Advanced API Testing

• Advanced API testing goes beyond basic request-response validation. It involves assessing an API’s functionality, performance, security, and reliability across real-world and edge-case scenarios. Involves validating complex workflows, handling batch operations, and performing API chaining to ensure comprehensive coverage.

Core Areas of Advanced API Testing

Functional Testing

• Positive Tests - Validate the correct output for valid inputs.

• Negative Tests - Verify how APIs handle invalid data, missing fields, or incorrect HTTP methods.

• Boundary Testing - Test the limits. ie: Maximum length of a string, Number of items in a payload.

• Data-driven Testing - Execute the same tests with multiple data sets. ie: Test user creation with different user roles.

Performance Testing

• Load Testing - Evaluate API behavior under expected user loads. ie: 1000 concurrent users.

• Stress Testing - Push the API beyond its capacity to identify breaking points.

• Spike Testing - Simulate sudden traffic spikes to evaluate performance.

• Latency Testing - Measure response time and ensure SLAs (Service Level Agreements) are met.

Security Testing

• Authentication and Authorization - Test APIs using valid, invalid, and expired credentials (OAuth 2.0, JWT, API keys).

• Injection Attacks - Check against SQL, XML, and JSON injections.

• Data Exposure - Ensure sensitive information. ie: Passwords and Credit Card numbers are encrypted and not exposed.

• Rate Limiting and Throttling - Validate protections against abuse and denial-of-service (DoS) attacks.

Integration and Contract Testing

• Contract Validation - Ensure APIs conform to OpenAPI (Swagger) specifications.

• Backward Compatibility - Ensure APIs maintain compatibility with older client versions.

• Third-party Integration - Validate API behavior with external services. ie: Payment gateways, SMS.

Complex API Testing Scenarios

Multi-Step Workflows (API Chaining)

• APIs often rely on sequential steps where the output from one request feeds the next.

User Registration Workflow

• Create User (POST /users)

• Verify Email (POST /users/{id}/verify)

• Login (POST /auth/login)

• Fetch User Profile (GET /users/{id})

Test Considerations

• Ensure each step depends on the correct prior response.

• Validate the user’s state at every stage. ie: Is Email verified field updated?.

• Handle partial failures. ie: If the current step fails, can you retry the previous step?.

Stateful and Session-Based Testing

• APIs that require session management ie: Shopping carts or Authentication demand multi-step verification.

User Session Workflow

• Login (POST /login) => Receive Token

• Access Secure Data (GET /profile with Token)

• Logout (POST /logout) => Invalidate Session

Test Considerations

• Check token expiration and session timeout.

• Verify session continuation across APIs.

• Simulate concurrent logins and session revocation.

• Store and reuse session tokens across requests.

• Verify behavior after logout. ie: 401 Unauthorized response.

Batch Operations (Bulk Requests)

• Batch APIs process multiple records in a single request.

Bulk Product Creation Workflow

POST /products/bulk
{
  "products": [
    { "name": "Item A", "price": 100 },
    { "name": "Item B", "price": 200 }
  ]
}

Test Considerations

• Partial Success - Ensure the API reports which items failed.

• Size Limits - Validate behavior on large payloads.

• Consistency - Ensure all or none are processed. ie: Atomicity.

• Performance - Measure response times for large batches.

• Edge Cases

  • Test with Empty payloads.

  • Test with Duplicates.

  • Test with Invalid items mixed with valid ones.

• Idempotency and Retry Mechanisms - APIs should handle retries without duplicating operations.

Payment Processing Workflow

• POST /payments with Idempotency-Key

Test Considerations

• Retry request during failure and ensure only one payment is processed.

• Ensure repeated requests don’t cause side effects.

• Verify unique operation via Idempotency-Key header.

Retry Valid Requests

• Verify consistency under network failures.

Test Considerations

• Handle concurrent requests and race conditions.

• APIs that handle simultaneous operations may suffer from race conditions.

• Testing tools such as JMeter or Cypress can be used to simulate concurrent users.

Inventory System Workflow

• Two users attempt to purchase the last item simultaneously.

• Ensure only one purchase succeeds.

Test Considerations

• Locking mechanisms (Optimistic / Pessimistic locking).

  • Optimistic lock - Data is read without locking it. Before updating, the application checks whether another transaction has modified the data.

  • Pessimistic lock - Locks data as soon as it's read to prevent other transactions from modifying it.

• Concurrent user simulation with multiple threads.

• Verify consistency under simultaneous operations.

Data Consistency and Atomic Transactions

• Ensure APIs maintain data integrity across operations.

Bank Transfer Workflow

• Debit User A’s Account

• Credit User B’s Account

Test Considerations

• Ensure atomicity either both steps succeed or fail together.

• Simulate system failures in the middle of a transaction.

• Verify rollback on errors.

Error Scenarios and Failure Handling

• APIs must gracefully handle errors with appropriate status codes.

Error Types

• 4xx: Client Errors. ie: 400 Bad Request, 403 Forbidden.

• 5xx: Server Errors. ie: 500 Internal Server Error.

Test Considerations

• Invalid payloads and missing fields.

• Unauthorized requests and expired tokens.

• Fault injection. ie: Simulate backend failure.

tip

Complete list of HTTP Response status codes via MDN.

Pagination, Sorting, and Filtering

• APIs handling large datasets often use pagination.

Product Listing Workflow

• GET /products?limit=10&offset=0&sort=name

Test Considerations

• Validate proper data segmentation.

• Edge case testing the Last page, Empty result, and Negative offsets.

• Measure API efficiency.

• Verify sorting orders.

• Simulate large-scale dataset navigation.

Time-Dependent APIs (Asynchronous Processing)

• Some APIs operate asynchronously. ie: Job Queues.

Image Upload and Processing Workflow

• POST /uploads => Returns 202 Accepted

• GET /uploads/{id}/status => Check progress

Test Considerations

• Handle asynchronous responses by using JS Async / Await.

• Polling mechanisms and timeouts.

• Simulate delayed responses.

• Eventual consistency. ie: Final vs. Intermediate state.

• Verify completion within Service level agreements.

External Dependencies (Third-Party Integration)

• APIs interacting with third-party services add another layer of complexity.

Payment Gateway Integration Workflow

• Initiate Payment

• Verify Webhook

• Check Reconciliation Status

Test Considerations

• Mock external services for isolated tests.

• Validate asynchronous callbacks.

• Handle third-party errors and rate limits.

Best Practices for Complex API Testing

• Automate Scenarios - Use tools like Postman, Cypress, and REST Assured.

• Environment Isolation - Use separate environments for DEV, QA, and Production testing.

• Data Management - Reset test data or use ephemeral environments for complex workflows.

• Contract Testing - Ensure consistency using OpenAPI or Swagger definitions.

• Continuous Testing - Integrate with CI/CD pipelines (Jenkins, GitHub Actions).

• Monitor Logs - Track API logs for better debugging and observability.

Module Review

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