API Testing: A Practical Guide

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What is an API?

  • An API (Application Programming Interface) is a software intermediary that enables two applications to communicate with each other. It comprises a number of subroutine definitions, logs, and tools for creating application software.
  • APIs carry data from servers (database) to the client (user-interface), meaning you can carry data from the backend and populate it to your front-end.
  • APIs are a collection of functions that are executable by other functions of the software application.

API vs. Web Service

  • All web services are APIs, but not all APIs are web services.
  • Web services might not contain all the specifications and cannot perform all the tasks that APIs would perform.
  • A Web service uses only three styles of use: SOAP, REST and XML-RPC for communication whereas an API may be exposed in multiple ways.
  • Web services require a network to operate, while APIs don’t need a network for operation.

Why is API Testing Important?

  • API testing verifies APIs that drive application logic.
  • It validates data flow between client and server.
  • API Testing ensures data accuracy.
  • It improves application performance.
  • API testing reduces bugs early.
  • APIs are the backbone of modern applications, making testing crucial.
  • API testing identifies issues before they reach the user.
  • API contracts are designed for this process, which helps how data is stored (like JSON, XML, Form-Data etc.).
  • Testers don't require the UI to be finished so that testing can start, because only the backend setup or API contracts are needed.

API Testing Objectives

  • Validate that the API performs expected operations.
  • Ensure the API is secure from unauthorized access.
  • Test API performance under high usage.
  • Verify proper error codes and messages for failed requests.

API Architectural Styles

  • REST (Representational State Transfer): An architectural style for developing web services which exploit the ubiquity of the HTTP protocol and uses HTTP methods to define actions. REST revolves around resources where every component is a resource that can be accessed through a shared interface using standard HTTP methods. Key characteristics of REST include it being stateless, having a layered system, and cacheability.
  • Other API architecture styles include: SOAP (Simple Object Access Protocol), GraphQL, WebSocket, Webhook, RPC/gRPC, and MQTT. 

Common REST API Methods

  • GET: Retrieves data from the server. Not necessarily requiring a request body. After request completes response is received. Use GET requests for safe and idempotent operations.
  • POST: Sends data to the server to create a new resource. Requires a Request body and provides a response once the request is completed. POST is neither safe nor idempotent, so be cautious about retrying failed POST requests.
  • PUT: Updates an existing resource or creates a resource if it does not exist. Requires Request body and provides response once request completed. PUT should be idempotent, meaning repeated requests with the same data should produce the same result.
  • DELETE: Removes a resource from the server. It can work with & without request body. A response is received once the request is completed. DELETE operations should also be idempotent.

API Testing Types

  • Functional Testing: Ensures APIs perform their intended functions.
  • Performance Testing: Tests response time, throughput, and scalability.
  • Security Testing: Validates authentication, authorization, and data protection.
  • Contract Testing: Checks API request-response structure against specifications.
  • Negative Testing: Tests API behavior with invalid inputs or scenarios.
  • Other types: UI testing, Runtime/ Error Detection, Penetration testing, Fuzz testing, Interoperability and WS Compliance testing.

API Testing Process

  1. Understand Requirements: Review API documentation to understand endpoints, request methods, and response formats. Identify key functionalities to test.
  2. Test Planning: Define test objectives, tools, and scenarios. Prioritize critical API functionalities for testing.
  3. Test Case Design: Write test cases for valid inputs, invalid inputs, boundary values, and performance scenarios. Include assertions for status codes, response times, and data validation.
  4. Test Execution: Run test cases using tools like Postman or automated scripts. Document results and track issues for resolution.

API Test Design Principles

  • Setup: Create objects, start services, initialize data, etc.
  • Execution: Steps to apply API or the scenario, including logging.
  • Verification: Oracles to evaluate the result of the execution.
  • Reporting: Pass, failed or blocked.
  • Clean up: Pre-test state.

API Testing Checklist

  • Validate HTTP response codes (e.g., 200 OK, 404 Not Found, 500 Internal Server Error).
    • 1xx: Informational Responses.
    • 2xx: Successful Codes.
      • 200 OK: The request was successful.
      • 201 Created: The request has been fulfilled, and a new resource was created.
      • 202 Accepted: The request has been accepted for processing, but the processing has not been completed.
      • 204 No Content: The server successfully processed the request, but there is no content to return.
    • 3xx: Redirection.
      • 302 Found: The resource is temporarily located at a different URL.
      • 304 Not Modified: The resource has not been modified since the last request, so the client can use the cached version.
    • 4xx: Client Error Responses.
      • 400 Bad Request: The server could not understand the request due to invalid syntax.
      • 401 Unauthorized: Authentication is required, and the user is not authenticated.
      • 403 Forbidden: The request is valid, but the server refuses to authorize it.
      • 404 Not Found: The server cannot find the requested resource.
      • 409 Conflict: The request could not be completed due to a conflict with the current state of the resource.
      • 429 Too Many Requests: The user has sent too many requests in a given amount of time.
    • 5xx: Server Error Responses.
      • 503 Service Unavailable: The server is not ready to handle the request, often due to being overloaded or down for maintenance.
      • 504 Gateway Timeout: The server was acting as a gateway or proxy and did not get a response in time from the upstream server.
  • Test API endpoints (GET, POST, PUT, and DELETE).
  • Verify JSON/XML schema consistency.
  • Ensure data accuracy matches database records.
  • Test authentication and authorization (e.g., JWT tokens).
  • Perform negative testing with invalid inputs and methods.
  • Load-test APIs for performance under traffic.
  • Validate meaningful error messages.
  • Ensure security with checks against SQL injection and XSS.
  • Test timeout and retry mechanisms for reliability.
  • Verify that the API request parameters are correctly passed to the API.
  • Verify that the API endpoint URL is correct.
  • Verify that the API response headers are correct.
  • Verify that the API returns an error message if the request payload is missing.
  • Verify that the API returns an error message if the requested resource does not exist.
  • Verify that the API returns a success message if the resource is created successfully.
  • Verify that the API returns a success message if the resource is updated successfully.
  • Verify that the API returns a success message if the resource is deleted successfully.
  • Verify that the API returns a success message if the resource is retrieved successfully.
  • Check for proper access controls to prevent unauthorized access to sensitive resources.
  • Validate the API's compliance with secure data transmission practices (e.g., HTTPS).

API Testing Tools

  • Postman: User-friendly for manual and automated API tests.
  • SoapUI: Ideal for SOAP and REST API functional testing.
  • RestAssured: Java-based library for automating API tests.
  • JMeter: Performance testing for APIs under heavy load.
  • Swagger (OpenAPI): Simplifies API documentation and testing.
  • Other tools mentioned include: Newman, Burp Suite, JIRA.

API Testing Challenges

  • Incomplete or unclear API documentation.
  • Handling dynamic input parameters.
  • Ensuring security.
  • Testing APIs in isolation.
  • Initial setup of API testing.
  • Updating the schema of API testing.
  • Testing parameter combinations and sequencing the API calls.
  • Validating parameters.
  • Test data management.

API Testing Best Practices

  • Start with simple functionality tests before diving into complex scenarios.
  • Use tools to automate repetitive tests and ensure consistency.
  • Validate all aspects, including headers, payloads, status codes, and response times.
  • Test with real-world data to ensure reliability.
  • Include negative and boundary testing scenarios.
  • Ensure your test cases cover all scenarios, including happy paths and edge cases.
  • API tests should not depend on each other to avoid false negatives.

API Documentation

  • API documentation is a complete, accurate technical writing giving instructions on how to effectively use and integrate with an API.
  • It is a compact reference manual that has all the information needed to work with the API, and helps you answer all the API testing questions with details on functions, classes, return types, arguments, and also examples and tutorials.
  • API documentation templates that are commonly used are Swagger, Miredot, Slate, FlatDoc, API blueprint, RestDoc, and Web service API specification.

Free Public APIs for Testing

  • JSONPlaceholder API: A free online REST API for testing and prototyping.
    • Base URL: https://jsonplaceholder.typicode.com/
    • Example Endpoints:
      • GET https://jsonplaceholder.typicode.com/posts
      • POST https://jsonplaceholder.typicode.com/posts
  • ReqRes: A hosted REST API that simulates user management scenarios.
    • Base URL: https://reqres.in/
    • Example Endpoints:
      • GET https://reqres.in/api/users
      • POST https://reqres.in/api/users
  • HTTP Stat.us: A simple service to return different HTTP status codes for testing.
    • Base URL: https://httpstat.us/
    • Example Endpoints:
      • GET https://httpstat.us/200
      • GET https://httpstat.us/404
  • The Dog API: A fun API that returns pictures and information about dogs.

Quick Summary: Key aspects of API testing

The following is a concise summary of the essential elements of API testing, based on the information provided above.

  • What is an API? An API (Application Programming Interface) enables two applications to communicate. It carries data from servers to the user interface and is a collection of functions executable by other software functions.
  • API vs. Web Service: Every web service is an API, but not every API is a web service. Web services require a network, while APIs can be on- or offline.
  • Why is API Testing Important? API testing validates data flow, ensures accuracy, improves performance, and reduces bugs. It's crucial because APIs are the backbone of modern applications.
  • API Testing Objectives: Validate performance, ensure security, test performance under high usage, and verify error codes.
  • API Architectural Styles: REST is a common style using HTTP methods. Other styles include SOAP, GraphQL, WebSocket, RPC/gRPC, and MQTT.
  • Common REST API Methods:
    • GET: Retrieves data. Safe and idempotent.
    • POST: Creates new resources. Not safe or idempotent.
    • PUT: Updates existing resources. Should be idempotent.
    • DELETE: Removes resources. Should be idempotent.
  • API Testing Types: Functional, performance, security, contract, and negative testing. Also UI, runtime/error detection, penetration, fuzz, and interoperability testing.
  • API Testing Process: Understand requirements, plan tests, design test cases, and execute tests.
  • API Test Design Principles: Setup, execution, verification, reporting, and clean up.
  • API Testing Checklist:
    • Validate HTTP response codes. (e.g., 200 OK, 404 Not Found, 500 Internal Server Error).
    • Test API endpoints.
    • Verify JSON/XML schema.
    • Ensure data accuracy.
    • Test authentication and authorization.
    • Perform negative testing.
    • Load-test APIs.
    • Validate error messages.
    • Ensure security with checks against SQL injection and XSS.
    • Test timeout and retry mechanisms.
  • API Testing Tools: Postman, SoapUI, RestAssured, JMeter, Swagger. Also Newman, Burp Suite, and JIRA.
  • API Testing Challenges: Incomplete documentation, dynamic parameters, security, isolation, setup, schema updates, parameter combinations, test data management.
  • API Testing Best Practices: Start simple, automate, validate all aspects, use real-world data, include negative and boundary testing, and ensure independence of tests.
  • API Documentation: Crucial for effective use and integration, including functions, classes, and arguments. Use tools like Swagger, Miredot, and Slate.
  • Free Public APIs for Testing: JSONPlaceholder, ReqRes, HTTP Stat.us, and The Dog API.

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