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System Design

Idempotent Consumers

One-Liner

Consumers that can safely process the same message multiple times.

What It Is

A design approach where message handlers tolerate duplicate delivery without incorrect behavior.

Why It Exists

At-least-once delivery is common; duplicates are inevitable.

How It Works

  • Deduplication keys
  • Idempotent writes
  • State-based guards

Tradeoffs

Pros

  • Increased reliability
  • Simplified retry logic

Cons

  • Additional state or storage
  • More complex handlers

Failure Modes

  • Partial side effects
  • Incorrect deduplication windows

Interview Traps

  • Assuming exactly-once delivery
  • Confusing idempotency with deduplication

Real-World Usage

Payment processing, event-driven systems, background jobs.

Anti-Patterns

  • Relying on broker guarantees alone
  • Message retries
  • Transactions