API Reliability: Scenario-Based Questions

Idempotency means that multiple identical requests produce the same effect. This is critical for reliability, especially in network retries, payment APIs, and distributed systems.

🔄 Why Idempotency Matters

• Prevents accidental duplication (e.g., duplicate charges).
• Enables safe retries in case of timeouts or client failures.
• Improves user experience and consistency in distributed environments.

🧱 Idempotency in HTTP

• GET, HEAD, PUT, DELETE are naturally idempotent.
• POST is not, but can be made idempotent with proper design.

🔧 Techniques to Achieve Idempotency

• Use an Idempotency-Key header for POST requests.
• Store keys + outcomes in a database/cache for deduplication.
• Ensure side effects (writes, emails, etc.) only occur once.

📦 Examples

• Stripe API: Requires Idempotency-Key for payments to avoid double charges.
• Create User: Return existing user if same email is sent twice.

✅ Best Practices

• Expire idempotency keys after a safe window (e.g., 24h).
• Document idempotency behavior in API docs.
• Return consistent response structure across identical retries.
• Log and monitor for duplicate key usage.

🚫 Common Pitfalls

• Storing partial results on first failure and retrying without validation.
• Confusing client retries vs retry logic in downstream services.
• Not persisting key outcomes — causing duplicate effects if server restarts.

📌 Final Insight

Idempotency adds safety and predictability to APIs. It’s a design discipline that enables better retries, user experience, and system resilience — especially under load or failure.