Top 30 API Testing Interview Questions

API testing is a software testing type to validate the functionality and correctness of application program interfaces (APIs). It also determines the performance, reliability, and security of APIs. It involves the use of software to send a call to an API under test and get the output. 

If you are planning to enter the field of API testing, you must be prepared for interviews. This article lists some of the most commonly asked API testing interview questions and answers. Whether you are a beginner or a seasoned API tester, our list covers everything. 

So, let us get started!

API Testing Interview Questions

1. What is an API? 

A collection of guidelines and procedures known as an API, or application programming interface, enables various software programs to exchange information and interact with one another. It specifies the techniques and data formats programs may use to request information and share data.

APIs provide for the smooth integration and interoperability of various software systems, allowing them to cooperate and exchange information or functionality. They give programmers a standardized method of gaining access to and using the functions and offerings of a certain piece of software, platform, or operating system.

2. How does an API work? 

An API enables interoperability and communication between various software programs by providing a set of guidelines and protocols. An application makes a request to the API in order to use the features or services offered by another application or system.

After analyzing the request and taking the necessary steps, the API responds with a response containing the desired data or carries out the requested function. The requestor program can easily analyze and handle the answer since it often comes in a structured format like JSON or XML.

3. What is API testing?

API testing evaluates an API’s (application programming interface) dependability, performance, and usability. It entails determining if the API conforms to the requirements and standards stated in its documentation, acts as expected, and appropriately handles different inputs and outputs.

The primary objective of API testing is to look at how different software programs interact at the API level. It ensures the API is operationally sound, produces the desired outcomes, and handles failures correctly.

4. What are the types of API testing?

The following are the common types of API testing –

• Unit Testing: It involves individually testing each function, method, or API component. It ensures that each piece of code works as intended and adheres to the standards. 
• Functional Testing: It examines an API’s endpoints and actions to ensure they work as intended. Input parameters, outputs, expected behaviors, and numerous API calls are all tested.
• Integration Testing: The main goal of integration testing is to examine how various APIs or parts interact. It makes APIs communicate and share data correctly. 
• Load Testing: Through load testing, an API’s performance is assessed under various load scenarios. Many concurrent queries are simulated to evaluate the API’s response times, throughput, and scalability.
• Security Testing: It involves validating the API for security vulnerabilities and ensuring it meets security standards. 
• Usability Testing: It evaluates the user-friendliness and simplicity of the API. It emphasizes elements like API documentation, simplicity of integration, readability of error messages, and general developer experience.
  
  

5. Which protocols are testable with API testing?

The following are the protocols that can be tested with API testing –

• HTTP (HyperText Transfer Protocol) and HTTPS (HyperText Transfer Protocol Secure)
• REST (REpresentational State Transfer)
• SOAP (Simple Object Access Protocol)
• GraphQL
• JMS (Java Message Service)
• UDDI (Universal Description, Discovery, and Integration)
  
  

6. Enlist the architectural styles used for creating an API. 

The common architectural styles used for creating an API are –

• HTTP for client-server communication
• XML/JSON (a formatting language)
• Simple URI (the address for the services)
• Stateless communication
  
  

7. State the advantages of API testing. 

Here are some remarkable advantages of API testing – 

• Early Detection of Issues: API testing enables the early identification of problems and flaws with the functionality of the API. Potential issues may be found and fixed in the early phases of development. 
• Faster Development Cycles: API testing promotes quicker development cycles by giving developers a simple interface to work with. As long as they follow the established API contract, teams are permitted to build several API-reliant components concurrently.
• Increased Test Coverage: API testing extends test coverage, as it supports testing at the service or backend level. Hence, it uncovers issues that UI testing leaves undetected. 
• Performance Optimization: Performance testing for APIs enables the detection of performance bottlenecks, scalability problems, or wasteful resource utilization.
• Cost-Effective and Less Time-Consuming: API testing consumes less time compared to GUI testing. Additionally, it requires less code for an API’s functionality. Hence, it is easier to set up and ensure maximum test coverage. 
  
  

8. What does API testing verify? 

API testing verifies the following – 

• API endpoint behavior and functionality.
• GET, POST, PUT, and DELETE request methods should be handled correctly.
• Validation of input parameters and management of edge situations.
• Response validation and anticipated data format verification.
• Handling errors and response codes.
• Mechanisms for security and authentication.
• Under various load circumstances, performance and scalability.
• Compatibility and integration with other systems or APIs.
• API consistency and stability across upgrades or versions.
  
  

9. What are some common tools used for API testing? 

Here is a list of some common tools used for API testing –

• Postman: It offers an easy-to-use interface for building, sending, and verifying API calls. It provides tools for test automation, request/response analysis, and creating test reports. 
• SoapUI: It is a complete API testing tool that supports both the SOAP and REST protocols. It enables the creation, execution, and analysis of tests for microservices, web services, and APIs. Data-driven testing, assertions, and test reporting are its capabilities.
• Apache JMeter: Originally intended for load testing, JMeter now includes API testing features. It may mimic numerous users interacting simultaneously and assess how well APIs work under various loads.
• Karate: Karate is an open-source application that combines BDD (behavior-driven development) principles, test automation, and API testing. It offers features like data-driven testing, assertions, and reporting and enables the authoring of tests in straightforward, legible language.
• JUnit: It is a popular testing framework for Java applications. It makes it simpler to build and run unit tests and check that particular code pieces are accurate by providing annotations and assertions. 
• TestNG: A testing framework called TestNG for Java applications expands and improves JUnit’s functionality. Data-driven testing, parallel test execution, test dependency management, and support for test configuration are just a few things it offers. TestNG offers a more adaptable and complete testing solution for Java programs.
  
  

10. How is API testing performed? 

Here is how the API testing is performed – 

• Test Planning and Preparation: Identify the API’s standards, specifications, and proper behavior.  Decide on the testing parameters and establish the test goals. Design the test scenarios and gather the required test information.
• Test Environment Set Up: Create the test environment by establishing the API endpoints, test servers, dummy servers, and any necessary test databases or external dependencies.
• Test Case Design: Create test cases that cover different API features, such as boundary conditions, error handling, and positive and negative situations. Indicate for each test case the anticipated inputs, API calls, and desired results.
• Test Execution: Run the defined test cases against the API. This entails making calls to the API endpoints using the proper methods (GET, POST, PUT, DELETE), along with the required input parameters. Record the output for further verification.
• Response Validation: Compare the actual and expected results of test cases. 

Additionally, you may need to subject the API to different types of testing based on the functionality.

11. What is an API test environment? 

A special setup or configuration used only for API testing is known as an API test environment. Developers and testers may use the controlled environment to run various tests on the API without disrupting the production or live environment. 

The standard API test environment duplicates the relevant parts, systems, and dependencies needed to evaluate the API’s functionality, performance, and integration.

12. What is an API framework?

An API framework is a collection of tools, libraries, and standards that offers an organized and standardized strategy for designing, developing, and testing APIs. It is sometimes referred to as a testing framework or development framework. It provides a framework and a collection of reusable components to make API creation and testing tasks easier and quicker.

13. How does API testing differ from unit testing? 

The below table explains the differences between API testing and unit testing – 

14. What is the limit of API usage?

The limit of API usage is generally imposed by the API provider in order to guarantee fair usage, preserve system performance, deter abuse, and safeguard the API infrastructure. According to the API provider and the API’s features, different limits are applied to the usage of API.

15. What is the difference between API testing and UI testing? 

API testing entails testing the application’s API (Application Programming Interface) independently from a graphical user interface. Its main objective is to verify the APIs’ functionality, dependability, security, and performance that the application exposes. 

Contrarily, UI testing, commonly referred to as GUI (Graphical User Interface) testing, focuses on evaluating the user interface of the application. It includes evaluating the application’s visual components, user interactions, and general user experience.

16. What types of bugs does API testing uncover?

The following are the bugs API testing can uncover – 

• Incorrect or unexpected API responses
• Inconsistent or invalid data formats (e.g., JSON or XML)
• Missing or incorrect error handling and error messages
• Authentication or authorization issues
• API endpoint or parameter validation failures
• Poor performance or scalability under different loads
• Inadequate handling of edge cases or boundary conditions
• Incompatibility or incorrect behavior with different API versions
• Integration issues with third-party APIs or systems
• Security vulnerabilities, such as injection attacks or improper access controls
  
  

17. What do you understand by API documentation?

API documentation refers to the extensive collection of materials and data that details an API’s features, applications, and technical standards. It acts as a reference manual for developers, giving them the information they need to comprehend and properly use the API in their software applications.

18. What should you consider while writing API documentation? 

The following factors you must consider while writing API documentation – 

• The purpose and scope of API documentation.
• Determine the target audience and their level of technicality. 
• Comprehensively document API endpoints, methods, and parameters. 
• Throw light on examples of API requests and responses. 
• Explain error handling and status codes.
• Describe data formats and conventions used in the API.
• Provide the guidelines, test scenarios, and best practices for testing the API. 
• Include a glossary section for technical terms.
• Use clear and concise language with appealing visuals. 
  
  

19. What is SOAP? 

The SOAP (Simple Object Access Protocol) protocol is used in web services communication to exchange structured data. It is a messaging protocol that enables online communication between programs written in various programming languages and operating systems.

20. What is REST API? 

An architectural design approach called REST (Representational State Transfer) API is used to create networked applications and services that interact with one another online. It depends on a set of guidelines and restrictions that encourage scalability, clarity, and interoperability.

21. What do you mean by resource in REST? 

Any content, such as a text file, image, video, audio, or any other dynamic information, is considered a resource in the REST architecture. Each resource has a unique URI/global ID used for identification. 

22. What different HTTP methods does REST support? 

REST supports the following HTTP methods – 

• GET – A method for obtaining a single resource or a group of resources from a server. 
• POST – A method for adding new resources to a server. It sends information to the server for processing, which might lead to the creation of a new resource.
• PUT – A method for updating an already-existing server resource. The new information sent in the request completely replaces the original resource.
• DELETE – This command is used to remove or delete a specified resource. 
• HEAD – It obtains the response’s headers rather than its entire content. It is frequently used to examine a resource’s information or status.
• OPTIONS – Returns the allowed methods, headers, and other server capabilities that are supported for a certain resource. It aids clients in comprehending the capabilities that the API has to provide.
  
  

23. State the difference between the PUT and POST methods. 

The POST method is used to create a new resource by sending data to a server. It is non-idempotent. This means each POST request will create a new resource on the server. Additionally, if you send the same POST request multiple times, the method will generate multiple resources with different identifiers. 

Conversely, the PUT method is used to modify an existing resource on the server by replacing it with new data provided in the request. It is idempotent, meaning sending the same PUT request multiple times will have the same effect on the resource as sending it once.

24. Explain latency in API testing.

The term “latency” in API testing describes the period of time it takes for a request to go from the client to the server and for the corresponding answer to be returned. It calculates the amount of time that passes between making an API call and receiving a response.

25. What is throughput in API Testing?

Throughput in API testing refers to the quantity or rate of requests that an API can process in a specific amount of time. It shows how many requests the API successfully processes and completes in a certain amount of time, usually expressed in requests per second (RPS) or transactions per second (TPS).

In evaluating APIs, throughput is a crucial performance statistic since it offers information about the API’s capacity and effectiveness. For applications with high traffic or demanding workloads, an API with a greater throughput is one that can handle more requests in a given amount of time.

26. How do REST API and SOAP differ from each other? 

The following are the differences between REST API and SOAP –

27. What major challenges or blockers do testers face during API testing?

The following are the challenges or blockers testers face while API testing – 

• Lack of Documentation
• Complexity and Learning Curve
• Environment Setup
• Handling Dependencies
• Data Management
• Dynamic Data
• Error Handling and Response Validation
• Security Testing
• Performance and Load Testing
• Continuous Testing and Integration
• Versioning and Compatibility
  
  

28. What is a web service?

A web service is a system or software that allows for online communication and application compatibility between various software programs. It permits standardized and platform-independent communication and data sharing between many systems.

Web services are commonly used to facilitate communication between client applications and server-side services using specific technologies and protocols. These technologies and protocols include HTTP (Hypertext Transfer Protocol), XML (eXtensible Markup Language), SOAP (Simple Object Access Protocol), and REST (Representational State Transfer).

29. What is the difference between an API and a web service? 

Here is a head-to-head comparison between an API and a web service – 

30. Can you explain the payload?

A payload is the body of the API request or response containing the transferred data or information. It includes the information that is actually being delivered or received, including any parameters, variables, or message data.

The payload contains the API’s data to carry out particular operations or actions. It may include a variety of data kinds, including binary data, form data, XML, and JSON (JavaScript Object Notation).

Conclusion 

These were some of the most commonly asked API testing interview questions and answers. If you have already learned the basics of API testing, this article will help you recollect the concepts and know what you must answer in interviews. 

If you have encountered any other questions in your API testing interview, do let us know in the comments, and we will add them to the above list. 

Good luck!