Backend Study Lab · Games
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Delivery Guarantee Timeline

Trace producer, broker, consumer, acknowledgment, crash, and redelivery boundaries to name the real delivery guarantee.

Concept
Message delivery semantics and acknowledgment timing
Difficulty
Advanced
Play time
7-10 minutes
Path
Async Workflows

Interactive scenario

Playable game area: Make the next backend decision

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Concept notes

Why this decision matters

Delivery Guarantee Timeline is focused practice for Message delivery semantics and acknowledgment timing. It explains why each answer is safe, risky, or production-ready.

Follow each message through the ambiguity windows that define its real processing guarantee.

Learning objectives

  • Distinguish delivery from processing and side effects.
  • Place acknowledgments around durable work.
  • Name exactly-once boundaries precisely.

How to play

  1. Inspect the event trace.
  2. Choose the guarantee-preserving response.
  3. Review why each crash window changes the outcome.

Scoring

  • Correct timeline decisions add 250 points.
  • Misses enter local review.
  • A perfect run scores 1,000.

Backend concept notes

Acknowledgment placement determines whether work can be lost or repeated.

Broker transactions do not automatically include external databases, payments, or email.

Common mistakes

  • Acknowledging before durable work.
  • Treating timeouts as definite failure.
  • Promising exactly once without naming a boundary.

Review misses from this game

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Use tools from the main site when you want to inspect real inputs.

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Time
8-11 minutes
Concept
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Time
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Concept
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  • queues
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Outbox Relay Recovery

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Time
8-12 minutes
Concept
Transactional outbox and relay recovery
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FAQ

Short answers for how this game fits backend interview and study practice.

Why can a message be delivered twice?

A crash after the side effect but before acknowledgment leaves the broker unable to prove completion.

Is at-least-once unsafe?

It is safe when consumers and downstream effects are designed for duplicate attempts.

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