10 API testing Interview Questions and Answers for qa engineers

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1. How do you ensure the API endpoints are designed properly and conform to industry standards?

When it comes to ensuring API endpoints are designed properly and conform to industry standards, there are a few key steps I take:

  1. First, I review industry best practices: Before designing or testing any API, it's important to have an understanding of current industry standards and protocols. I regularly review resources like the OpenAPI Specification and industry-specific guidelines, such as HL7's FHIR standard for healthcare APIs.
  2. Second, I work closely with development teams: Collaborating with developers and architects early in the process helps ensure that APIs are designed in a way that aligns with project requirements and best practices. I ensure APIs are properly documented and include all necessary information for consumption by other teams.
  3. Third, I use automated testing tools: Automated testing helps identify issues and ensure that APIs adhere to industry standards. I use tools like Postman, SoapUI, and Swagger for testing and monitoring the performance of the endpoints.
  4. Fourth, I use synthetic monitoring to track API performance: To ensure that our APIs are designed optimally, I use synthetic monitoring to track KPIs, such as response times and availability. Using these metrics, we can ensure API endpoints meet industry standards and are performing as expected.

By keeping a close eye on industry standards and working closely with development teams, using automated testing tools, and leveraging synthetic monitoring to track performance, I'm able to ensure that API endpoints are designed properly and meet all required standards. This approach has consistently led to successful API integrations and a high level of developer satisfaction.

2. What is your approach to API testing?

As an API tester, my approach is to ensure that the API is tested thoroughly and accurately. My process involves the following:

  1. Understanding the API's functionality and purpose
  2. Creating a list of all possible inputs and expected outputs
  3. Designing test cases that cover all possible scenarios
  4. Executing the test cases and verifying the results against the expected outputs
  5. Using automation tools to run repeatable tests and reducing human error
  6. Performing load testing to determine the API’s performance under heavy usage
  7. Checking for security vulnerabilities and ensuring data is protected
  8. Using monitoring tools to continually test the API and ensuring any issues are identified and resolved promptly

Through this approach, I have consistently been able to identify and resolve issues early on in the development process, which has saved my team time and resources. In my most recent role, implementing this approach led to a 25% reduction in bug reports from production and a 50% decrease in the time it took to discover and resolve issues.

3. How do you handle authentication, authorization, and access control for APIs?

As an API tester, handling authentication, authorization, and access control are critical to ensure the security of the API being tested. I follow a comprehensive approach to achieve this.

  1. Authentication: To ensure that users are who they claim to be, I implement two-factor authentication, which involves a password and a time-based one-time password (TOTP). TOTP ensures that even if an attacker has stolen the password, they still can't access the API. I also use OAuth2.0, which allows users to log in using their social media or email accounts.
  2. Authorization: I ensure that authorized users only have access to the resources they need. For instance, if a user has access to the employee database, they should not be able to modify payroll data. To implement this, I use role-based access control, where I assign roles to users, and each role has access to specific resources. I also make use of attribute-based access control, where I control access based on the user's attributes, such as department or location.
  3. Access Control: To restrict access to particular functionalities, I ensure that access control is in place. I use API gateways and proxies to implement this, where I restrict access based on IP addresses. I also use multi-factor authentication to ensure that only authorized users can access certain functionalities.

To ensure that authentication, authorization, and access control are working correctly, I use automated testing tools. I use tools such as JMeter and SoapUI to test the API's security features. I also conduct penetration testing to identify vulnerabilities in the API and provide detailed reports to the development team.

Through this comprehensive approach, I can ensure that the APIs I test are secure, and unauthorized access is prevented.

4. What is the process you follow to verify the data being returned by an API?

When it comes to verifying the data being returned by an API, my process involves the following steps:

  1. Understand the API documentation: I always ensure I have fully understood the API documentation and know exactly what data is supposed to be returned by the API.

  2. Test the API: I then test the API to verify that the data being returned matches what is documented.

  3. Check response status: I check the response status code to ensure that the API was successful in returning the data.

  4. Validate data: I validate the data returned to ensure it is in the expected format, such as JSON or XML.

  5. Verify data accuracy: I verify the accuracy of the data by cross-referencing it with other data sources, if applicable.

  6. Performance testing: Lastly, I perform load testing and stress testing to ensure that the API can handle a large amount of requests and returns data efficiently.

As a result of following this process, I have been able to identify and resolve issues with APIs before they are released into the production environment. In one instance, I discovered that an API was returning inaccurate data due to two fields being swapped in the API code. Upon identifying the issue, I worked with the development team to correct the issue, and ultimately the API was released without errors.

5. How do you handle testing APIs that have frequent updates?

Handling testing APIs that have frequent updates can be challenging, but there are a few strategies I would utilize:

  1. Communication: Consistent communication with the development team is critical to staying up-to-date on the latest API changes. This includes attending daily stand-up meetings and asking for clarification on any changes that may affect our testing.
  2. Test Automation: Automation testing tools such as Postman, can help streamline the testing process, especially when dealing with frequent API updates. Test suites can be created ahead of time and automatically executed whenever updates occur, reducing the manual testing time.
  3. Regression Testing: Regression testing ensures that changes or updates to APIs don't negatively impact the existing functionality. Regression testing would make sure that the endpoint’s functionality remains consistent, in the sense that the output itself hasn't changed.
  4. Load Testing: Frequent updates to an API could cause performance issues. By performing load testing, we can ensure that the API performs optimally, even after frequent updates. For example, I worked on a project where we increased the max limit of requests per minute with updates. Load testing allowed us to ensure that the endpoints were still processing requests at the same speed as before.
  5. Collaboration: As a remote worker, I know that collaboration is essential when handling frequent updates to APIs. This means working closely with the development team, product owners, and possibly other testers to ensure that every detail is updated and tested correctly.

By using these strategies, I'm confident that I can effectively handle testing APIs that have frequent updates. At my previous job, we implemented these strategies and were able to increase our test coverage by 50%, which helped us catch more bugs earlier in the development process.

6. What tools and techniques do you use to monitor and test API performance?

When it comes to monitoring and testing API performance, there are a variety of tools and techniques that I use to ensure optimal functionality. One of my go-to tools is New Relic, which allows me to monitor API performance in real-time and quickly identify any potential issues.

  1. Load testing is another crucial technique that I use to ensure API performance is up to par. I utilize tools like JMeter to simulate traffic to the API and test how it handles different levels of load. For example, I recently load tested an API for a social media platform and found that it was able to handle up to 10,000 requests per second without any major issues.
  2. API Fortress is another tool that I have found valuable for monitoring performance. This tool allows me to set up automated tests that run continuously to ensure APIs are functioning as expected. With API Fortress, I have been able to catch and fix issues before they impact users.
  3. To monitor API logs and identify issues, I use tools like Kibana and Logstash. These tools allow me to search through logs and pinpoint specific errors or patterns of behavior that might be causing performance issues. Recently, I used Kibana and Logstash to identify and fix a memory leak issue that was causing slow response times for an API.
  4. I also make use of various API monitoring services such as Pingdom, UptimeRobot and Site24x7. These services provide continuous monitoring from different locations around the world and alert me if the API is down or experiencing issues. Recently, I configured UptimeRobot to monitor an API for a weather app and was able to receive alerts before any of the users noticed issues.
  5. Finally, for more specific testing needs, I make use of Fiddler and Postman. These tools allow me to send and receive sample requests and responses and fine-tune the API as needed. For example, I recently used Postman to test an API for a finance app and was able to validate the accuracy of the data returned.

By utilizing a combination of these techniques and tools, I have been able to ensure top-notch API performance for a variety of projects. In a previous role, the team and I increased API response times by 20% after implementing load testing and log monitoring tools.

7. How do you handle testing APIs that are dependent on other APIs or third-party services?

Handling APIs that depend on other APIs or third-party services requires a strategic approach. First, I ensure that I thoroughly understand the dependencies involved and how the APIs interact with each other.

  1. Next, I create an end-to-end test plan that covers all of the API dependencies involved. This plan includes identifying potential failure points and designing tests that verify the functionality of each API component in isolation.
  2. After establishing test conditions, I begin testing with the API layer closest to the system under test, verifying that all expectations are met.
  3. Once I am comfortable that the dependent API is reliable, I move to testing the system's API layer. This ensures that if any errors arise, I can identify and address the problem before it impacts end-users.
  4. When testing third-party services, I use mock services to create test environments that simulate the necessary dependency endpoints. These mock endpoints allow me to test more easily and detect any issues before deploying to production.
  5. Finally, I use monitoring tools to continuously monitor API performance to detect and rectify any faults. This provides me with data on the API's usage and a clear picture of its reliability.

This approach has proven effective as it has reduced API downtime by 30% and enhanced overall system performance by 20%.

8. What is your understanding of API versioning and how do you approach testing different versions of an API?

API versioning is the process of managing different versions of an API, which allows developers to maintain backward compatibility with existing applications while introducing new features and updates. In my previous role as a QA Engineer at XYZ, we tested multiple versions of our API by implementing the following approach:

  1. First, we identified the different endpoints and parameters that were modified or added in the new version. We also documented any deprecated endpoints or parameters that were removed.
  2. Next, we created a test plan that focused on testing the modified endpoints and parameters in the new version, as well as any backward compatibility issues with the previous version.
  3. We also ensured that all existing test cases were still passing in the new version before releasing it to the development team for further testing.
  4. To verify backward compatibility, we ran regression tests on the previous version to make sure it still worked with applications that were built on it.
  5. We also used automated tools to monitor the performance and response time of the API in both versions, and compared the results to make sure there were no negative impacts on the end user experience.

Using this approach, we were able to successfully release multiple versions of our API without any major issues or negative impacts on our users. In fact, we were able to improve the performance and functionality of our API with each new release, which resulted in a data-driven increase in user engagement and satisfaction.

9. Can you explain the importance of documentation in API testing, and how do you ensure API documentation is up-to-date?

Documentation is a crucial part of API testing because it provides a clear understanding of the API's purpose, functionality, and usage. Comprehensive and up-to-date documentation can save time and prevent misunderstandings between teams when developing and testing an API. Proper documentation can also improve the overall quality of the API by helping developers and testers identify and fix issues quicker.

  1. One way to ensure API documentation is up-to-date is to make it a part of the build process. This means that the documentation should be reviewed and updated every time there is a change in the API code.
  2. Using automated documentation tools is another way to keep API documentation up-to-date. These tools allow for real-time updating of the documentation every time there's a change in the API code. Swagger Editor is an example of an automated documentation tool that can help ensure updated documentation.
  3. It is also important to make the documentation accessible and easy to read. Using detailed examples, concise language, and organized structure in API documentation can make it easier for both developers and non-developers to understand and utilize the API.
  4. Finally, it's important to get feedback on the documentation from developers, testers, and other stakeholders. This can help identify any areas that need improvement or clarification.

By ensuring that API documentation is up-to-date, comprehensive, and accessible, testing teams can proactively identify and address issues and ultimately save time and effort during the testing process, leading to better API performance and a more seamless development process.

10. How would you handle a scenario where an API is not returning the expected results or is returning incorrect data?

Handling Incorrect Data Returned by API

API testing involves ensuring that all API functions are working correctly, including verifying that the API is returning the expected data. In the scenario where an API is not returning the expected results or is returning incorrect data, the following steps can be taken:

  1. Check the API documentation to ensure that the expected results match what the API is supposed to return.
  2. Verify that the testing environment has not changed since the last successful testing run, such as a change in authentication requirements or server settings.
  3. Test the endpoint with different inputs, including edge cases, to identify patterns or areas that may be causing the incorrect data to be returned. During this process, any issues found should be documented and communicated to the development team as soon as possible.
  4. Inspect the server logs to identify any errors or exceptions that may be impacting the API's performance.
  5. Utilize debugging tools such as Postman, Swagger or Fiddler to see if an issue can be found with the request being sent or the response being received.
  6. Collaborate with the development team to identify any potential issues with the backend code, and work towards making necessary updates to fix the issues.

In a previous role, I worked on an API project that returned inconsistent results when users attempted to filter data. By using Postman to test different inputs and review the returned responses, I was able to identify an issue with the backend code. By collaborating with the development team, we discovered that the filter function was not completely developed and was returning an incorrect result. We were able to fix the issue and reduce the inconsistency between our data filtering and the API's returned results. By consistently testing the API and communication with the development team, it is possible to ensure that API data is accurate, precise and meets the expectations of the end-users.


Congratulations on familiarizing yourself with 10 essential API testing interview questions and answers for 2023. Remember, to make yourself stand out from the competition, you need to write a captivating cover letter that highlights your experience and strengths as a QA engineer. Don't forget to write a

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