The Problem of Duplicate Transactions
A payment system suffered an issue where some users were charged twice when retrying a failed transaction.
The problem? The API wasn’t idempotent, meaning repeated requests caused unintended side effects.
The solution? Idempotency, retries, and deduplication mechanisms to ensure API reliability.
What is Idempotency?
Idempotency ensures that multiple identical API requests produce the same result without unintended side effects.
Example: A user clicks “Pay Now” twice due to a slow response. Without idempotency, they might be charged twice.
Key Benefits:
Prevents duplicate actions (e.g., duplicate payments, order submissions).
Ensures consistency across distributed systems.
Improves API reliability under network failures.
How to Implement Idempotency in APIs
Use an Idempotency Key:
Clients send a unique key with each request.
Servers store the key and ensure each request is processed only once.
Store Processed Requests:
Maintain a cache/database of previously processed requests.
If a duplicate request arrives, return the stored response.
Example: Stripe’s API requires an Idempotency-Key for payment transactions.
POST /payment
Headers:
Idempotency-Key: 12345-xyz
Handling Retries – Ensuring API Reliability
APIs should handle transient failures gracefully using retry mechanisms.
Common Failures:
Network timeouts – Temporary disconnections.
Server overload – Too many simultaneous requests.
Rate limits – Exceeding API request quotas.
Best Practices for Retrying Requests
Exponential Backoff:
Increase wait time between retries (e.g., 1s → 2s → 4s → 8s).
Prevents overwhelming the server.
Retry on Specific Errors:
Retry only on 5xx errors (server issues), not 4xx errors (client mistakes).
Use Circuit Breakers:
Stop retrying if failures continue to prevent system overload.
Request Deduplication – Eliminating Unintended Duplicates
Sometimes, network retries or client-side bugs cause duplicate API calls.
Example: A message is accidentally sent twice in a chat app due to a retry.
Techniques to Prevent Duplication
✔ Idempotenct Keys: Ensure each request is processed once. ✔ Unique Request Identifiers: Use a transaction ID to track processed requests. ✔ Database Constraints: Enforce uniqueness at the database level (e.g., unique order IDs).
Real-World Use Cases
1. Payment Systems
Idempotency prevents double charges.
Retries ensure transactions complete during failures.
2. E-Commerce Orders
Deduplication prevents multiple order placements.
Idempotency ensures users don’t receive duplicate shipments.
3. Messaging & Notifications
Message brokers deduplicate events to prevent repeated alerts.
Exponential backoff retries undelivered messages.
Conclusion
Designing reliable APIs requires idempotency, retry strategies, and request deduplication.
Idempotency ensures repeated requests don’t cause unintended side effects.
Retries with exponential backoff prevent unnecessary server load.
Deduplication eliminates duplicate transactions and messages.
Next, we’ll explore Graph Databases & NoSQL Alternatives – Neo4j, MongoDB, DynamoDB, Time-Series DBs.
