API Testing Interview Questions

API testing has been called the "future of software testing" because of its advantages in core functionality testing, speed, language independence, and GUI integration. In this post, the most frequently asked interview questions for freshers and experienced API testers are explored.

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If you're looking for API Testing Interview Questions for Experienced or Freshers, you are at the right place. There are a lot of opportunities from many reputed companies in the world. According to research API Testing has a market share of about 16.7%. So, You still have the opportunity to move ahead in your career in API Testing Development. Mindmajix offers Advanced API Testing Interview Questions 2023 that helps you in cracking your interview & acquire a dream career as API Testing Developer.

We have categorized API Testing Interview Questions - 2023 (Updated) into 2 levels they are:

Top 10 Frequently Asked API Testing Interview Questions

  1. What does API testing mean?
  2. What is the test environment of API?
  3. What is SOAP?
  4. What’s the procedure to test API’s?
  5. Define API testing with Runscope
  6. What are the main principles of API test design?
  7. Explain the working of API Builder
  8. How to perform API testing?
  9. Define the caching mechanism
  10. What is Payload in REST API?
If you want to enrich your career and become a professional in API testing, then enroll in "API Testing Training".This course will help you to achieve excellence in this domain.

API Testing Interview Questions And Answers

Types of API Testing

Testing TypeDescription
Unit testingIndividual operation functionality testing.
Functional testingBlock of unit test results tested together.
Load testingAnalyzing performance and functional ability when the load is applied.
Runtime error detectionManual and automated executions to detect any sort of errors or data leaks.
Security testingTo verify the ability to safeguard access and data.
Web UI testingGUI test to identify proper executions of any transactions.
Interoperability testingValidate performance with Interoperability Profiles.

Basic API Testing Interview Questions For Freshers

1. What does API testing mean?

API stands for Application Programming interface details regarding how some software components must act together. In general terms, API testing is a set of procedures and functions allowing the creation of apps accessing data or features of an operating system or application. All in all, testing of such procedures is acknowledged as API testing.

Related Article: API Testing

2. What are the names of tests executed on APIs?

There can be numerous reasons behind executing API testing and there are a number of tests that can be performed on APIs. Some common API test examples are as follows:

  • Any data structure which demands proper validation can be updated by API
  • In case some interruption arises during the process or any other event is prompted, you can call another API
  • Tests for inputting condition-based return values can be performed
  • With the help of these tests, some resources can be easily modified like process killing, an update of the database, etc.
  • The return values can also be tested which can be even null or are with wrong results

3. What is the procedure to perform API testing and what exactly needs to be checked?

During the API testing process, a request is raised to the API with the known data. This way you can analyze the validation response. Basically, things that must be checked during performing API testing are:

  1. Accuracy of data 
  2. Schema validation
  3.  HTTP status codes
  4. Data type, validations, order, and completeness 
  5. Authorization checks 
  6. Implementation of response timeout
  7. Error codes in case API returns, and
  8. Non-functional testing like performance and security testing.

4. What is the test environment of API?

To set up the test environment of API is not a cakewalk. It is a bit complete and demands the configuration of the database as well as a server according to the need of the software. There is no availability of GUI (Graphical User Interface) in this form of testing.

Once the installation process is over, API is verified for proper functioning. During the whole process API that is invoked by the initial environment is being set up with distinct parameters to examine the test results.

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5. What approach should be followed for API testing?

There are certain factors that determine the API testing approach. Let’s check them below:

  1.  Defining the accurate input parameters
  2.  Verifying the calls of the blend of two or more than two value-added parameters 
  3.  Defining the basic functionality and scope of the API program
  4.  Writing suitable API test cases and making use of testing techniques like equivalence class, boundary-value, etc. to verify the functionality 
  5. Testing case execution 
  6. Testing result comparisons with the results expected 
  7. Verifying behavior of API under conditions like the connection with files etc.

6. Define the basic difference between API testing and UI level testing?

UI testing means the testing of the graphical interface. Its focus is basically on the feel and looks of an application. Within user interface testing, things like how the user interacts with app elements such as images, fonts, layouts, etc are checked.

On the other hand API, testing allows communicating between two different software systems. During this testing, a software system that implements an API includes sub-routines or functions that can be performed by other software systems.

7. Name the common protocols used in API testing.

Many protocols are there that can be used in API testing. These are as follows:


8. Name different tools used for API testing.

There are many testing tools available that can be used for API testing. A few of them are:

Postman, SoapUi Pro, Curl, and Alert site API monitoring

9. What is SOAP?

The term SOAP refers to Simple Object Access Control. In simple terms, it is an XML-based protocol that helps in exchanging information among computers. 

10. What’s the procedure to test API’s?

For testing API’s one must follow the below-mentioned steps:

  1.  Make a selection of the suite you like to add the API test case to
  2. Now choose the test development mode
  3. Next demands the development of test cases for the required API methods 
  4. After this, you need to configure the control parameters of the application and then test the conditions
  5. Once done with all the previous steps, configure method validation
  6. Now is the time for the execution of the API test
  7. After this, you can check test reports and filter API test cases 
  8. Last but not least, sequence all API test cases. That’s it!
Related Article: Manual Testing Interview Questions

11. What is REST API?

REST API is a set of functions helping developers in performing requests along with receiving responses. Through HTTP protocol interaction is made in REST API. 

The term REST refers to Representational State Transfer. In a very short span of time, it has become an effective standard for API creation.

12. What are the areas that need to be taken care of while writing API documents?

The main areas that need your concentration while writing API documents are as follows:

  1. You need to check the source of the content 
  2. Plan or sketch of your document
  3. The delivery layout of the same
  4. Information needed for each of the functions available in the document
  5. Lastly, automatic document creation programs

13. What is an API framework?

The API framework is easy to understand. During the process, the config file is used to hold the configurable parts as well as to value the test run. Besides, within the config file, automated test cases should be represented in the format of a parse table. During the process of API testing, it is not mandatory to test each API as a result the config file contains some sections whose API is activated for all that specific run.

14. What do you mean by input injection? Explain different ways of doing it.

The term Input injection is the act to stimulate user input. User input can be simulated in many different ways such as:

  1.  Direct Method Invocation 
  2. Invocation with the help of accessibility interface
  3.  Doing simulation with the help of low-level input
  4.  Doing simulation with the help of a device driver 
  5.  Doing simulation with the help of a robot

15. Define API testing with Runscope.

To test APIs, Runscope is used. It is basically a web application providing backend services as well as an easy-to-use interface.

16. Explain the major challenges that come while API testing.

The list of major challenges that come while API testing is:

  1.  Parameter Combination
  2.  Parameter Selection and
  3.  Call Sequencing

17. What are the main principles of API test design?

There are various principles of API test design. Those are as follows:

  1. Setup: this includes the creation of objects, start services and initialize data, etc.
  2. Execution: during this principle, there are steps to follow API or scenario as well as logging
  3. Verification: for evaluating the execution outcome there are oracles 
  4. Reporting: keep a tab on the pass, blocked or failed
  5. Clean up: this shows the pre-test state

18. Explain the types of bugs that can be found using API testing?

API is capable of finding many types of bugs that includes:

  1. Stress 
  2. Security
  3. Duplicate or missing functionality 
  4. Reliability 
  5. Unused flags
  6. Incompatible error handling 
  7. Multi-threaded issues, and 
  8. Improper errors

19. Name various tools used for API test automation.

While doing API testing and Unit testing, both targeting source code. In case an API method is making use of code based on .NET then other tools that are providing support must have .NET

There are various automation tools for API testing:

  1. HP UFT
  2. Soap UI
  3. JUnit for Java
  4. NUnit for .Net

20. What is the API documentation?

For any foundation, there is always a need for good documentation. Similarly, API documentation provides a quick reference to access working or library within a program.

While walking through any such documents, a proper plan is a must along with a proper sketch or layout for delivery, there is a need for the content source, information regarding each and every function, etc.

There are various API documentation tools like Doxygen and JavaDoc. Below you can check the various categories in which each and every function is being documented that are revolving around the parameters like:

  1. Function description
  2. Type and syntax of the error message that may occur
  3. Syntax, elements, and sequence needed for each parameter
  4. Links regarding functions

21. List some templates for API documentation that are most used.

There are various API documentation templates that are making the whole process really simple and easy. Check them below:

  1. Swagger
  2. Miredot
  3. Slate
  4. FlatDoc
  5. API blueprint
  6. RestDoc
  7. Web service API specification

22. Explain the difference between API testing and Unit Testing.

  • Where Unit testing is a form of white-box testing, API testing is a form of black-box testing. 
  • Unit testing is performed prior to the process of including the code in the build. On the other hand, API testing is done after the build is prepared for testing.
  • In Unit testing, the source code is drawn in the form of testing while in API testing the source code is not drawn in.
  • In Unit testing, there is a limited scope of testing as a result only basic functionalities are measured for the purpose of testing. Subsequently, in API testing there is a wide scope of testing, thus all the issues that are functional are measured for the purpose of testing.
  • Unit testing is done by the testers and wherein every functionality is separately tested. While The API testing is done by the testers for the purpose of end-to-end testing of the whole functionality.

23. Define TestApi?

TestApi can be explained as the test building blocks library which is indispensable for testers and developers to create testing tools and automated test suites.

24. Explain everything about warnings and API errors.

When something is not going as per expectations like when the outcome is not as predicted then the occurrence of errors can be seen and the same warnings are explained in the form of a message in a proper format. Within a single module, there can be one or many warnings.

A wide range of warnings that can form are:

Missing module warning and parameter validation warning 

A wide range of errors that can form are:

25. Explain the working of API Builder.

API Builder is a PLSQL utility that includes 4 square files. To place API parameters and to begin the technique only one report is liable. API builder allows you to create and make use of API endpoints that can be guzzled by any client application.

There are several components that lead to the making of API Builder. During the working, files and formed brief tables as well as master bundles for creating the output code. Lastly, the fourth record generates a spooled output of the code into a record relating to output_script_.sq.

API Testing Interview Questions For Experienced

26. What are the benefits of API testing?

  • Provides application access without the user interface 
  • Provision for easy test maintenance
  • Less time for resolution
  • Speed and coverage of testing
  • Protects from malicious code and breakage
  • Cost-effective/ reduces testing cost
  • Technology independent 

27. What are the challenges faced in API testing?

Just like other software testing techniques, API testing also faces some challenges like:

  • The main challenge is sequencing API calls.
  • Strong coding knowledge needed for testers.
  • No GUI is available to test the application, which makes it difficult while giving inputs.
  • Testers must be aware of parameter combinations and validations.
  • Exception handling functions also be tested.
  • Validating and verifying different systems is difficult for testers.

28. Explain how to document each function in the API document? 

Description: Small description of what a function does

Syntax: Syntax about the parameter of the code, the sequence in which they occur, required and optional elements, etc.

Parameters: Functions parameters

Error Messages: Syntax of error messages

Example Code: Small snippet of code

Related Links: Related functions

29. What are the tools used for API documentation?

The free tools used for API documentation are ReDoc, Swagger UI, and DapperDox.

30. What’s the difference between API and Web Service?

APIWeb Services
API is a set of protocols and definitions which allow one application to interact with another application.A web service is a way for two machines to interact with each other over a network. 

API can interact through REST, SOAP. CURL, and XML-RPC calls as well.

Also, through DLL, JAR, XML over HTTP, JSON over HTTP, etc.

A web service uses three styles for communication, such as SOAP, REST, and XML-RPC.
All APIs are not web services.All web services are APIs
APIs don’t need a network for operationWeb services always need a network for operations
API can perform all the operations which web service can't achieve.
Web services cannot perform all the tasks that API would perform.

31. What are the architectural styles used for creating a Web API?

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

32. How to perform API testing?

API testing is a type of integration testing used to test API to validate the functionality, performance, and security of the application.

API testing should perform the following testing methods:

  • Discovery testing - This testing manually executes the set of calls documented in the API.
  • Usability testing - It verifies whether the API is functional, user-friendly, and does API integrates with another platform or not.
  • Automated testing - It comes while creating a set of scripts or a tool to execute API regularly.
  • Security testing - It recommends what authentication type is needed and also checks sensitive data encryption.
  • Documentation - It's the final phase for a deliverable. The team makes sure the documentation provides enough data to interact with the API.

33. Why is API testing the most preferred for Automation testing?

API testing is considered most suitable for automation testing because:

  • It effectively verifies all the functional paths of the system under test.
  • Provides faster feedback.
  • It presents the most stable interface.

34. Name a few API documentation templates?

There are several API documentation templates that make the entire process simple, leverage best practices, and will make API users satisfied. They are:

  • RestDoc
  • RAML
  • Miredot
  • Calamum 
  • Swagger
  • API Blueprint 
  • Web Service API Specification Doc Template
  • FlatDoc
  • apiDoc  
  • Slate 

35. What to be considered to create great API documentation?

  • Plan your docs
  • Include fundamental sections
  • Be consistent and avoid jargon
  • Include interactive examples and other resources
  • Maintain your docs
  • Delivery layout
  • Information needed for every function in the document
  • Automatic document creation programs

36. What are the differences between SOAP and REST API?



SOAP stands as Simple Object Access Protocol.

REST stands as Representational State Transfer.

It’s largely based and uses only HTTP and XML

It supports different data formats such as HTML, plain text, JSON, XML, and more. But the most preferred format to transfer data is JSON.

It’s a protocol

It’s an architectural pattern

SOAP uses WS-security and SSL( Secure Socket Layer) for security 

On the other hand, REST has SSL and HTTPS for security.

37. What is messaging in RESTFUL Web Services?

RESTFUL Web Services uses HTTP protocol as a source of communication between client and server. The technique when a client sends a message in the form of an HTTP request, and the server responds in the way of an HTTP response is called Messaging. These messages comprise metadata and message data, i.e., information related to the message itself.

38. What are the main components of an HTTP request?

  • Action showing HTTP methods like PUT, GET, DELETE, POST.
  • Uniform Resource Identifier (URI), which is the identifier for the resource on the server.
  • HTTP version which represents the HTTP version like- HTTP V1.1.
  • Request Header used for carrying metadata to the HTTP request message. 
  • Request Body describes resource representation or message content.

39. Which HTTP protocols are supported by REST?

  • GET - Requests data from the defined resource. 
  • PUT - Replaces the current representation of the target resource with the request payload.
  • POST - Sends data for a server to create or update the resources. POST requests are never cached or bookmark.
  • OPTIONS - Specifies the communication option for the target resources.
  • DELETE - Removes the specified resource.
  • HEAD - HEAD requests for a response that is similar to GET requests, but without the response body.

40. What is URI? What is the purpose of a web-based service, and what is its format?

Uniform Resource Identifier (URI) is a string of characters used for unambiguous identification of resources and extensibility through the URI scheme.

The purpose of this web-based service is to locate a resource on server hosting.

A URI’s format is <protocol>://<service-name>/<ResourceType>/<ResourceID>. 

41. Define the caching mechanism.

A caching mechanism is a practice to store data temporarily and retrieve data from a high-performance data store either implicitly or explicitly.

Caching mechanism improves performance by copying the asset requested and obtaining the cached copy instead of the original later.

42. What’s the difference between PUT and POST operations in Rest API?

The PUT method is a call when you have to modify a single resource, which is part of resource collection.POST method is a call when you have to add a child resource under resource collection.
The PUT method is idempotentPOST method is not idempotent
PUT for UPDATE operations.POST for CREATE operations.
If the PUT request is used more than one time, the results will remain the same.If a POST request is used multiple times, the results will be different.
PUT works as specific.POST work as abstract.

43. Can we use GET requests instead of PUT to create a resource?

PUT or POST are used for creating resources. GET is used only for requesting data from a specified resource.

44. What are the commonly used HTTP methods for RESTful services?

  • GET - Retrieves data from a server at the specified resource.
  • HEAD - Works the same as the GET method, but the server replies without the body.
  • POST- Creates a new resource
  • PATCH - Allows partial modifications to a resource
  • PUT - Replaces all current representations of the target resource 
  • DELETE - Removes the defined resource
  • OPTIONS - Returns the HTTP methods supported by the server for the specified URL

45. What is Payload in REST API?

The Payload in REST API is the actual data pack that is sent with the GET method in HTTP. It’s the crucial information that you submit to the server when making an API request. 

The payload is denoted using “{}” in a query string, and it can be sent or received in multiple formats.

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About Author

Ravindra Savaram is a Technical Lead at Mindmajix.com. His passion lies in writing articles on the most popular IT platforms including Machine learning, DevOps, Data Science, Artificial Intelligence, RPA, Deep Learning, and so on. You can stay up to date on all these technologies by following him on LinkedIn and Twitter.

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