Top 20 API Testing Interview Questions with Answers

API Testing is an essential part of software testing, and it requires specific skills and knowledge to be conducted effectively. API testing can be challenging, and it requires a deep understanding of how an application programming interface (API) works. During the interview process, employers may ask potential candidates about their experience with API testing and their knowledge of API testing concepts, tools, and techniques.

In this article, we will provide the top 20 API testing interview questions with answers to help you prepare for your next API testing job interview.

1. What is API testing, and why is it important?

API testing is the process of testing an API to verify that it works correctly, is reliable, and meets its intended functional and performance requirements. It is important because APIs are the backbone of modern software applications and provide critical functionality that allows different systems to communicate and share data with each other.

2. What is the difference between SOAP and REST APIs?

SOAP (Simple Object Access Protocol) and REST (Representational State Transfer) are two popular types of APIs. SOAP is a protocol that uses XML messages to communicate between systems, while REST is an architectural style that uses HTTP requests to transfer data. REST APIs are simpler and faster than SOAP APIs and are widely used in modern web applications.

3. What is HTTP, and why is it essential in API testing?

HTTP (Hypertext Transfer Protocol) is a protocol used to transmit data over the internet. It is essential in API testing because most APIs use HTTP requests to communicate, and understanding the HTTP protocol is critical to understanding how an API works.

4. What are the types of API testing?

There are several types of API testing, including functional testing, security testing, performance testing, and load testing. Functional testing verifies that the API meets its intended functional requirements, while security testing checks for vulnerabilities that could be exploited by attackers. Performance testing tests the API’s response time and throughput, while load testing checks how the API behaves under high load.

• Functional testing: Testing the functionality of individual APIs.
• Load testing: Testing the performance of APIs under different load conditions.
• Security testing: Testing the security of APIs to ensure that they are protected against attacks.

5. What is API documentation, and why is it important?

API documentation is a set of instructions and guidelines that explain how to use an API. It is essential because it helps developers understand how an API works, how to interact with it, and what the API’s limitations are. Good documentation can also help reduce support costs and increase developer adoption.

6. What is an API endpoint, and how do you test it?

An API endpoint is a specific URL that represents a particular function or resource in the API. To test an API endpoint, you need to send HTTP requests to the endpoint using a tool like Postman or cURL and check the response for correctness.

7. What is an API contract, and how is it tested?

An API contract is a document that defines the rules and expectations of an API. It specifies the inputs and outputs of the API and the expected behavior of the API under different conditions. To test an API contract, you can use tools like Swagger or OpenAPI to validate the API’s compliance with the contract.

8. What is API versioning, and why is it important?

API versioning is the practice of maintaining different versions of an API to support backward compatibility and allow for changes to the API without breaking existing clients. It is important because it allows developers to add new features to an API without disrupting existing clients, which can save time and reduce costs.

9. What is API mocking, and how is it useful in testing?

API mocking is the process of creating a fake API for testing purposes. It is useful in testing because it allows developers to test their code without having to wait for the real API to be available. It can also help isolate bugs and reduce dependencies on external systems.

10. What are the common tools used for API testing?

Some common tools used for API testing are:

• Postman
• SoapUI
• JMeter
• Assertible
• RestAssured

11. What is REST API?

REST (Representational State Transfer) API is a type of API architecture that uses HTTP protocol for data communication. REST APIs typically use HTTP methods like GET, POST, PUT, and DELETE to perform CRUD (Create, Read, Update, Delete) operations on data.

12. What is API throttling?

API throttling is a process of limiting the number of requests that can be made to an API within a specified time period. It helps to manage API traffic and prevent overload or denial of service attacks.

13. What is API gateway?

API gateway is a layer that sits between the client application and the backend API services. It typically handles tasks like request routing, load balancing, authentication, and caching to improve the performance and security of API services.

14. What is API version control?

API version control is a process of managing different versions of an API to ensure backward compatibility and avoid breaking changes. It typically involves using version control tools like Git to manage changes to API code and documentation.

15. What are the best practices for API testing?

Some best practices for API testing are:

• Use automation tools to speed up testing and improve accuracy.
• Test both positive and negative scenarios to ensure complete coverage.
• Test API security and performance under different load conditions.
• Document test cases and results to facilitate collaboration

16. What is the difference between HTTP and HTTPS?

HTTP (Hypertext Transfer Protocol) is a protocol that is used for communication between web servers and clients, while HTTPS (Hypertext Transfer Protocol Secure) is a secure version of HTTP that uses SSL/TLS encryption to protect the data.

17. What is the difference between PUT and POST methods in REST API?

PUT and POST are two different methods used in REST API to update or create a resource, respectively. PUT is used to update an existing resource, while POST is used to create a new resource.

18. What is a web API?

A web API (Application Programming Interface) is a set of protocols, routines, and tools that allow different software applications to communicate with each other over the web.

19. What are the key differences between API and Web Service

APIs and web services are both used for communication between software systems, but there are some key differences between the two. Here are some of the main distinctions:

1. Architecture: APIs are often designed as a set of protocols and tools for building software applications, while web services are built using a standardized architecture (such as SOAP or REST) that provides a well-defined set of rules for how software systems should communicate.
2. Transport protocol: APIs can use different transport protocols for communication, such as HTTP, MQTT, and AMQP. Web services, on the other hand, typically use HTTP or HTTPS as the transport protocol.
3. Data formats: APIs can support a variety of data formats, such as JSON, XML, and CSV. Web services, on the other hand, typically use XML as the data format.
4. Communication type: APIs are often used for synchronous communication, where one system sends a request and waits for a response from the other system. Web services can be used for both synchronous and asynchronous communication.
5. Language and platform independence: Web services are designed to be language and platform-independent, meaning that they can be used by systems built with different programming languages and running on different platforms. APIs can also be language and platform-independent, but this is not always the case.
6. Security: Web services often have built-in security features, such as authentication and encryption, that are based on the standardized architecture they use. APIs may or may not have built-in security features, depending on how they are designed and implemented.
7. Level of abstraction: APIs can be more abstract and higher-level than web services, as they are often designed to provide a simpler and more intuitive interface to software systems. Web services, on the other hand, can be more low-level and granular, as they are designed to expose specific functionality of a software system to other systems.

20. What is the procedure to start API testing

The procedure for API testing involves understanding the API’s functionality, identifying test scenarios, creating test cases, executing the tests, verifying responses, reporting defects, retesting and regression testing, and performing performance testing.