🧠 What is Dedupe Storage / Idempotency Keys?

Deduplication storage is a technique where a system stores unique keys (idempotency keys) to prevent processing the same request or message multiple times.

• Helps achieve idempotent operations
• Often used in payments, APIs, messaging systems, and batch processing
• Essential when at-least-once delivery is used

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⚡ Why It’s Needed

In distributed systems, failures and retries can cause:

• Duplicate API requests
• Duplicate messages from queues
• Double payments or orders

Solution: Generate a unique idempotency key per request, store it, and check before processing.

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🧩 How Idempotency Keys Work

1️⃣ Request Flow

1. Client generates a unique idempotency key (e.g., UUID)
2. Sends request to server with key

3. Server checks dedupe storage:

   • Key exists → return previous result
   • Key does not exist → process request, store key + result

2️⃣ Storage Options

• In-memory cache (Redis) → fast, short TTL
• Database table → persistent, for long-term idempotency
• Dedicated dedupe storage service

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💻 Example: Payment API (Java + Redis)

String idempotencyKey = request.getHeader("Idempotency-Key");

// Check Redis
if (redis.exists(idempotencyKey)) {
    return redis.get(idempotencyKey); // Return previous response
}

// Process payment
PaymentResult result = processPayment(request);

// Store result in Redis with TTL (e.g., 24h)
redis.set(idempotencyKey, result, 24, TimeUnit.HOURS);

return result;

• Redis TTL ensures storage doesn’t grow indefinitely
• Guarantees no duplicate processing even if client retries

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🔑 Best Practices

1. Unique & unpredictable keys

   • Usually UUIDv4 or hash of request body + user ID

2. Short TTL for ephemeral requests

   • e.g., 24 hours for payment retries

3. Atomic storage check-and-set

   • Prevent race conditions in high concurrency

4. Store result, not just key

   • Allows returning exact same response for retries

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🧩 Analogy

Think of it like a ticketing system 🎟️:

• Each ticket has a unique number (idempotency key)
• If someone tries to buy again with the same ticket number, the system says:

“You already bought this ticket — here’s your receipt”

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✅ Summary

Concept	Purpose	Storage	Use Case
Idempotency Key	Prevent duplicate processing	Redis, DB, dedicated store	Payments, API requests, messaging
Deduplication Storage	Tracks processed keys/results	Fast & atomic	At-least-once messaging, retries, micro-batching

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