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API Testing Interview Questions

Last Updated : 19 Sep, 2023
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API testing is a subdomain of software testing that focuses on the functionality, dependability, performance, and security of Application Programming Interfaces (APIs). An API is essentially a collection of protocols and tools that enable different apps to connect with one another. API testing is regarded as the future of software testing because of its advantages in core functionality testing, time effectiveness, language independence, and GUI integration.


API Testing Interview Questions 2023

This article covers a list of the top 50 API testing interview questions that are commonly asked in interviews for freshers, Intermediate and experienced candidates. The questions are divided into three categories:

The article also provides answers to each question, along with explanations and practical Example with PostMan. This will help you to understand the concepts behind the questions and to prepare for your interview.

API Testing Interview Questions For Freshers

1. What is an API?

Application Programming Interface (API) is useful for communication between different software systems. It acts as an interface between two applications and allows two software systems to communicate with one another.

  • It is a collection of functions that can be executed by another software program.
  • It takes the request from the user and gives the response without exposing the internal details.

2. What is API Testing?

API testing is a type of software testing that aims to test the API directly and as a part of integration testing to determine if it meets the expectations for functionality, performance, and security. It aims to analyze multiple endpoints such as databases, web services, etc.

3. List the differences between API and Web Service.



Web Service


API is an Application Programming Interface that acts as an interface between two applications.

Web services are a type of API that must be accessed through a network connection.

Protocols Support

It provides support for HTTP/s protocol.

It provides support for HTTP protocol.

XML Support

API supports XML and JSON.

Web service supports XML.

Hosting Platform

It can be hosted on IIS and self.

It can be hosted on IIS.


It is used as an interface for communication between two applications.

It is used for REST, SOAP, and XML-RPC for communication.

4. How do APIs work?

  • API takes requests from the user.
  • It processes the request which may involve data validation, data processing, database interaction, etc.
  • The response is sent back to the source.

5. What are the different types of API testing?

  1. Functional Testing: This aims to verify the API functions correctly and meets the required specifications.
  2. UI Testing: This aims to validate that the API is returning the expected results and in the correct format.
  3. Load Testing: This type of testing aims to test the APIs performance and stability under stressful conditions.
  4. Security Testing: This type of testing aims to identify the security-related vulnerabilities and flaws in the API and ensures that the API meets the required security standards.
  5. Validation Testing: This type of testing aims to ensure that the API is returning the expected results and in the correct format.
  6. Runtime and Error Detection: This type of testing aims to ensure that the API can handle runtime errors and exceptions by testing for network timeouts, memory leaks, etc.
  7. Penetration Testing: This is a type of security testing that involves simulating attacks from hackers to detect vulnerabilities and weaknesses in the API.
  8. API Hacking: It is a security testing technique that exploits vulnerabilities in an API, Attackers can target API endpoints to gain access to data or disrupt services.
  9. Fuzz Testing: It involves giving invalid inputs into the API to test its ability to handle unexpected input and recover from errors.

6. What protocols can be tested using API Testing?

  • HTTP
  • REST
  • SOAP
  • JMS
  • UDDI

7. What are the advantages of API Testing?

  • Faster bug fixes: API testing works quickly to provide a solution, thus it helps to diagnose problems earlier in the development of the system allowing for efficient bug detection and removal.
  • Reduced costs: API testing provides quick release of results, easier to maintain, thus reducing the cost of testing.
  • Universal language support: API testing provides support for many different languages like Javascript, Ruby, Python, and PHP. Formats like XML, and JSON are supported during API testing.
  • Better test coverage: API testing provides better test coverage as it drills down to the layers beneath the UI to ensure databases are communicating correctly with one another.

8. What are the disadvantages of API Testing?

  • Technical skills: API testing requires skilled and trained testers
  • Time-consuming: API testing is time-consuming as it requires time to develop and execute test scripts for APIs.
  • Limited documentation: Limited documentation of the API makes it difficult for the testers to understand how the API should behave in response to a particular input.
  • Cost: API testing using automated tools can be expensive and requires a significant budget thus increasing the cost of testing.
  • Security: APIs can introduce security vulnerabilities if not properly tested and secured.

9. List the differences between API Testing and Unit Testing.


API Testing

Unit Testing


API testing is used to test the API to ensure that it meets the expectations for functionality, performance, security, etc.

Unit testing is used to test each unit and to ensure that each unit performs as expected or not.

Carried out

It is carried out by QA team.

It is carried out by developers.

Type of testing

It is mostly black-box testing.

It is white-box testing.


It is broader in scope.

The scope is limited.

When executed

It is executed after the build is finished.

It is performed prior to check-in.

10. What needs to be verified in API testing?

  • API testing helps to verify the correctness of data and responses. The API should return the return correct data and resposnses according to the specified requirements and expected behavior.
  • API testing should verify the functionality and behavior. The API should perform its intended functions accurately.
  • HTTP status codes of the API needs to verified.
  • Error codes if the APIs are expected to throw an error.
  • Authorization details.

11. What is Latency in API testing?

Latency in API testing is how long it takes for the request to reach the server. The shorter the response time, the better the user experience.

12. What is an API framework?

API framework is a foundation on which a software developer can build applications for a specific platform. It is defined by the configuration file which consists the list of all APIs that is reqquired to be activated and activated for a particular program run.

13. Mention common HTTP methods used in API testing.

  • POST: It is used to create new data or resources on an API.
  • GET: It is used to retrieve data or resources from an API.
  • PUT: It is used to update existing data or resources on an API.
  • PATCH: It is used to update existing data or resources from an API partially.
  • DELETE: It is used to delete data or resources from an API.

14. What is the role of headers in API testing?

Headers in API testing plays an important role in providing additional information that is not included in the request body. Herders are important in controlling the behavior of the server in handling the request, to provide authentication information, or to provide metadata about the request or response.

15. What is the purpose of authentication in API testing?

API authentication is a combination of technology or process that verifies the identities of users who want access to an API. It involves the use of software protocol to verify identity of the requester before granting access to protected resources.

API Testing Interview Questions for Intermediate

16. What is API versioning, and why is it important in API testing?

API versioning involves specifying a version number in the API endpoint or headers to ensure the backward compatibility. It manages the changes in the API over time.

  • It ensures the correct version of the API is being tested.
  • It ensures that the changes in the API do not break the existing client applications.

17. What is the purpose of HTTP status codes in API testing?

HTTP status codes indicate whether a specific request successfully completed or not. These are embedded in the HTTP header of a page to tell the browser the result of its request.

18. What is the purpose of the request and response body in API testing?

The request body is used to send and receive data such as input parameters, or data to create/ update resources via the REST API. The response body is the data API send to the client.

19. What is the purpose of the request and response headers in API testing?

The request headers are used to specify a content type, caching, authentication, etc. The response header can provide information about the server, caching, etc.

20. What is API mocking?

API mocking is practice of simulating the behavior of an API endpoint during testing without actually invoking the real API. It is beneficial during the development stage.

21. Why API Mocking is used in API Testing?

  • Mock APIs are useful when applications have dependencies with external APIs.
  • Mock APIs are useful for testers to plan and validate test executions and for developers to do unit testing and identify the initial development stages.
  • Mock APIs are useful in situations where the complete API needs to be made available for consumer testing before committing it to development stage.

22. What are the advantages of using API mocking in API testing?

  • Development stage: API mocking is useful to simulate external dependencies and unexpected behavior.
  • Functional stage: API mocking is useful to prepare the API to handle extreme cases detect potential failures and verify whether the API works as expected or not.
  • Non-functional stage: API mocking makes it possible to evaluate the performance and response times of APIs in various scenarios.
  • Allows API demonstration: API mocking allows developers to demonstrate an API to clients before deploying the software.

23. What is API security testing, and why is it important?

API security testing is the process of testing the vulnerabilities in the API. This is done through penetration testing or manual scanning of the APIs. API security testing is important:

  • Cloud-based applications rely on APIs to exchange data and interact with each other. Any security vulnerabilities can have far-reaching consequences.
  • Online operations and businesses rely on APIs to integrate different systems and services. This causes potential security risks.
  • Organizations rely on security solutions that are built for web apps to detect and secure from API threats. Such solutions cannot detect unique vulnerabilities and gaps in the APIs.

24. How do you ensure the security of APIs in your tests?

  • Use secure communication protocols like HTTPs to encrypt data transmitted between client and server.
  • It is important to properly implement authentication and authorization mechanisms to ensure that only authorized users can access the API endpoints and perform the authorized actions.
  • Ensure that all the input data in the API requests are properly authorized, validated, and sanitized to prevent injection attacks and cross-site scripting attacks.

25. What are the different types of error responses in API testing?

  • Validation errors: Validation errors occur when the API request does not meet the validation criteria defined by the API.
  • HTTP error status codes: These can be client-side status codes or server-side status codes. There are standard HTTP status codes like 4xx or 5xx that indicate errors in the API request or response.
  • Custom error messages: These are the customer error messages that are returned by the API in case of errors or exceptions.

26. How do you handle error responses in your API tests?

  • Clear error messages: One of the best practice to handle error responses is to provide clear and helpful error messages that explain the cause of the error and the solution to the error.
  • Error logging: Best practice is to implement error logging and monitoring tools that can help track, analyze, and resolve errors in API testing.
  • Use consistent error codes: It is important to use consistent and meaningful error messages that indicate the type and severity of the error.

27. What is input validation?

Input validation is the process of analyzing the inputs and disallowing the inputs that are considered unsuitable. It ensures that all input data in API requests are properly sanitized and validated to prevent injection attacks.

28. Why input validation is important in API testing?

Input validation is important in API testing as it prevents malicious data from entering the system. It is important to prevent injection attacks and other malicious activities. This can be done manually or using automated tools.

29. What is cross-site request forgery (CSRF)?

Cross-site request forgery is an attack that forces user top execute unwanted action in the web application in which they are authorized to perform actions. This attack exploits the trust the web application has in an authenticated user.

API Testing Interview Questions for Experienced

30. How CSRF can be prevented in API testing?

  • CSRF tokens: CSRF can be prevented by using CSRF tokens. These are the random tokens that need to be unique per user session and should be of large random value to make it difficult to guess.
  • SameSite cookies: SameSite is a browser security mechanism that determines when a website’s cookies are included in the requests originating from other websites.
  • Referer-based validation: HTTP referer header can be used to defend against CSRF attacks by verifying the request originated from the application’s own domain.

31. What is API contract testing?

API contract testing is a type of testing that aims to monitor the API conversation that takes place between the API consumer and the API producer. It can be performed using tools like Postman, Swagger, etc.

32. Why API contract testing important?

  • System maintenance and Scaling are difficult when multiple modules are involved.
  • The integration testing is slow in development and returning results, especially when communication modules need to be tested.
  • Debugging is also a hectic task when multiple modules are involved. It may happen that one is debugging an error due to a module apart from the one that is being tested.

33. What is API performance testing?

API performance testing is a type of software testing that evaluates the API performance like response time, throughput, etc under certain conditions. It can be done using tools like JMeter, Gatling, etc.

34. Why API performance testing important?

  • API performance tetsing helps to improve the API’s overall performance and stability.
  • It helps to provide insight into the API’s overall performance thus helping to identify the areas of strength and weakness.
  • It helps to identify the performance issues and determine the impact of changes.
  • It helps to ensure that the API is flexible and can handle the demands of the real world.

35. What is API monitoring?

API monitoring is the practice of monitoring the API to gain visibility into performance, availability, and functional correctness. These tools helps to analyze the performance of applications and improve poorly performing APIs.

36. Why API monitoring important?

  • They provide measurements of how long a routine takes to execute, how often it is called, and how much of total time is spent in executing the transaction.
  • It is important to ensure availability, performance, and security of the APIs.
  • It helps to guarantee a dependable and effective user experience.
  • It helps to track the availability of the critical APIs.

37. What is API virtualization?

API virtualization is the process of using a tool that creates a virtual copy of the API mirroring all of the specifications of the production API and using this virtual copy for testing.

38. Why API virtualization is used in API testing?

  • It is used in API testing as it allows for testing much earlier in the development process, removing the key bottlenecks that would otherwise delay production.
  • It is helpful to isolate dependencies, simulate responses, and ensure consistent behavior.

39. What is the purpose of API documentation in API testing?

API documentation is a technical document that describes the API in detail. It includes instructions on how to effectively use and integrate API and provides updates regarding API’s lifecycle such as new versions. It is primary resource explaining what is possible with the API and how to get started with the API.

40. How do you prioritize API test cases for regression testing?

  • Select test cases with frequent defects.
  • Select test cases with critical functionalities.
  • Select test cases with frequent code changes.
  • Cover end-to-end test flows.
  • Select risk-based testing approach.

41. How to handle versioning in API testing?

Versioning in API testing can be handled by maintaining backward compatibility for existing APIs, providing clear versioning information, and updating tests accordingly.

42. List the challenges faced while performing API testing.

  • Building irrelevant tests: Building tests without considering how the APIs will be consumed may be quicker in short term.
  • Initial setup of API testing: Setting up an API testing setup requires certain level of expertise and dedication among the team members.
  • Not including API dependencies: Failure to include API dependencies as a part of API testing strategy can be a critical API testing challenge.
  • Not validating data: It might be possible that API tests pass successfully but APIs are not returning the correct data in their responses. It is important to verify that the API is returning correct data.

43. What are the different bugs that can be found in API testing?

  • Duplicate or missing functionality.
  • Improper messaging.
  • Multi-threading issues.
  • Security and performance issues.
  • Reliability issues.

44. What do you understand by Input injection?

In input injection, a malicious application can inject input into the user interface to mimic user interaction through Android’s accessibility APIs. It can lead to denial of service or a full webserver compromise.

45. Is it possible to hack API while testing?

Yes, it is possible to hack API while testing as requests are being sent over the internet which mostly follows HTTP protocol which is a text-based protocol. Hence, it is important to perform security testing of the APIs to ensure safer systems.

46. How do you perform API Load Testing?

API load testing is a type of performance testing that is done to check the application’s capability to perform under various user loads. It is done by simulating many users hitting the API at the same time to identify if the application is capable of handling the load by maintaining the consistency in the response times and not impacting the functionality.

47. What is the test environment of API?

The test environment is an environment where the testing team analyzes the quality of the API. The test environment can include a test database, a localized gateway, a load balancer, and a server.

48. What is the procedure to perform API testing?

The API testing flow basically includes three main steps:

  • Send the request with the necessary input data.
  • Get the response having the output data.
  • verify that the response returned is as expected in the requirement.

49. When writing API document, what must be considered?

  • Use clear language.
  • Include reference documentation.
  • Incorporate documentation into process.
  • Provide quickstart guides.

50. What tools could be used for API testing?

  • Katalon: This is easy-to-use tool that supports REST, SOAP requests, and SSL client certificates. It also enables test import from Swagger, Postman, WADL, and WSDL.
  • JMeter: Apache JMeter is an open-source, 100% Java application that is created for performance testing. This tool enables automatic working with CSV files.
  • REST-assured: REST-assured is an open-source Java domain-specific language that enables testing REST services. It supports POST, GET, PUT, DELETE, PATCH, and HEAD requests.
  • SoapUI: SoapUI is an automated testing tools for Soap and REST APIs. It is an open-source tool that allows to create tests effortlessly with drag and drop, point-and-click.
  • Postman: Postman is an application that is used for API testing. It is a standalone platform that is used to build, test, design, modify, and document APIs.


API testing is a critical part of the software development lifecycle. By testing APIs, you can ensure that they are working correctly and that they are meeting the requirements of the application.

In this article, we have discussed some of the most common API testing interview questions. These questions are designed to test your knowledge of API testing concepts, tools, and techniques. If you are preparing for an API testing interview, it is important to familiarize yourself with these questions. By doing so, you can increase your chances of success in the interview.

API Testing Interview Questions – FAQs

1. What is API testing?

API testing is the process of testing application programming interfaces (APIs). APIs are the intermediary between different software components. They allow different applications to communicate with each other.

2. What are the different types of API testing?

There are many different types of API testing, such as:

  • Functional testing: This type of testing verifies that the API is working as expected and that it is returning the correct results.
  • Security testing: This type of testing verifies that the API is secure and that it is not vulnerable to attacks.
  • Performance testing: This type of testing verifies that the API can handle the expected load and that it is not slowing down the application.
  • Stress testing: This type of testing verifies that the API can handle extreme loads and that it does not crash.
  • Regression testing: This type of testing verifies that changes to the API have not affected its functionality.

3. What are the benefits of API testing?

The benefits of API testing include:

  • Increased confidence in the quality of the API.
  • Reduced risk of bugs and errors in the API.
  • Improved performance and scalability of the API.
  • Increased security of the API.
  • Reduced development and maintenance costs.

4. What are the challenges of API testing?

The challenges of API testing include:

  • The complexity of APIs.
  • The lack of documentation for APIs.
  • The dynamic nature of APIs.
  • The need to test multiple combinations of parameters.
  • The need to test APIs in different environments.

5. What are the tools used for API testing?

There are many different tools available for API testing, such as:

  • Postman
  • SoapUI
  • JMeter
  • SoapTest
  • Apigee
  • Katalon Studio

6. How to automate API testing?

API testing can be automated using a variety of tools and frameworks. Some of the popular tools for automating API testing include:

  • Postman
  • SoapUI
  • JMeter
  • SoapTest
  • Apigee
  • Katalon Studio

7. What are the best practices for API testing?

Some of the best practices for API testing include:

  • Start with a clear understanding of the API.
  • Use a variety of testing techniques.
  • Automate as much testing as possible.
  • Test in different environments.
  • Track and report on test results.

8. How to prepare for an API testing interview?

To prepare for an API testing interview, you should familiarize yourself with the different types of API testing, the tools used for API testing, and the best practices for API testing. You should also practice answering API testing questions.

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