x faster and from the same place with Testsigma

26. What is API Request and Response?

API request: An API request is a request made from a client, such as an application or website, to a server to access data or services.

API response: An API response is the data returned by an API when a request is made. This data can be in the form of JSON, XML, or other data formats.

Sample request and response below:


GET /users/1 HTTP/1.1

Host: sample.com


HTTP/1.1 200 OK

Content-Type: application/json


   “id”: 1,

   “username”: “sample_username”,

   “email”: “sample@test.com”


27. What is API Security Testing?

API security testing checks the security of an API, which is a vital component of any program. An API must be evaluated against typical attack vectors such as Cross-Site Request Forgery (CSRF), SQL Injection, and the Remote File Inclusion (RFI) attack to help assure its security. Authentication and authorization mechanisms should also be examined. Tests should be performed to confirm that the API is not subject to malicious agents exploitation and that it can offer the desired level of control and access to the data it is supposed to serve.

28. What is API Mocking and How is it Used in API Testing?

API mocking is the process of simulating the behavior of an API without really having access to it. This is important for API testing since it allows developers to evaluate the functionality and performance of their application without relying on the external API’s availability. Developers may also create edge cases and evaluate how their program reacts to unexpected input. You can proceed with testing when the APIs are unstable and underdeveloped. Finally, it is useful since it eliminates the need for manual calls while testing against a live API, allowing for quicker test cycles.

29. What do You Understand About Latency in an API?

Latency in an API is the time taken for a request to be completed and for the response to be sent back by the application. It is the amount of time it takes for a user to send a request to the server and receive a response. Latency is usually measured in milliseconds (ms) – the amount of time it takes for a single request/response cycle to complete. The higher the latency, the slower the API will be to respond.

30. What do You Understand by Error Handling on an API?

Error handling is the process of responding to and dealing with errors that occur while interacting with an API. Returning error messages with information about what went wrong, such as an appropriate HTTP status code and an error message with a description of what caused the issue, is one method of managing errors. This can assist users in debugging and understanding the issue, making it easier to resolve. Another method is to offer extensive error logging so that developers may examine the data and determine where the problem occurred. Finally, strategies for alerting developers to possible concerns, such as putting up alerts to notify them when an unexpected occurrence occurs, might be beneficial. This may also be used to trace down and explore the source of a problem.

31. What is the Importance of Documentation for an API?

API documentation is critical for every software development project since it allows developers to explain the API’s capabilities and parameters to other developers and end users. It also minimizes debugging time and guarantees that everyone is on the same page when it comes to the API’s functionalities, which helps to eliminate misunderstandings between developers and users. Furthermore, thorough and precise documentation may aid in optimizing an API’s usability and increasing its adoption rate. Furthermore, API documentation may be utilized by developers to better the development process by knowing how the API works and what coding approaches should be employed. Lastly, API documentation may be a great resource for end users since it gives extensive information on how to use the API as well as troubleshooting tips.

REST (Common Web API Testing Interview Questions)

Here are some commonly asked REST API interview questions,

32. What is REST?

REST stands for Representational State Transfer. It is an architectural style used for designing networked applications, particularly web services. REST has become a popular choice for designing APIs due to its simplicity, scalability, and compatibility with the HTTP protocol. RESTful APIs are built on the principles of REST.

33. What is a RESTful Web Service?

A RESTful web service is an implementation of the REST architecture that enables communication and data exchange between different systems over the web.It is a stateless, client-server architecture where web services are considered as resources that can be identified using unique URIs. To interact with these web services, REST client applications can use HTTP GET/POST methods.

34. What are Some Key Characteristics of REST?

Some key characteristics of REST (Representational State Transfer) are,

  • Client-Server Architecture
  • Statelessness
  • Cacheability
  • Layered System
  • Code-On-Demand
  • Uniform Interface

To Conclude:

We hope this collection of API testing interview questions and answers has given you a better idea of what to expect in an interview setting when discussing your experience with API testing. 

Overall, there are many aspects of API testing that are important for an interviewer to understand when assessing a candidate’s skillset. Being able to answer API Testing Interview questions related to these topics will give a candidate a great advantage in their interview and show employers that they have a strong understanding of how APIs work and why they are important for creating reliable applications. Always strive to explain with real-life examples, and this does not imply that you can talk a lot 🙂. Keep it short and simple.

Please keep in mind there’s no cheat sheet or pdf file that you can read and go get a job. You definitely should get your hands dirty and learn the concepts yourself. Try building sample projects and keep learning. 

With the right preparation and knowledge in hand, you’ll be well-equipped to tackle any challenges that come your way during your next interview!

Good Luck with your interview!!

Some suggested blogs for reading about API testing:

API Testing Basics – A Comprehensive Guide for Beginners 

Unlocking Types of APIs & Protocols: Choose the Best for Your Project



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