Idempotent operations produce the same result regardless of how many times they're called. This is essential for reliable APIs that handle retries and network failures.
Why Idempotency Matters#
Network Failures:
- Request sent but response lost
- Client retries not knowing if it succeeded
- Without idempotency: duplicate charges, orders, etc.
With Idempotency:
- Client can safely retry
- Server detects duplicate requests
- Same result returned for repeated calls
HTTP Method Idempotency#
Naturally Idempotent:
- GET: Read operations
- PUT: Replace resource
- DELETE: Remove resource
- HEAD: Read headers
- OPTIONS: Read options
NOT Idempotent by Default:
- POST: Create resource (needs idempotency key)
- PATCH: May or may not be idempotent
Idempotency Keys#
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Payment Processing Example#
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Database-Level Idempotency#
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Optimistic Locking#
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Event Processing Idempotency#
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Retry Handling#
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Best Practices#
Implementation:
✓ Generate keys client-side (UUIDs)
✓ Include user context in key
✓ Set appropriate TTL (24-48 hours)
✓ Handle "processing" state
Error Handling:
✓ Return same response for duplicates
✓ Handle parameter mismatches
✓ Log duplicate requests
✓ Clean up on failure
Performance:
✓ Use fast storage (Redis)
✓ Index idempotency columns
✓ Set TTL to auto-cleanup
✓ Consider eventual consistency
Conclusion#
Idempotency is essential for reliable APIs. Use idempotency keys for non-idempotent operations, store responses for replay, and design your data models to handle duplicates gracefully. This enables safe client retries and improves system reliability.