API testing is a type of software testing that validates application programming interfaces (APIs) directly, rather than testing through the user interface. It checks functionality, reliability, performance, and security of APIs — including REST and SOAP services — and is a core skill for QA engineers.

What are the most common HTTP methods tested in API testing exams?

The five most-tested HTTP methods are GET (retrieve data), POST (create a resource), PUT (replace a resource), PATCH (partial update), and DELETE (remove a resource). Exam questions often ask you to identify the correct method for a given scenario.

What HTTP status codes should I know for an API testing exam?

Key status codes include: 200 OK, 201 Created, 204 No Content, 400 Bad Request, 401 Unauthorized, 403 Forbidden, 404 Not Found, 405 Method Not Allowed, 409 Conflict, 422 Unprocessable Entity, 429 Too Many Requests, 500 Internal Server Error, and 503 Service Unavailable.

What tools are commonly used in API testing?

Postman is the most widely used GUI tool for manual API testing, while REST-assured (Java) and requests (Python) are popular for automation. Other common tools include SoapUI (SOAP/REST), Insomnia, Newman (Postman CLI), Karate, and k6 for performance/load testing of APIs.

What is the difference between REST and SOAP APIs?

REST (Representational State Transfer) is an architectural style that uses standard HTTP methods and typically exchanges JSON data. SOAP (Simple Object Access Protocol) is a protocol that uses XML messages and supports built-in security (WS-Security) and ACID transactions. REST is lighter-weight and more common in modern web services; SOAP is preferred in enterprise and financial systems.

What authentication methods are tested in API testing exams?

Common authentication types include API Keys (sent in headers or query strings), Basic Auth (Base64-encoded credentials), Bearer Token / JWT (JSON Web Token), OAuth 2.0 (authorization framework), and HMAC signature-based auth. Exam questions often ask you to identify the correct auth header format or the security risks of each approach.

API Testing Automation

Ace your API testing interview or QA certification with free practice questions on REST, HTTP methods, status codes, Postman, SOAP, and test automation.

API - Testing By David ChenApr 24, 202629 min read

XML API for Testing

The Cloud Storage XML API programmatically provides a RESTful interface for managing Cloud Storage data. XML supports more operations than the JSON API does. XML API for testing allows a quick and easy way to test your application by accessing all of your content in Cloud Storage easily and quickly when you are on the go or at work or want to avoid writing some complex code.

This Cloud Storage XML API is only available as part of the Google Cloud Platform Console. It's ideal for tasks that require processing large amounts of data. It's designed to be used in a variety of applications, including backend services to support mobile or web applications, workflow processing systems that need to process input files along with their associated metadata in an automated way, or media asset management systems where users aggregate information from multiple sources into an accessible database format.

What is API Testing?	API Testing validates the behavior, reliability, performance, and security of application programming interfaces to ensure they return correct responses and handle requests properly.
Why is API Testing important in modern applications?	API Testing is critical because most modern systems are service-based, and failures at the API layer can break integrations, microservices, and user-facing workflows even when the UI looks stable.
What skills are measured in an API Testing certification exam?	The API Testing exam typically measures your understanding of HTTP, REST, authentication, test design, automation tools, and how to analyze responses, logs, and defects in API-driven systems.
Who should take the API Testing exam?	The API Testing exam is ideal for QA engineers, SDETs, developers, and test leads who work with backend services and want to prove their skills in automated and manual API Testing.

What is API Testing?

API Testing validates the behavior, reliability, performance, and security of application programming interfaces to ensure they return correct responses and handle requests properly.

Why is API Testing important in modern applications?

API Testing is critical because most modern systems are service-based, and failures at the API layer can break integrations, microservices, and user-facing workflows even when the UI looks stable.

What skills are measured in an API Testing certification exam?

The API Testing exam typically measures your understanding of HTTP, REST, authentication, test design, automation tools, and how to analyze responses, logs, and defects in API-driven systems.

Who should take the API Testing exam?

The API Testing exam is ideal for QA engineers, SDETs, developers, and test leads who work with backend services and want to prove their skills in automated and manual API Testing.

📄API Testing Practice Test Questions and Answers PDF

Api Testing Tools - API Testing study guide

Error Codes in API Testing

Error codes in API testing are a pain to deal with. They're not always obvious and can appear during any phase of the testing process — during testing, staging, or production. Here are the common API testing error codes: For more preparation, try our ISTQB practice test. For more preparation, try our AWS practice test.

Error Code	Description
badRequest	The API request is incorrectly constructed or invalid. As a result, the API server was unable to comprehend the request.
notModified	The If-None-Match header's condition was not satisfied. This answer indicates that the requested document has not been changed and that a cached response should be obtained. Examine the If-None-Match HTTP request header value.
unknownApi	The API that the request is attempting to access is not recognized.
temporaryRedirect	Resend your request to the URL given in the Location header of this response to have it processed.
notFound	The requested operation failed because the required resource could not be located.

What are the challenges faced in API testing?

When evaluating API implementations, software testers encounter several distinct challenges. Here are the common challenges in API testing:

Parameter Combination - APIs manage system communication by assigning data values to parameters and transferring those parameters via data requests. All potential parameter request combinations must be tested in the API to test for faults.
Parameter Selection - API calls result in varying levels of information and functionality. The system response itself may be available in the API documentation, but the number of parameters required to make the call differs. Additional parameters may be necessary to create a more robust response.
Call Sequencing - API calls may be sequenced in any order, meaning earlier API calls must complete successfully before subsequent API calls can be made. This presents additional test challenges when working with multiple APIs. Errors may occur due to timing issues, logic errors, or difficulty debugging the call sequence.

API Practice Test Questions

Prepare for the API - Testing exam with our free practice test modules. Each quiz covers key topics to help you pass on your first try.

API Authentication and Security

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API Documentation & Specification

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API REST API Fundamentals

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API REST API Testing

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API Testing Questions and Answers

About the Author

David ChenMS, CISSP, CEH, AWS-SAA, Azure Expert

Senior Cloud Architect & Cybersecurity Certification Trainer

Stanford University

David Chen holds a Master of Science in Computer Science from Stanford University and has earned over 25 professional certifications across AWS, Microsoft Azure, Google Cloud, cybersecurity, and enterprise architecture domains. He works as a solutions architect and now focuses on helping IT professionals pass cloud, security, and technical certification exams.