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API Testing Interview Questions

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Most Frequently Asked API Testing Interview Questions

  1. What is an API?
  2. What are web services?
  3. What are the main differences between an API and a web service?
  4. What is API testing?
  5. What are the types of APIs you have worked with?
  6. What are the limits of API usage?
  7. Differences between REST and SOAP APIs.
  8. What are the main components you consider when testing an API?
  9. Name some common protocols used in API testing.
  10. Common HTTP methods used in API testing.

When applying for a position as a tester, there are several phases involved, with the interview being one of the most important. Keeping this in mind, we’ve compiled a list of the top 52 most frequently asked interview questions, along with solutions.

All of the API testing interview questions listed below have been compiled by test professionals to help you in your preparation. These questions have already been asked in interview rounds by the top companies. Going through these will give you enough confidence and give you an idea of what to expect in the interview.

Basic API Testing Interview Questions

1. What is an API?

An API acts as an intermediary between different software systems, enabling them to communicate and share data. These are a collection of functions that are executable by other functions of the software application.

2. What are web services?

Web services are a type of API specifically designed to communicate over the web. They utilize standard web protocols like HTTPS and are accessible over the internet, enabling interoperability between various systems and platforms.

3. What are the main differences between an API and a Web Service?

APIs are a broader concept defining how different software components interact, while web services are a type of API specifically designed for web-based communication using protocols like HTTP.

4. What is API Testing?

API testing involves verifying the functionality, reliability, performance, and security of APIs. It focuses on testing the individual parts of the software that interact with other software components, ensuring they work as expected.

5. What are the types of APIs you have worked with?

APIs come in various types, including RESTful APIs, SOAP APIs, GraphQL APIs, and more. Each type has its protocols, standards, and usage scenarios, and experience with these demonstrates familiarity with diverse API ecosystems.

6. What are the limits of API usage?

API usage can be restricted by factors like rate limiting (a maximum number of requests allowed within a certain time frame), access permissions (user-based or role-based), data usage limits, and server load capacities.

7. Differences between REST and SOAP APIs.

REST APIs (Representational State Transfer) use simpler and more lightweight protocols like HTTP, making them more flexible and scalable. SOAP APIs (Simple Object Access Protocol) are more rigid, relying on XML for communication and demanding more bandwidth and overhead.

8. What are the main components you consider when testing an API?

Testing an API involves examining various elements, such as endpoint functionality, request methods, parameters, response codes, data formats (like JSON or XML), error handling, and security measures.

9. Name some common protocols used in API testing.

Commonly used protocols in API testing include HTTPS, REST, SOAP, GraphQL, and TCP/IP, each catering to different types of API communication needs.

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10. Common HTTP methods used in API testing.

API testing frequently involves utilizing HTTP methods like GET (retrieve data), POST (submit data), PUT (update data), DELETE (remove data), PATCH (partially update), OPTIONS (retrieve supported operations), and HEAD (retrieve headers only).

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11. How do we validate API responses?

API response validation includes checking response codes (like 200 for success), ensuring data accuracy against expected results, validating schema compliance, inspecting headers, and monitoring response times.

12. Which tool is used for API testing?

Various tools facilitate API testing, such as Postman, SoapUI, RestAssured, Insomnia, and others, providing interfaces to create, automate, and analyze API tests.

13. Advantages of API testing

API testing accelerates testing cycles, detects issues early, allows for the reuse of test cases, simplifies integration testing, and enhances overall test coverage, ensuring robust software functionality.

14. Disadvantages of API testing

Challenges in API testing include complexity in testing specific functionalities, reliance on accurate documentation, difficulties in testing UI-related aspects, and initial setup time for test automation.

15. What is an API endpoint?

An API endpoint refers to a specific URL or URI (Uniform Resource Identifier) where an API can be accessed. Each endpoint represents a unique interaction point in the API.

16. What is an API testing framework?

An API testing framework provides a structured approach and tools for designing, implementing, and executing API tests, streamlining the testing process for better efficiency and repeatability.

17. What is the use of authentication in API testing?

Authentication in API testing ensures that only authorized users or systems have access to the API, maintaining security and preventing unauthorized usage or data breaches.

18. Common authentication methods used in API testing

Authentication methods in API testing commonly include OAuth, Basic Authentication, API keys, token-based authentication, and JWT (JSON Web Tokens), each offering various levels of security and access control.

19. What is the API documentation?

API documentation is a detailed guide that outlines how to use an API. It includes information on endpoints, parameters, request/response formats, authentication methods, and usage examples, aiding developers in effectively utilizing the API.

20. Which is the most used template for API documentation?

The OpenAPI Specification (formerly known as Swagger) is widely adopted as a standard template for API documentation. It provides a structured format for describing RESTful APIs, making them easily understandable and accessible for developers.

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Intermediate API Testing Interview Questions

21. What are the differences between API testing and unit testing?

API testing focuses on testing APIs as a whole, verifying functionality, performance, security, and integration. Whereas, unit testing specifically targets individual components or units of code. Unit tests examine the functionality of small parts of the code in isolation to ensure they work as expected.

22. Explain the significance of the HTTP status code in API testing.

The significance of HTTP status codes in API testing is that they indicate the outcome of a request made to an API. They range from 1xx to 5xx, with each code representing a specific response. For instance, a 200 status code signifies success, 404 indicates a not-found error, and 500 denotes an internal server error. Understanding these codes helps testers identify issues and understand the state of the API.

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

API testing encounters various error responses such as Bad Request, Not Found, Forbidden, Unauthorized, Bad Gateway, Gateway Timeout, Too Many Requests, etc. These responses signify issues on either the client or server side, helping testers diagnose and rectify problems efficiently.

24. Write about parameterization in API testing.

Parameterization involves using different sets of inputs in API tests to validate the behavior of an API with varying data. It allows testing multiple scenarios by substituting parameters, ensuring comprehensive test coverage and validation of different data combinations.

25. Difference between stateful and stateless API.

Stateful APIs maintain the state or context of previous interactions, relying on stored information from previous requests. On the other hand, stateless APIs treat each request as an independent transaction, not storing any client state between requests.

26. How do you test authentication and authorization in APIs?

Testing authentication involves verifying that valid credentials grant access to protected API resources. Authorization testing ensures that authenticated users have appropriate permissions to access specific functionalities or resources within the API.

27. What is mocking in API, and what’s its benefit?

Mocking in API testing involves simulating parts of the API that aren’t easily accessible or controllable during testing, like external services or complex dependencies. 

The benefit of mocking an API is that it provides controlled responses, allowing testers to isolate and test specific functionalities without relying on external factors, leading to faster, more reliable tests.

28. How do you unit test asynchronous APIs?

Testing asynchronous APIs involves techniques like using callbacks, promises, or async/await functions to handle and verify asynchronous behaviors.

Unit tests for asynchronous APIs focus on validating the correct execution flow, handling errors, and ensuring expected outcomes for asynchronous operations.

29. What are the best approaches to testing asynchronous API calls?

Effective testing of asynchronous APIs involves strategies like using timeouts, employing callbacks or promises for assertions, using stubs or mocks to control asynchronous behavior, and verifying correct error handling and async data processing.

30. What is API security testing?

API security testing involves evaluating the security aspects of APIs to identify potential vulnerabilities and threats. It includes testing for authorization flaws, authentication issues, data exposure risks, injection attacks, and other security concerns that could compromise the integrity and confidentiality of data exchanged through APIs.

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

Ensuring API security in tests involves several steps:

  • Identifying potential vulnerabilities through comprehensive testing
  • Ensuring secure development practices in the API codebase
  • Employing specialized tools to detect security flaws and vulnerabilities
  • Testing user access control and ensuring only authorized access to resources
  • Simulating attacks to assess the API’s resilience against real-world threats

32. Does the API tester need coding knowledge to perform API testing?

No, knowing the coding language isn’t always mandatory for API testing, but it’s quite beneficial. 

Understanding programming languages (like JavaScript, Python, etc.) can help in scripting tests, creating automated test suites, and handling complex scenarios efficiently. It allows testers to manipulate requests, validate responses, and automate repetitive tasks effectively.

33. How is automation testing done for asynchronous systems?

Automation testing for asynchronous systems involves:

  • Using specialized testing frameworks and libraries that support asynchronous operations
  • Employing callbacks, promises, or async/await functions to handle asynchronous behavior
  • Implementing timeouts and handling asynchronous data flows to ensure the expected outcomes
  • Creating test cases that cater to the specific behavior of asynchronous operations

34. Why is API testing considered the most suitable form for automation testing?

API testing is highly suitable for automation due to several factors:

  • APIs offer a defined interface and standardized protocols, making automation easier.
  • APIs have consistent behavior, allowing for the creation of reliable and repeatable test cases.
  • API testing is less dependent on UI changes, making tests more stable and easier to maintain.
  • Automation in API testing accelerates the testing process, ensuring faster feedback loops and quicker identification of issues.

35. What is latency in API testing?

Latency in API testing refers to the delay or time taken for a request to be sent from the client to the server and for the corresponding response to be received. It measures the time interval between the initiation of the request and the receipt of the response, impacting the performance and responsiveness of the API. 

Advanced API Testing Interview Questions

36. Explain the concept of fuzz testing in API security and how it contributes to identifying security vulnerabilities in APIs.

Fuzz testing involves systematically injecting an API with invalid, unexpected, or random data to identify potential vulnerabilities. By intentionally providing inputs that deviate from the expected, fuzz testing aims to expose weaknesses in input validation, error handling, or boundary conditions. 

This method helps discover security loopholes such as buffer overflows or injection attacks, enhancing the API’s robustness against malicious exploits.

37. What is API contract testing, and why is it important?

API contract testing is a validation process ensuring that an API complies with its predefined contract or specifications, typically established through documentation or contracts like OpenAPI (formerly Swagger). 

The importance of API contract testing lies in guaranteeing that the API functions as intended, enabling seamless integration between different services or clients. By verifying endpoints, data formats, and responses against the contract, this testing ensures consistency and reduces the risk of integration failures.

38. How do you manage versioning challenges in APIs?

For managing versioning challenges, some strategies involve using semantic versioning (Major.Minor.Patch), clearly documenting changes and deprecations, employing URL or header-based versioning, and providing backward compatibility for a certain transition period. This ensures existing users can smoothly migrate to newer versions while minimizing disruptions.

39. What are some techniques for handling API timeouts in tests?

To manage timeouts effectively during testing, there are various techniques, including:

  • Setting appropriate timeout thresholds based on expected response times
  • Implementing retries with increasing intervals to handle transient failures
  • Utilizing circuit breakers to prevent overwhelming the API
  • Simulating diverse network conditions to ensure robustness under varying circumstances

40. Explain HATEOAS in RESTful APIs and its impact.

Hypermedia as the Engine of Application State (HATEOAS) is a constraint in REST architecture that allows clients to interact with the API entirely through hypermedia links within the responses. 

This enables clients to dynamically navigate the API’s functionalities by following hyperlinks provided in each response, reducing the dependency on fixed, out-of-band information and enhancing the discoverability and flexibility of the API.

41. What are the strategies for API testing in microservices?

The strategies for API testing in microservices involve individual microservice testing in isolation, contract testing to ensure compatibility between services, chaos engineering to test resilience against failures, security testing at each service boundary, and end-to-end testing to validate the entire microservice architecture.

42. What is API performance testing?

API performance testing is the evaluation of an API’s responsiveness, scalability, and reliability under various load conditions. Performance testing involves measuring response times, throughput, concurrent user handling, and resource utilization to ensure the API meets defined performance benchmarks.

43. How do you measure the performance of APIs in your tests?

For measuring API performance in tests, several metrics such as average response time, throughput (requests per second), error rates, latency distribution (percentiles), and resource consumption (CPU, memory) are essential. 

Monitoring tools like JMeter, Gatling, or New Relic assist in capturing and analyzing these metrics during performance testing.

44. How do you manually test WebSocket APIs?

Manually testing WebSocket APIs involves using WebSocket clients, browser extensions, or specialized tools to send and receive messages, validate message formats, simulate different scenarios, and observe how the API responds to various inputs, ensuring its functionality and responsiveness.

45. How do you use JSON schema validation for API design?

JSON schema defines the structure, data types, and constraints of JSON payloads exchanged through APIs. Integrating JSON schema validation in API design ensures that the transmitted data adheres to the defined schema, reducing errors, enhancing interoperability, and facilitating consistent data validation across different services or clients interacting with the API.

46. What are the different HTTP return codes? Explain 3 codes that you use frequently.

HTTP return codes convey the status of an HTTP request. Some of the common codes include:

  • 200 (OK): Successful request
  • 404 (Not Found): Resource not found
  • 500 (Internal Server Error): Server-side error

47. What does 1xx,2xx,3xx,4xx,5xx mean as an error code?

HTTP error code ranges:

  • 1xx- Informational
  • 2xx- Success
  • 3xx- Redirection
  • 4xx- Client error
  • 5xx- Server error

48. Is it possible to hack the API while testing?

Yes, testing can inadvertently expose vulnerabilities if not conducted responsibly. Using malicious inputs or manipulating requests during testing might uncover security flaws. It is crucial to test within controlled environments, implement proper authorization, and use sanitized test data to avoid security breaches.

49. What is API performance testing?

API performance testing involves assessing an API’s speed, scalability, and stability under varying load conditions to ensure it meets performance criteria and user expectations, thereby optimizing the user experience.

50. How do you load test and tune performance on API?

Load testing simulates real-world scenarios by subjecting the API to high traffic volumes to evaluate its behavior under stress. Performance tuning involves optimizing resources, code, and configurations based on load test results to improve the API’s performance and scalability.

51. How can API virtualization help in testing?

API virtualization in testing creates a simulated environment where APIs, especially those under development or not readily available, can be tested independently. 

Virtualization allows testing to proceed without dependencies on actual services, facilitating early development and testing stages.

52. What's the difference between API virtualization and mocking?

API virtualization creates a complete simulation of the API’s behavior and responses, enabling testing of complex scenarios.

Mocking provides pre-programmed responses for specific API interactions without replicating the entire functionality, making it suitable for simple or isolated tests without involving the entire API ecosystem.

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About the Author

Technical Manager - DevOps

Akshay Shukla, a Technical Manager in DevOps, is an experienced professional with a rich background in Cloud Computing, Software Development, and Talent Acquisition. Proficient in Azure, Kubernetes, Docker, and more, he excels at designing scalable solutions and automating workflows. He leads his team with unwavering commitment and also has a passion for crafting business content.