Top 15 API Testing Interview Questions and Answers for Freshers and Experienced

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In today’s competitive world, businesses of all sizes realize the importance of API testing for creating an effective customer experience. They are adopting API testing at a fast rate, resulting in the creation of lucrative career opportunities for skilled professionals.

Our API testing interview questions blog will help you to master the field through the carefully curated set of frequently asked API interview questions and answers.

This list of interview questions on API will also benefit both freshers and experienced candidates who want to gain fundamental as well as advanced knowledge in web API testing.

Q1. What is an API? How does it work?

Ans. API stands for Application Programming Interface. It is a software interface that enables two applications to communicate and exchange data with each other. It is a set of functions that can be executed by another software program.

Let us now understand how an API works. When you use an application on your smartphone, the application connects to the Internet. It sends data to a server. The server will retrieve that data, interpret it, and perform the required actions. Then it sends it back to your phone. The application interprets that data and provides you with the information in a readable way without exposing the internal details.

Examples of API:

  • Google maps
  • Third-party payment processing

Q2. Define API testing.

Ans. API (Application Programming Interface) has predefined sets of routines, protocols, and tools which guide how some software components should interact with any other component. API is a collection of functions that enable the creation of applications that access the features or data of an application or operating system. The testing of these functions is called API testing.

Below are some of the tests which are performed on API:

  • Verify what APIs return
  • Return value as per the input condition
  • Verify what API trigger
  • Verify whether API updates the data structure

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Q3. What are the different types of API testing?

Ans. API testing types:

  • UI Testing
  • Validation Testing
  • Unit Testing
  • Functional Testing
  • Load Testing
  • Runtime and Error Detection
  • Security Testing
  • Penetration Testing
  • Fuzz Testing
  • Interoperability and WS compliance Testing

Q4. What are the benefits of API testing?

Ans. The benefits of API testing are:

1. Test for Core Functionality

API testing allows accessing the application without the UI (User Interface). The core functionality of the application is tested before the GUI tests. It helps in identifying the minor issue which can become bigger during the GUI testing.

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2. Time Effective

API Testing does not need GUI to be ready. It can be performed earlier in the development cycle. API testing is less time-consuming than GUI testing. API tests offer test results quickly. They significantly accelerate development workflows. This helps in speeding up the feedback loop and identifying issues faster.

3. Reduced Cost

API testing requires less code and provides better and faster test coverage compare to GUI test automation. You can identify issues at an early stage. It is easier and comparatively inexpensive to fix the issue at the early stages. This reduces the cost of the testing project.

4. Language-Independent

The data in API testing is exchanged data using XML or JSON. These transfer modes are completely language-independent. It means that you can select any core language when using automated testing services for the application.

5. Easy Integration with GUI

API testing allows highly integrable tests. This is useful if you plan to perform functional GUI tests after API testing. For example, simple integration will allow for the creation of new users within the application before the start of a GUI test.

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Q5. What are the tools used for API testing?

Ans. Some of the popular API testing tools for 2022 are:

  • SoapUI
  • PostMan
  • Karate DSL
  • Rest Assured
  • Alertsite API monitoring
  • Apigee
  • Assertible

Q6. Explain some protocols used in API Testing.

Ans. API protocols facilitate a standardized exchange of information. They provide a collection of defined rules that specifies the accepted data types and commands. Below are some of the common protocols used in API testing:

  • HTTP
  • REST
  • SOAP
  • JMS
  • UDDI

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Q7. What are the 7 principles of an API test design?

Ans. The 7 principles of API test design are the following:

1. Exhaustive Testing

Exhaustive testing is impossible. An optimal amount of testing based on the risk assessment of the application is needed.

2. Defect Clustering

Defect Clustering applies the Pareto Principle to software testing. According to this principle, a small number of modules contain most of the defects detected. Approximately 80% of the defect are found in 20% of the modules. By experience, one can identify such risky modules. There are some drawbacks to this approach. If the same tests are repeated again and again, the test case will not find new bugs.

3. Pesticide Paradox

If the tester conducts the same set of repetitive tests, it will become useless for identifying new defects. Thus, the test cases need to be regularly reviewed and revised. New and different test cases must be added to discover more defects. Testers need not depend on existing techniques. They need to improve the existing method to make testing more effective.

4. Testing shows the presence of defects

As per this principle, testing talks about the presence of defects not about the absence of a defect. Software testing reduces the number of undiscovered defects which remain hidden in the software. Even if no defects are found, it does not prove that the software or system is 100% defect-free. Teams should accept this concept. They should make efforts to manage client expectations.

5. Absence of error-fallacy

The software which is 99% defect-free may be still unusable. Such a case may occur if the system is tested for the wrong requirement. Software testing is not about finding the defects but also ensuring that software meets the business needs. The absence of error-fallacy means discovering and fixing defects will not help if the system build is unusable and fails to fulfill the requirements of the user.

6. Early Testing

Early Testing is crucial in the software lifecycle. Testing should start early in the software development lifecycle. This helps in identifying the defects in the requirement or design phase in the early stages. It is easier and comparatively inexpensive to fix a defect in the early stages of testing.

7. Testing is context-dependent

It means that the types and methods of testing performed can completely depend on the context of the software or systems. For example, the method of testing an e-commerce site will be different from a testing method of a commercial website.

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Q8. What is the difference between API testing and UI testing?

Ans. The differences between API testing and UI testing are:

API TestingUI Testing
API testing enables the exchange of data and communication between two software systems.It involves the testing of the graphical user interface (GUI).
API testing is the testing between the backend or server-side of the application and the backend of another application.It is the testing between users and the frontend or client-side of the application.
It is focused on the testing of business logic, data responses, and performance challenges.UI testing concentrates on how users interact with an application and its elements such as fonts, buttons, colors, and layouts.
API testing focuses on the business logic layer of the software architecture and not on the look and feel of an application.It focuses on the look and feel of an application.
difference between API testing and UI testing

Q9. What is SOAP?

Ans. SOAP stands for Simple Object Access Control. It is an XML-based protocol for exchanging information between computers. It is a communication protocol for communicating through the Internet. SOAP is platform-independent and language-independent and can be used for broadcasting a message.

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Q10. What is REST API?

Ans. REST stands for Representational State Transfer. REST API is an API that conforms to the principles of the REST architectural style. It allows for interaction with RESTful web services. This is why REST APIs are sometimes referred to as RESTful APIs.

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Q11. What is the difference between SOAP and REST API?

Ans. The differences between SOAP and REST API are:

SOAP stands for Simple Object Access Protocol.REST stands for Representational State Transfer.
It is a protocol and follows a strict standard to allow communication between the client and the server.It is an architectural style that follows six constraints – Uniform Interface, Client-Server, Layered System, Stateless, Cacheable, Code on Demand.
SOAP only uses XML for exchanging information.REST is not restricted to XML. It permits different data formats such as Plain text, XML, HTML, and JSON.
SOAP cannot use REST.REST can use SOAP protocol.
Difficult to implement.Easy to implement.
Requires more bandwidth.requires less bandwidth.
difference between SOAP and REST API

Q12. What are the types of bugs found in API testing?

Ans. The different types of bugs found in API testing are:

  • Duplicate or missing functionality
  • Reliability
  • Security
  • Unused flags
  • Inconsistent error handling
  • Performance
  • Multi-threading issues

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Q13. What is the difference between Web Service and API?

Ans. Following are some of the differences between web service and API:

  1. Every web service is an API. The vice versa may not be true. This means that not every API is a web service.
  2. It is not necessary that a web service can perform every task that an API would perform.
  3. A web service uses three styles of use including REST, SOAP, and XML-RPC for communication. On the other hand, an API may be exposed to in more ways.
  4. Web services always need a network to operate whereas APIs do not need a network for operation.

Q14. What are the limits on API usage?

Ans. Many providers set up limits on API usage. It is important to have an estimate of your usage and understand its impact on the overall cost of the offering. You need to consider the issue may arise depending on how data is leveraged.

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Q15. What are API restrictions?

Ans. API restrictions enable you to specify exactly which Google Cloud APIs can be called through the API key. You should add both client and API restrictions to every API key. It is possible to specify one and more services in the API restrictions.


Due to its advantages such as language independence and GUI integration, API testing is considered the future of software testing. It is a must for you to have a good understanding of API testing before you enter the software testing world.

We hope this list of commonly asked API testing interview questions will help you test your knowledge and clear your next interview successfully


Avinash is the Founder of Software Testing Sapiens. He is a blogger and Software Tester who has been helping people to get thier Jobs over a years Now.

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