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Top 20 Advanced API Testing Interview Questions and Answers


API testing is considered as the future of software testing because of its endless advantages, time effectiveness, language independence, or GUI integration etc. Today, API testing has become a must-have requirement for all software testing projects to ensure the maximum product quality.

If you also wanted to get into API testing profile then you definitely need to face interview. For this reason, we have given a complete set of frequently asked API interview questions answer in this blog. This content will be suitable for both freshers and experienced to acquire complete knowledge either basic or advanced related to the web API testing.

A. API Testing interview questions and Answer

1). How can you define the API testing for software projects?

API testing for any software project specifies how different components of a project should interact with each other. Testing the functionality of these components or functions is named as the API testing.

2). Name API testing tools that are frequently used by developers?

The API testing tools that are frequently used by developers include – PostMan, Alertsite API monitoring, SoapUI Pro etc.

3). Do you know about the common tests performed on the APIs?

Here is the list of common tests that are performed on APIs –

  • You should verify the API first and check either it is updating any data structure or not.
  • You need to check either API returns anything.
  • As per the given parameters or values, the values returned by APIs need to check.
  • Verify the API either it triggers any other event or calling another API.

4). What is a SOAP web service?

SOAP means Simple Object Access Control Protocol and this is an XML-based protocol that is used to exchange information between computer machines.

5). What do you understand by the REST API?

Read: How to Improve Test Case Writing Skills?

Rest API is defined as the set of functions that helps a developer in sending requests and receiving responses. In this protocol, the interaction is always made through an HTTP protocol. The meaning of REST is Representable State Transfer that has become a defacto standard for API creation these days.

6). How will you differentiate the API testing and the UI testing from each other?

UI or User-Interface testing is used to check the graphical interface of an application or software program how user interacts and reacts to different elements like fonts, images, layouts etc. This testing is majorly focused on look and feel of an application.

At the same time, API enables the communication among two different software components. Any software system implementing API contains functions or sub-routines that can be executed any other software system.

7). How will you differentiate the API testing and the Unit testing from each other?

Here are the major differences between API testing and the Unit testing. Let us discuss on each of them one by one –

API Testing

  • This type of testing is owned by the QA team.
  • This is an example of black box testing.
  • API testing is related to checking the full functionalities of a system so that it can be perfectly used by end users.
  • API testing is performed once the build is ready and authors don’t have the access to source code anymore.

Unit Testing

  • This type of testing is owned by the development team.
  • This is an example of white box testing.
  • Unit testing is performed to check either each unit is isolated and performing as per the expectation.
  • Unit testing is performed after each of the modules is ready before it is converted to a build.

8). When you are writing an API document, what are the major areas to focus on?

Here is the list of major areas that you should focus on while writing an API document –

  • Focus on the content source
  • Sketch or document the plan well.
  • Layout delivery
  • Detailed information about each of the functions
  • Automatic document creation programs

9). For any API document, how can we document different functions?

  • Syntax – Here, you have to write the syntax for the parameter of the code in the same sequence as they occur, highlight necessary elements, optional elements etc.
  • Description – Give a quick description of each of the functions
  • Error Messages – Here you need to give syntax of error messages.
  • Parameters – Give proper details about function parameters.
  • Related Links – Connection fields
  • Example Code – A small snippet of the code

10). Mention the list of tools that are needed for the documentation.

Read: 2020 Top 20 QTP Interview Questions and Answers

For Java Code, you can use JavaDoc and for the .Net code, you can use Doxygen.

B. Advanced API Testing Interview Questions and Answers

11). What is the basic process to test an API? Explain based on your previous experiences.

Here are the basic steps to follow to test an API –

  • First of all, decide the suite where you wanted to add the API test case.
  • Now select the test development code too.
  • Now you have to develop test cases for needed API methods.
  • This is the time to configure the app control parameters.
  • Configure the test conditions too.
  • Validate the methods and configure them
  • Execute APIs, check the test reports, filter the test cases, arrange the test cases etc.

By following these steps in the same sequence as given, this is easy to test an API successfully.

12). What do you understand by the API Framework?

There is no need to give an introduction to the API Framework as it is self-explanatory. When you are testing APIs then you don’t have to work on each API independently but you can use config files in that case where details for all APIs are given and can be used if needed.

13). Explain the working of API builder.

API builder is PL SQL program that is made up of four SQL files where one file is responsible for starting the process, two files are used to create the temp tables or the master package, and the fourth file will help in generating the final output.

14). What is a Test API?

Test API is a set of test APIs or a library of utility that helps developers or testers in creating testing tools and automated test cases for .Net or WIN 32 applications. It also offers a set of basic building blocks, data structures, data types, algorithms etc.

15). What are the different types of Input Injection and what are the different ways of stimulating user input?

Input injection is a popular act of simulating user inputs and it can be done in popular ways as listed below –

Read: Black Box Testing vs White Box Testing – What are the Key Differences?
  • By direct method invocation
  • With the help of an accessibility interface
  • By simulating low-level input
  • By simulating a device driver
  • Simulation using a robot

16). What are the different types of challenges faced by developers during API testing?

The different type of challenges faced by developers during API testing include –

  • Developers are not sure how to select the parameters?
  • They are not sure how to combine the parameters?
  • Developers are not sure of call sequencing.

17). How to perform the API testing with Run Scope?

Run Scope is a web application that supports the backend services and easy to understand user interface too for testing APIs.

18). What are the different principles of API testing design?

The major principles include – Setup, Execution, Verification, Reporting, and Clean up etc.

19). What are the possible number of bugs can be detected with API testing?

These are duplicate content errors, stress, tough error conditions, reliability issues, performance issues, inconsistency, multi-threading issues, not used flags, improper errors and more.

20). What are the tools that should be used for API test automation?

The automation tools that are frequently used of API test automation include – JUnit for Java, SOAP UI, HP UFT, or NUnit for .NET etc.


The above-mentioned advance API questions will be very helpful for you when an interviewer will focus on finding out the functional knowledge of the domain.

It is well understood that you should have practical knowledge when working with any technology.

Read: Top 30 Manual Testing Interview Questions to know in 2020

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