API testing interview questions and answers for QA Engineers

Summary

In this video, Sergio Kenko, a senior QA engineer and manager, addresses popular API testing interview questions from the past three months. He explains API testing fundamentals, covering tools like Postman, the differences between API and UI testing, environmental variables, tokens, HTTP response status codes, and test automation. He also touches on performance testing tools like k6 and JMeter and advises on how to honestly handle questions about tools you haven't used professionally. Sergio offers practical tips for aspiring QA engineers, including resources for further learning and a breakdown of common interview scenarios.

Takeaways

• 📚 **Popular API Testing Questions**: The video discusses 10 commonly asked API testing interview questions based on recent interviews from students who secured job offers.
• 🧑‍💻 **How to Test API**: API testing involves sending requests using tools like Postman or Insomnia, verifying response status codes, body, speed, and handling positive/negative cases. Authentication and authorization should also be tested.
• 🤔 **Why API Testing Is Important**: API testing verifies business logic and server-side data processing, which is quicker than UI testing and useful for systems without UIs, like weather data services.
• 🔍 **API Testing vs UI Testing**: API testing focuses on reliability, performance, and security without involving the user interface, while UI testing verifies graphical elements and user interactions like clicks and inputs.
• 🚀 **Automating API with Postman**: Postman allows easy API automation using snippets to verify status codes and autofill code, making it beginner-friendly for those without much experience.
• 🔑 **Environmental Variables & Tokens**: Environmental variables store values like base URLs or API keys, making it easy to switch between environments. Tokens act as temporary identification strings during sessions to authenticate users.
• 🧾 **Common HTTP Status Codes**: The speaker highlights frequently encountered codes like 200 (OK), 201 (Created), 400 (Bad Request), 401 (Unauthorized), 403 (Forbidden), 404 (Not Found), and 500 (Server Error).
• 🛠 **Automating API Calls**: When creating an API test automation framework, the key steps are gathering requirements, selecting appropriate tools, and setting up the framework from scratch, using libraries like Axios for API testing.
• 🧪 **Testing POST Requests**: Testing POST requests requires sending payloads, verifying status codes (e.g., 201), checking response bodies, and ensuring functionality with both positive and negative test cases.
• 📊 **API Performance & Metrics**: The key performance metrics in API testing are response time, error rates, and test coverage, which help track server behavior and the extent of API test automation coverage.

Q & A

What is API testing, and why is it needed?

API testing is done to verify the business logic, data processing, and reliability of the server-side components independent of the user interface. It is crucial because it allows for faster testing than UI testing, and in some cases, there might not even be a UI, such as with companies that provide data via APIs.

How do you test an API, and what tools can be used?

To test an API, you send requests using API clients like Postman or Insomnia. You verify the response by checking status codes, response body, speed, and performance. Both positive and negative test cases should be considered, along with testing authentication and authorization.

What is the difference between API testing and UI testing?

API testing focuses on reliability, performance, functionality, and security of APIs, while UI testing is concerned with the graphical user interface and user experience. API testing verifies server-side operations, while UI testing is done from the user's perspective.

How can you automate API testing using Postman?

Postman allows you to automate API testing by writing scripts in its 'Tests' section. You can use Postman’s Snippets to easily generate automated tests, such as verifying status codes. Postman helps automate testing of requests and responses, making it easier to manage API tests.

What are environmental variables and tokens in API testing?

Environmental variables store values like base URLs, API keys, or tokens, allowing easy switching between environments (e.g., dev to QA or production). A token is a form of identification issued by the server after login, used in subsequent requests to authenticate the user without requiring credentials again.

What are some common HTTP response status codes you should know for API testing?

Common HTTP status codes include: 200 (OK), 201 (Created), 400 (Bad Request), 401 (Unauthorized), 403 (Forbidden), 404 (Not Found), and 500 (Server Error). These codes indicate the success or failure of an API request and provide insight into server-side issues.

How would you automate API calls and create an API test automation framework?

To automate API calls, you would first gather requirements and choose a tool like Axios or any other API testing client. After selecting the right tool, you would proceed by setting up the framework to include test scripts for requests, responses, and validations.

How do you test a POST request?

To test a POST request, gather requirements, include expected payload and headers, and send the request. Verify the status code, response body, and response time. Perform both positive and negative test cases, checking for error handling, authorization, and data validation.

What kind of metrics are important for API testing?

Key API testing metrics include response time (how long the server takes to respond), error rate (percentage of failed requests), and test coverage (how many APIs are covered by automated tests). These metrics help assess the performance and reliability of the APIs.

Have you ever done API performance testing, and how would you approach it?

While the speaker has not done professional API performance testing, tools like K6 and JMeter can be used for performance, load, and stress testing. These tools simulate different levels of traffic to measure how well the API performs under various conditions.