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✅ API Testing Guide

API testing ensures your services behave as expected under different conditions. It's essential for validating business logic, security, performance, and integration.

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🧪 Common Types of API Testing

1. Functional Testing

• Ensures the API endpoints behave as expected.
• Verifies response codes, headers, payloads.

🔧 Example:

• GET /users returns 200 and JSON body with user data.

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2. Validation Testing

• Confirms the correctness of data sent and received.
• Checks schema, data types, field validation.

🔧 Example:

• POST /users with a missing email should return 400 Bad Request.

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3. Load Testing

• Measures how the API handles a high volume of requests.
• Detects bottlenecks or limits.

🔧 Tools: JMeter, k6, Artillery.

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4. Stress Testing

• Pushes the API beyond expected limits to find the breaking point.
• Helps determine system robustness.

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5. Security Testing

• Ensures the API is protected from attacks like XSS, SQL injection, CSRF, etc.
• Tests auth tokens, rate limiting, role-based access.

🔧 Tools: OWASP ZAP, Postman, Burp Suite.

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6. Integration Testing

• Verifies the interaction between different APIs or between API and DB/external services.

🔧 Example:

• Ensure creating a user also initializes settings in another microservice.

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7. Unit Testing

• Tests individual functions or endpoints in isolation.
• Simulates dependencies via mocks or stubs.

🔧 Tools: Jest (JS), PHPUnit (Laravel), JUnit (Java), pytest (Python).

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8. Regression Testing

• Ensures new changes haven't broken existing functionality.
• Run after updates, especially before deployment.

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9. Runtime/Error Testing

• Validates proper error handling during edge cases or invalid input.
• Checks if error messages, codes, and logging are correct.

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10. UI/API Contract Testing

• Ensures the frontend and backend communicate under a shared schema.
• Useful in microservices or frontend/backend decoupled architectures.

🔧 Tools: Pact, Postman schema validation, Swagger/OpenAPI validators.

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🧰 Popular API Testing Tools

Tool	Use Case
Postman	Manual, automated functional tests
REST Assured	Java-based API testing
Supertest	Node.js/Express testing
Insomnia	Exploratory testing
JMeter	Load/stress testing
k6	Performance testing (code-based)
Swagger	Contract validation
SoapUI	SOAP/REST enterprise testing

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✅ Best Practices

• Test both positive and negative scenarios
• Keep environments isolated (staging vs prod)
• Use mock servers for early testing
• Automate testing in CI/CD pipelines
• Validate status codes, headers, and response time
• Use assertions for exact field values
• Include token/authorization scenarios

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🔄 Example (Postman Test Script)

// Tests tab in Postman
pm.test("Status code is 200", function () {
    pm.response.to.have.status(200);
});

pm.test("Body contains user ID", function () {
    var jsonData = pm.response.json();
    pm.expect(jsonData.id).to.exist;
});

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