20 API Interview Questions and Answers To Ace the Interview

By Indeed Editorial Team

Updated July 12, 2022 | Published January 3, 2020

Updated July 12, 2022

Published January 3, 2020

When applying for an Application Programming Interface (API) software engineering job, you may need to demonstrate that you have a firm grasp of API, as well as API testing, SOAP and REST. As with any interview, it’s important to prepare answers ahead of time to ensure that you effectively communicate the skills and knowledge necessary for the job. This is especially true when interviewing for a technical position. 

In this article, we list 20 API interview questions, describe why employers may ask you these questions and provide sample answers to help you prepare for a successful interview. 

API interview questions with sample answers

Your interviewer may ask you many technical questions when interviewing for an API software engineering position. Here are 20 API interview questions that a hiring manager may ask you and some sample answers:

1. What’s an API?

An interviewer may ask this question to see if you have the basic knowledge to succeed in the field. At the very least, offer your interviewer a brief description of what API is. You can also expand the answer to include an example of a time that you’ve previously used it.

Example: “An API is a software that allows two applications to communicate with each other.”

Related: 15 Computer Science Jobs That Pay Well

2. What are some styles for creating a Web API?

This question can demonstrate your comfort and familiarity with working with API. To really differentiate yourself from other candidates, consider offering more than simply answering the question. Include your preference or a story about a time when you used a specific architectural style.

Example: “Common Web API architectural styles are XML/JSON as a formatting language, stateless communication, basic URI as the address for the services and HTTP for communication between the client and server. Personally, I prefer XML over JSON.”

Related: Types of Database Languages and Their Uses (Plus Examples)

3. What is API testing?

If an interviewer asks you this question, they may be looking to determine if you can apply fundamental concepts into practice. Provide a clear definition as well as some examples. You can also describe the importance of this testing.

Example: "API testing is a type of software testing that determines if the developed APIs are functional, reliable and secure. Some of the common API testing types are validation, security, UI, functional, load, penetration, runtime/error detection, fuzz and interoperability and WS Compliance."

Related: 7 API Development Skills for Web Developers

4. What are the advantages of API Testing?

Describing advantages can demonstrate that you’ve thought critically about your role. Make sure that your answer reflects a thorough comprehension of the advantages of API testing. You should show the interviewer that you know how and when to use it in your work.

Example: “API testing gives access to the application without needing a user interface. This allows you to detect the minor issues before they become big problems during GUI testing. Also, API testing is typically less time-consuming than GUI testing because it uses less code. As a result, it offers a more effective and efficient test coverage.

Another benefit is that the data transfers using XML or JSON. These modes of exchange are language-independent, allowing users to select any coding language when choosing automation testing services. Additionally, API testing easily integrates with GUI testing.”

Related: Job Search Guide: Product Management and Software Engineering

5. What’s the procedure for performing API testing?

Show the interviewer the steps you would take when performing API testing. This can showcase your ability to complete the entire process. Take your time and be sure to list each action.

Example: “When performing API testing, you’ll first choose the suite where you’d like to add the API case that you wish to test, then you choose the test development mode. After that, you create test cases for the desired API methods, configure the control parameters and test conditions of the application as well as the method of validation. Then you can perform the API test. Once the test is complete, you check the test reports, filter and sequence all of the API test cases.”

Related: 10 API Testing Tools for Developers (Including Their Benefits)

6. What are the primary challenges of API testing?

While you can keep your answer positive, be honest about the aspects of API testing that you find most challenging. Try not to focus on overly negative or challenging experiences. Your answer can also convey an intimate knowledge of the software. 

Example: “I find selecting and combining parameters and sequencing calls to be the most challenging parts of API testing.”

7. What are some tools used for API testing?

Though interviewers are asking for a list of tools, what they really want is your input about them. Make sure to mention your opinion when you answer. This can prove that you have an understanding of what makes effective API testing.

Example: “A few popular tools are Katalon Studio, Postman, SoapUi Pro, Tricentis Tosca and Apigee. Personally, I prefer SoapUi’s interface. It’s quick and really easy to use.”

8. What kinds of bugs does API testing find most commonly? 

Use this answer to prove that you have more than just a theoretical knowledge of API. Your experience working with this software can be just as valuable as the technical training to a potential employer. Try to focus on specific situations where you’ve encountered bugs and describe how you dealt with the issue.

Example: “I have often used API testing to find several different issues, such as missing or duplicate functionality, failure to handle errors effectively and seamlessly as well as any performance, stress, multi-threading, reliability or security issues. However, improperly implemented and improper errors, unused flags and inconsistent error handling are some of the other errors found through API testing.”

Related: What Are API Functions? (Types, Components and 3 Examples)

9. What’s the difference between API and Web services?

API and Web services serve different functions. Your answer can convey that you recognize when it’s appropriate to use each. Consider listing the basic definition of both as well as their key differences.

Example: “Web services must be over the web and have three styles of communication: SOAP, REST and XML-RPC. They always need a network to operate. However, APIs have multiple methods of communication. A network is unnecessary for their operation, and they don’t have to be exposed over the Web.”

Related: Software Engineer Cover Letter Sample

10. What’s SOAP?

This is another question that an interviewer may ask to see if you have adequate industry understanding. In your answer, state the definition for “SOAP. You can also describe why you’d want to use it.

Example: "SOAP, also known as Simple Object Access Protocol, is an XML-based messaging protocol. It aids in the exchanging of information between computers. You utilize SOAP API to make, find, delete or update records. In instances where there are more than 20 different calls, SOAP API can help conduct searches and manage passwords by adapting the protocol to whatever language supports web services.”

Related: 33 SOAP Interview Questions (With Sample Answers)

11. What’s REST API?

An interviewer may want to see if you understand the different types of API. Be sure to include your reasoning for using REST along with its definition. This shows that you’ve considered all options.

Example: “REST, or Representational State Transfer, is a set of functions that help developers perform requests and receive responses. You can perform interaction through HTTP Protocol. REST is stateless, so the server has no status or session data. With an effectively-applied REST API, you can restart the server in between two calls. Web services typically use the POST method to perform operations. REST, however, uses GET to access resources.”

Related: Common REST API Interview Questions (and Example Answers)

12. What’s the difference between SOAP and REST?

Comparing two similar concepts is an important critical thinking skill. Successfully answering this question can show your interviewer that you can weigh several options simultaneously. This is especially useful when troubleshooting. 

Example: “There are several differences between SOAP and REST. First, SOAP is a protocol through which two computers can communicate by sharing XML, while REST is a service designed for network-based software architecture. Additionally, SOAP supports only XML format, and REST supports a lot of different data formats. SOAP is unable to support caching, and REST can.”

Read more: What Is SOAP API? (And How It Differs From REST API)

13. What factors help inform your decision on which style of Web services—SOAP or REST—to use? 

This question can provide an opportunity for you to prove your qualifications and understanding of API. If applicable, mention an instance when this decision was particularly difficult or important. You can include your honest, evidence-informed option in your answer. 

Example: “REST is usually preferable because of its simplicity, performance, scalability and support across many data formats. However, SOAP is a viable choice when service requires an advanced level of security and reliability.”

14. What tests can help with APIs?

Demonstrate your knowledge and understanding of API by addressing the question before supplying your answer. This can show an interviewer that you understand why you perform tests. This demonstrates that you think critically about the choices you make at work.

Example: “Tests can and should help with APIs for several reasons, including testing the return values or inputting conditions.”

15. What kind of testing environment is beneficial for API?

When answering this question, provide some personality. Setting up the testing environment is difficult. eel free to share your opinion to communicate your intimacy with the process.

Example: “Setting up the API testing environment can be difficult because you have to configure both the server and the database without the use of GUI.”

16. What’s the difference between UI and API testing?

This question checks that you’re familiar with the different types of software testing. It also shows that you know when to use each. Consider all differences before providing an answer.

Example: “UI, or user interface, testing focuses on examining the graphical interface of an application, such as how the user can interact with its elements. API testing, on the other hand, sets up a mode of communication between two software systems, allowing them to share sub-routines and functions.”

Related: 15 Coded UI Interview Questions (With Example Answers)

17. What’s input injection?

This is a rather fundamental aspect of API, so keep your answer concise. A drawn-out response could signal an incomplete or unsure understanding of the process. To remain precise, try to plan your answer beforehand.

Example: “Input injection refers to the simulation of user input.”

18. What are some ways that you can simulate user input?

There are several ways to employ input injection, but just share a few examples with your interviewer. They know the information. They want to make sure that you do too.

Example: “You can accomplish input injection by utilizing a robot, a device driver or low-level input, just to name a few.”

19. What’s Runscope?

Interviewers may ask about many concepts related to API. Be sure to explain what the application is. You can also add what it provides to the API testing process.

Example: “Runscope is a Web application used to test APIs by supplying an accessible interface and backend services.”

20. Explain API documentation

Documentation is key when performing API testing. Make sure that you communicate the necessity of the process while describing it. This shows that you understand all the required steps.

Example: “Good API documentation is vital to the process. It supplies a quick reference while working within a program. It provides the plan, delivery layout, sources the content and details every function within the system.”

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