Top 30+ API Testing Interview Questions (2024)

The only thing more challenging than understanding the API itself is figuring out what the interviewer really wants from you. If you've ever found yourself wondering if an API interview is meant to test your technical skills or if it's just a mind-bending puzzle from a sci-fi movie, you're definitely not alone.

From the moment you're asked to describe your experience with API testing, you’re thrown into a whirlwind of technical jargon and hypothetical scenarios. You might start off discussing REST and SOAP, only to be hit with further questions that feel like you're decoding a cryptic message. And when it comes to live coding challenges, get ready for APIs that will throw as many curveballs as a cat on a caffeine kick.

As you deal with rate limits and unpredictable responses, keep in mind: the real goal is to blend technical skills with a light touch of humor. So, if you've ever wondered what an API testing interview is all about and how to come out of it with your sanity intact, come along as we answer more than thirty most common questions asked in QA interviews.

1 - What are APIs?

APIs (Application Programming Interfaces) allow different software systems to communicate with each other. They use standard rules and protocols, such as RESTful, SOAP, and GraphQL, to facilitate data exchange between servers and software components. API endpoints use code to allow data exchange between different software components.

Also Read: Professional Courses and Certifications for QA Testers & Developers

2 - What are API endpoints? 

An API endpoint is typically a URL string that acts as a point of contact between API clients (frontend apps), such as web apps, mobile apps, or other services to communicate with servers. Using the API endpoints, you can perform various actions such as:

• data retrieval, 
• posting data, 
• manipulating data, 
• deleting data, or 
• updating it. 

API endpoints normally comprise the:

• base URL, 
• path, and 
• query parameters.

3 - What is API Testing?

API testing evaluates that software APIs work well in terms of functionality, performance, security, and reliability. It's a black-box software testing method and a key part of the API lifecycle (planning, designing, development, testing, deployment, and maintenance of APIs).  

Read More: API Testing 101: A Comprehensive Guide

4 - What are the popular types of APIs?

Here are the noteworthy API types that would normally come across while developing and testing software:

• Web APIs — provide standard rules and protocols typically used while developing and testing web apps. Web APIs use a two-way data exchange mechanism through HTTP methods. Prime examples of WebAPIs include RESTful, SOAP, GraphQL, and gRPC.
• Library APIs — are predefined sets of functions, rules, or frameworks that can be used to write apps typically based on a specific programming language. Popular examples include React.js (JavaScript Library) and Requests (Python Library). 

Also Read: Top 50+ React Interview Questions

5 - What are the types of API testing?

Prominent types of API testing include:

• Unit testing
• API documentation testing
• Functional testing (includes contract testing, component testing, and scenario testing)
• Integration testing
• Regression testing
• Security testing
• End-to-end testing
• Load and performance testing

Also Read: Unit Testing vs. Integration Testing

6 - What are the popular tools for API testing?

Popular API testing tools include:

• Postman — Widely used for manual and automated API testing.
• Apache JMeter — Open-source tool for performance and load testing.
• Swagger — Provides API documentation and testing features with OpenAI specs.
• REST Assured — For testing RESTful APIs in Java.
• Karate Labs —Framework for API testing, API mocking, and API performance testing.
• SoapUI — Tool for testing SOAP, REST APIs, and GraphQL.
• Katalon Studio — Comprehensive test automation tool with API testing capabilities.

Check out the complete article on API testing that covers the top features of all the tools mentioned above.

7 - What are the benefits of API testing?

Among many, here are some top benefits of API testing:

• Quality assurance — API testing ensures that software APIs are defect-free, performant, secure, and reliable.
• Early detection of defects — API testing with the API-first approach helps eliminate defects as soon as they are introduced. 
• Reduced costs — Well-tested APIs can significantly reduce long-term costs associated with reworks and API bug fixes in live apps. 
• Faster release cycles — API testing with the shift left testing approach can speed up software updates and new version release cycles.

Also Read: Quality Assurance vs. Quality Control: Explained

8 - What is the difference between API testing and unit testing?

Here’s a summarized table that shows API testing vs. unit testing:

9 - What is a request-response cycle?

The request-response cycle is the data exchange mechanism by which user-facing clients (such as web apps, mobile apps, or services) talk to the servers using API endpoints. 

In a typical request-response cycle, the client sends a request to an API endpoint, the server processes the request, performs the necessary actions, and generates a response. This response is then transmitted back to the client. This interaction enables clients to retrieve data, update info, or execute specific actions on the server side. Simply put, APIs act as the communication bridge between the client and the servers. 

10 - What is JSON and how is it used in API testing?

JavaScript Object Notation, or JSON is a popular data representation format and is typically used for data exchange in APIs. It's often preferred for data exchanges between the client and server in REST APIs and provides a standardized way to represent, request, and receive data. 

In API testing, JSON helps define the expected request and response formats, as well as validate the data received from the APIs. Developers and QA engineers can create JSON payloads to send with API requests and compare the received responses against expected JSON structures.

11 - What are REST APIs?

REST, or Representational State Transfer is a popular architectural style for APIs that follows a set of guidelines and principles for developing web services and apps. REST APIs typically use HTTP methods like GET, POST, PUT, DELETE, and PATCH to interact with resources. 

Top features of REST APIs include:

• Statelessness — meaning each request from a client to a server must contain all the info necessary to understand and process the request. The server doesn't maintain a session state between requests.
• Client-Server Architecture — that focuses on clear separation of concerns between the client and server. The client handles the user interface and user experience, while the server manages data and business logic.
• Cacheability — of responses for improved performance and reduced server load.

12 - What are SOAP APIs?

SOAP, or Simple Object Access Protocol, is a messaging protocol standard for exchanging structured information in the form of XML between systems. 

With SOAP APIs, you get:

• XML-encoded — SOAP messages that allow for structured and self-describing data exchange.
• Enveloped structure — that defines headers and body elements for metadata and payload respectively.
• Support for WSDL — to describe SOAP services that provide detailed info about available operations, parameters, and data types.
• Transport independence — to function over protocols like HTTP, SMTP, and JMS.
• Built-in security features — through WS-Security that allows for authentication, confidentiality, and integrity of messages.

13 - What are HTTP methods in REST APIs?

Common HTTP methods include:

• GET — fetches data from the server.
• POST — sends a request to the server to create a new resource or to submit info.
• PUT — replaces the entire resource file with the updated/new file.
• DELETE — deletes the specified resource file from the server.
• PATCH — modifies the target resource file.
• HEAD — similar to GET request, but fetches only the header data from the target resource.
• OPTIONS — returns the HTTP method that the server supports for the target resource.

14 - What is an API Key?

An API key is a unique identifier used to authenticate clients (e.g., apps or users) with an API to access its resources. API keys are often used along with other authentication methods, such as:

• JWT bearer tokens,
• HTTP basic authentication, and
• OAuth authentication.

15 - What is the difference between a monolith and a microservice?

Monoliths are single, self-contained apps — typically deployed as a single unit — that bundle all components, including the user interface, data access, and business logic, into a single package. 

• For example, an e-commerce app where all functionalities like product catalog, shopping cart, checkout, and user management are part of a single codebase.

Microservices are a software architecture style that structures an app as a collection of loosely coupled services, each running in its own process and communicating with each other through lightweight methods, often HTTP-based APIs. These services are independently deployable and scalable. 

• For example, an e-commerce platform where product catalog, order management, payment processing, and recommendation engine are separate microservices, each handling specific functionalities.

16 - What are API testing frameworks?

API testing frameworks provide predefined rules and functions for performing API testing. They offer tools and methods that may vary in customization, from coding-heavy to user-friendly interfaces. Examples include:

• Postman — is a renowned platform for API development, with features for testing, monitoring, and collaboration.
• Apache JMeter — is a Java-based open-source tool preferred for performance testing with features for API testing. 
• SoapUI — is an open-source web service testing app for SOAP and REST APIs.
• Karate Labs —  is a popular API testing framework that focuses on behavior-driven development (BDD), API mocking, and API performance testing.

Also Read: Top 10 Automation Testing Frameworks and Tools

17 - What is the difference between authentication and authorization in API testing?

Authentication and authorization are often used together (sometimes interchangeably too). However, these terms aren’t the same and refer to two very different workflows for ensuring app security. 

In the context of API testing, authentication is the process of verifying the identity of the client making the API request, typically using credentials like API keys, bearer tokens, or username and password. Whereas, authorization refers to the process of determining what actions or resources the authenticated client is allowed to access based on predefined permissions or roles.

18 - What are request and response headers in API?

Request and response headers are key-value pairs that provide additional info about an HTTP request or response and clearly facilitate communication between the client and server. 

Here’s a breakdown of info that each contains:

Request Headers

• Content-Type — Specifies the format of the request body (e.g., JSON, XML, form-data).
• Authorization — Contains credentials for authentication (e.g., API key, token, basic auth).
• Accept — Indicates the media types the client can accept in the response.
• User-Agent — Identifies the client application or browser.
• Cache-Control — Controls caching behavior for the request.

Response Headers

• Content-Type — Specifies the format of the response body.
• Content-Length — Indicates the size of the response body.
• Set-Cookie — Sets a cookie in the client's browser.
• Location — Redirects the client to a different URL.
• Cache-Control — Controls caching behavior for the response.

Also Read: [Updated] How to Clear DNS Cache on Chrome with Chrome://Net-Internals/#DNS

19 - What are query parameters?

Query parameters are key-value pairs that can be appended to the end of an API endpoint URL to modify the behavior of the request or filter the response data. They are typically used to provide additional info to the server and are separated from the base URL by a question mark (?).

20 - How are query parameters used in APIs?

Query parameters are optional and an API can still function with any query parameters attached. However, they allow for more flexibility and customization of the request, typically used for pagination, sorting, and filtering the returned data based on given criteria in the query string.

For example, in the URL https://api.example.com/v1/products?page=2&limit=10, the query parameters are page=2 and limit=10. 

Here’s a breakdown of the above example:

• After the base URL (https://api.example.com/v1/products), a question mark (?) indicates the start of the query parameters, and multiple parameters are separated by an ampersand (&).
• The appended parameter has a key (page) with a corresponding value (2), connected by an equals (=) sign. It’s followed by a second set of parameters as key-value pairs (limit=10) to set the number of results.

The server interprets this as a request for the second page of products, with a maximum of 10 results per page. To sum up, you have the:

• Base URL — https://api.example.com
• API version  — /v1
• Path — /products
• Query Parameters — ?page=2&limit=10

21 - What is API mocking?

API mocking involves creating simulated versions of APIs that are still under development. It allows developers and testers to configure predefined responses to test and develop applications before the actual APIs are available. This helps identify issues early and ensures smooth integration when the production APIs are implemented.

Also Read: How to Write Test Cases in Software Testing?

22 -  How is mocking used in API testing?

API mocking provides a solution for creating controlled testing environments for developers and testers to work with by simulating API responses without relying on the actual API. 

Here are a few noteworthy use cases for API mocking:

• Parallel development —  allows front-end developers to continue building and testing UIs, while the actual back-end APIs are still in development. This ultimately reduces dependencies allowing teams to work without waiting for the actual APIs to be ready.
• Consistent testing — can be performed against a stable set of responses, reducing disruptions caused by network issues or API downtimes.
• Edge cases — can be tested for the app to return specific error messages or status codes and graceful handling of edge case scenarios.

Also Read: Top 24 Chrome Extensions for Software Testers

23 - What are the common API authentication methods used while API testing?

Common API authentication methods include:

• HTTP Basic Authentication — Sends username and password in plain text (base64 encoded) with each request. Insecure for sensitive data.
• API Key Authentication — Uses a static key sent with each request to identify the caller. Simple but less secure than token-based methods.
• JWT Bearer Token Authentication — Passes a self-contained token containing claims about the user. Stateless and often used for microservices.
• OAuth 2.0 Authentication — Delegates authorization to a third-party app, allowing users to grant access to their data without sharing credentials. Commonly used for social logins.

24 - What is fuzz testing?

Fuzz testing is a technique for discovering vulnerabilities and bugs in applications by inputting random, unexpected, or invalid data. This process aims to uncover security loopholes, crashes, and unusual behaviors by testing how the system handles such data. Fuzz testing helps identify weaknesses and ensure robustness in software applications.

25 - What is the difference between API input validation and input sanitization?

API input validation ensures that user input meets specific criteria (e.g., type, format) before processing it. This prevents invalid or malicious data from entering the system. For example, validation might check if an email address is in a proper format.

Input sanitization, on the other hand, involves cleaning or neutralizing potentially harmful characters or data to ensure it is safe for processing. This step is used to remove or escape characters that might be used in injection attacks. For instance, sanitization might strip out or escape HTML tags from user input.

26 - What are cookies and how are they used while API testing?

Cookies are small pieces of data stored on the client side and sent to the server with each request. They are used to maintain session state, track user interactions, and store preferences. In API testing, cookies can be used to test authentication flows, session management, and the handling of user-specific data during interactions with APIs.

27 - What is the difference between component testing and unit testing?

While they might sound very similar, they aren’t the same. Here’s a summarized table that shows component testing vs. unit testing:

Also Read: What is White Box Testing?

28 - What is contract testing?

Contract testing is a type of functional testing in APIs that verifies the interaction between the API service providers and the API consumers ensuring that both parties (provider and consumer) adhere to a predefined contract for API usage. This agreement specifies the expected API requests and responses to prevent integration issues by validating that changes in one service do not break the functionality of another service that relies on it.

29 - What are service providers and consumers in APIs?

A service provider is a software app offering specific functionalities or data through an API by defining the API's capabilities and establishing the rules for how other systems can interact. 

• For example, OAuth 2.0 (service provider) offers authorization services for apps to access user data.

A service consumer is a system that uses an API to access the functionalities or data provided by a service provider. It sends requests to the provider API and receives responses according to the defined rules. 

• For instance, a web app (service consumer) might use an OAuth 2.0 provider's API to authenticate users.

Also Read: What are End Users?

30 - What are the common bugs while testing APIs?

Among many, here are the common bugs typically encountered while testing APIs:

Functional Bugs

• Incorrect functionality — when API might not behave as expected.
• Missing functionality — that may have been overlooked while writing APIs. 
• Duplicate functionality — or redundant API endpoints.

Data-Related Bugs

• Data integrity issues — due to incorrect data types, formats, or values leading to data inconsistencies or missing data.
• Data validation errors — because of insufficient input validation.

Performance Issues

• Response time issues — leading to slow performance or impacted system responsiveness, such as slow or unsuccessful database queries and higher network latencies.
• Improper error and exception handling issues — leading to reworks or bad user experience.

Security Vulnerabilities

• Unauthorized resource access — due to weak authentication or authorization mechanisms.
• Vulnerabilities for injection attacks — through malicious code injection (e.g., SQL, XSS).

Others

• Rate limiting issues — due to incorrect or inconsistent rate limits impacting API performance and user experience.
• API versioning issues — leading to poor version control, compatibility problems, and unexpected behavior.

Also Read: What is Bug Lifecycle? 

31- What is CSRF?

Cross-Site Request Forgery, or CSRF is a security vulnerability that allows an attacker to trick a user into performing actions on web apps in which they are authenticated, without their consent. This can lead to unauthorized actions being taken on behalf of the user, such as:

• changing account settings, 
• making transactions, or 
• changing passwords.

32 - What are the common security threats related to APIs?

Common security threats to APIs include:

• SQL and XML injection — attacks inject malicious code into input fields to disrupt or hijack APIs.
• DDoS attacks — or Distributed Denial of Service attacks overwhelm an API with excessive traffic, leading to server downtimes. They are especially dangerous for apps or services that rely on real-time server data transactions.
• Cross-site scripting (XSS) — can occur in APIs if user input is not properly handled, potentially leading to malicious script execution.
• Data exposure —  may expose sensitive data, such as API keys, tokens, or personal user information from unprotected APIs.
• Boken authentication and authorization — security flaws in API code that can lead to unauthorized access by bad actors.
• Incorrect security configurations — for APIs are another common threat.
• Insufficient rate limiting — can increase the chances of DDoS attacks or other API abuses.

33 - What is the difference between a DDoS attack and an XSS attack?

34 - What is hashing in API security?

Hashing is a one-way cryptographic function commonly used in API security to protect sensitive data like passwords that transform plain text data into a fixed-size string of characters called a hash. This hash cannot be easily reversed back to the original data, but it can be used to verify its integrity.

Hashing helps prevent attackers from accessing user passwords even if they breach the database. They would only see the hashed values, which are useless without the hashing algorithm to reverse them.

Here's how hashing works in APIs:

• When you create an account in an app, your password is hashed before being stored in the database.
• During login, the entered password is hashed again and compared with the stored hash.
• If both hashes match, it indicates a successful login as the entered password likely matches the original one.

35 - What is the difference between hashing and encryption?

Here’s a summarized table that shows hashing vs. encryption: