Top 50 API Testing Interview Questions and Answers (Detailed Guide for 2025)

Top 50 API testing interview questions and detailed answers:

Basic Questions

  1. What is API testing?
    API testing involves validating the functionality, reliability, performance, and security of an API. It ensures that the API behaves as expected by testing the endpoints, request/response structures, and error handling.

  2. What are APIs?
    APIs (Application Programming Interfaces) are sets of protocols and tools that allow different software applications to communicate with each other. They define the methods and data formats for interaction between systems.

  3. How is API testing different from GUI testing?
    API testing focuses on verifying the back-end functionality and business logic, ensuring correct data exchange and processing. In contrast, GUI testing deals with testing the front-end user interface and user experience elements.

  4. What are common protocols used in API testing?
    Some common protocols for API testing are REST (Representational State Transfer), SOAP (Simple Object Access Protocol), GraphQL, XML-RPC, and JSON-RPC. REST is the most commonly used protocol today, especially for web services.

  5. What tools are commonly used for API testing?
    Popular API testing tools include Postman (for manual testing), SoapUI (for SOAP and REST APIs), JMeter (for load testing), RestAssured (for automated testing with Java), and Katalon Studio (for automated API testing).

HTTP Basics

  1. What are HTTP methods used in RESTful APIs?
    The most commonly used HTTP methods in RESTful APIs are:

    • GET: Retrieves data from the server.
    • POST: Sends data to the server to create a resource.
    • PUT: Updates an existing resource or creates a resource.
    • DELETE: Removes a resource from the server.
    • PATCH: Partially updates a resource.
    • OPTIONS: Returns the allowed HTTP methods for a resource.
    • HEAD: Similar to GET but does not return the response body.

  2. What is the difference between PUT and POST?

    • PUT is idempotent, meaning repeated requests with the same data will result in the same outcome (e.g., updating a resource).
    • POST is used to create new resources and is not idempotent; repeated requests may create multiple resources.

  3. What is the difference between 200 and 201 status codes?

    • 200 OK: Indicates that the request was successful and the server returned the expected response.
    • 201 Created: Indicates that the request was successful and resulted in the creation of a new resource.

  4. What are common HTTP status codes in API testing?
    Some of the most common HTTP status codes are:

    • 200 OK: Successful request.
    • 201 Created: Resource created successfully.
    • 400 Bad Request: The request could not be understood due to malformed syntax.
    • 401 Unauthorized: Authentication is required or has failed.
    • 404 Not Found: The resource could not be found.
    • 500 Internal Server Error: A generic error message when the server fails.

  1. What is an API endpoint?
    An API endpoint is a specific URL or URI that exposes a resource or service in an API. It defines the path to access a particular function or set of data.

REST vs. SOAP

  1. What is REST?
    REST (Representational State Transfer) is an architectural style that uses stateless communication, typically over HTTP, to perform operations on resources using standard HTTP methods. It uses lightweight formats such as JSON or XML for data exchange.

  2. What is SOAP?
    SOAP (Simple Object Access Protocol) is a protocol for exchanging structured information using XML. It requires a predefined message format and supports both request/response and asynchronous communication.

  3. Key differences between REST and SOAP?

    • REST is lightweight, stateless, uses HTTP methods, and typically uses JSON for data exchange.
    • SOAP is heavier, requires strict message formats (usually XML), supports stateful operations, and is more rigid in its standards.

Request/Response Basics

  1. What is a request payload?
    A request payload is the body of the request message that carries the data sent from the client to the server, especially in POST and PUT requests. It can include data in JSON, XML, or other formats.

  2. What is a response body?
    The response body is the part of the HTTP response that contains the data returned by the server after processing the request. It is usually in JSON or XML format and contains the requested resource or error message.

  3. What is a query parameter?
    Query parameters are key-value pairs appended to the URL in a GET request. They are used to filter, sort, or modify the request, such as /api/users?age=30&gender=male.

  4. What are headers in API requests?
    Headers are metadata sent along with the request or response. They contain information such as content type (e.g., Content-Type: application/json), authorization credentials (Authorization: Bearer <token>), and other configuration settings.

  5. What is JSON and why is it used?
    JSON (JavaScript Object Notation) is a lightweight, easy-to-read format for exchanging data between a client and server. It is preferred in API communication because of its simplicity and readability compared to XML.

  6. What is XML?
    XML (eXtensible Markup Language) is a markup language used to encode documents and data in a format that is both human-readable and machine-readable. It is commonly used in SOAP APIs.

Authentication and Security

  1. What is API authentication?
    API authentication verifies the identity of the client or user attempting to access an API. This can be done using API keys, OAuth tokens, or basic authentication (username/password).

  2. What is OAuth?
    OAuth is an open-standard authorization protocol that allows clients to access resources without exposing user credentials. It uses tokens for secure access, making it ideal for API authentication.

  3. What is the difference between authentication and authorization?

    • Authentication verifies who you are (e.g., username and password).
    • Authorization determines what resources you can access after authentication (e.g., permissions or roles).

  4. How do you test API security?
    Security testing includes verifying authentication mechanisms, ensuring data is encrypted in transit (using HTTPS), checking for common vulnerabilities (like SQL injection and XSS), and ensuring proper access control to resources.

  5. What is token-based authentication?
    Token-based authentication uses tokens (such as JWT or OAuth tokens) for secure communication. The client receives a token after successful login, which is used for subsequent requests instead of sending login credentials each time.

Advanced Concepts

  1. What is API rate limiting?
    API rate limiting restricts the number of API requests a client can make within a certain period (e.g., 100 requests per minute). This prevents overloading the server and ensures fair usage.

  2. What is CORS?
    CORS (Cross-Origin Resource Sharing) is a mechanism that allows resources to be requested from a domain different from the one serving the web page. It helps to secure cross-origin requests.

  3. What is API mocking?
    API mocking simulates an API's behavior without actually calling the backend service. It is useful for testing when the API is under development or unavailable.

  4. What is Swagger?
    Swagger is a framework for designing, documenting, and consuming RESTful APIs. It provides a user interface to interact with API endpoints and can generate client libraries based on the API definition.

  5. What is the difference between synchronous and asynchronous APIs?

    • Synchronous APIs require the client to wait for the response before proceeding.
    • Asynchronous APIs allow the client to make a request and proceed without waiting for the response; the response is delivered when the operation completes.

  6. What are idempotent methods in REST?
    Idempotent methods are those that return the same result when called multiple times with the same input. For example, GET and PUT are idempotent, while POST is not.

  7. What is HATEOAS in RESTful APIs?
    HATEOAS (Hypermedia as the Engine of Application State) is a constraint of REST where the server provides links in its responses, guiding the client on possible actions it can take next.

Testing Scenarios

  1. What is a positive test case in API testing?
    A positive test case tests an API with valid input data, ensuring it performs as expected, such as returning the correct status code and data.

  2. What is a negative test case in API testing?
    A negative test case involves testing an API with invalid input data or conditions to ensure the API handles errors correctly and returns appropriate error messages or status codes.

  3. How do you handle API exceptions?
    API exceptions are handled by testing the error response from the server, including status codes, error messages, and logging. Ensure the API returns the correct error response in case of invalid input or server failure.

  4. What is contract testing in API testing?
    Contract testing verifies that the API conforms to its defined contract, such as ensuring the API request and response formats align with the specification.

  5. How do you validate API performance?
    Performance

testing of APIs is conducted using tools like JMeter or LoadRunner to measure response times, throughput, and scalability. The API should handle the expected load efficiently.

API-Test Case Example



  1. How do you test APIs without a GUI?
    API testing without a GUI can be done using tools like Postman or by writing automation scripts with libraries like RestAssured, HTTPClient, or Request in various programming languages.

Practical Concepts

  1. What is Postman?
    Postman is a popular tool for testing and developing APIs. It allows you to create and send HTTP requests, automate tests, and analyze responses, making it an essential tool for manual API testing.

  2. What is RestAssured?
    RestAssured is a Java library that simplifies the process of testing REST APIs. It allows you to write tests in Java to validate the correctness of RESTful API responses, such as checking status codes and response body data.

  3. What is API chaining?
    API chaining refers to using the output from one API request as input for another. For example, the authentication token returned from a login API can be used in subsequent API requests.

  4. What is a smoke test in API testing?
    A smoke test is a preliminary test to ensure the basic functionality of an API is working, such as checking if the endpoints are reachable and if the expected response is returned.

  5. What is data-driven testing in APIs?
    Data-driven testing involves running the same API test with different sets of input data to ensure the API behaves as expected under various conditions.

  6. What is the purpose of API documentation?
    API documentation defines how the API should be used, including endpoints, request/response formats, error codes, authentication details, and usage examples. It is crucial for developers to understand how to interact with the API.

Debugging & Monitoring

  1. How do you debug API issues?
    Debugging API issues involves checking the server logs, inspecting headers, verifying request/response formats, and ensuring that the correct HTTP methods are used. You can also use tools like Postman and cURL to debug requests.

  2. What is API monitoring?
    API monitoring involves tracking the health and performance of APIs in a production environment. This includes monitoring uptime, response times, and detecting anomalies in real-time to ensure consistent service.

  3. What is an API gateway?
    An API gateway is a server that acts as a single entry point for client applications to access various backend services. It routes requests to the appropriate service and can also handle security, load balancing, and caching.

Miscellaneous

  1. What is gRPC?
    gRPC is a high-performance, open-source RPC (Remote Procedure Call) framework developed by Google. It uses HTTP/2 for better performance and supports bi-directional streaming.

  2. What is a Webhook?
    A webhook is an HTTP-based callback that allows one system to notify another system about an event, such as an API sending a message to another service when an action occurs.

  3. How do you test API versioning?
    API versioning can be tested by verifying that different versions of an API return correct and backward-compatible responses. The versioning can be tested by appending version identifiers (e.g., /v1/users) to API endpoints.

  4. What is the importance of response time in API testing?
    Response time is critical in API testing because it impacts the performance of an application. Fast response times lead to better user experiences and are crucial for scalability.



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