Day 8Level 1 — Core Mental Models
Idempotency & Delivery Guarantees: At-Least-Once, Exactly-Once, Effectively-Once
Learn: 45 minMap: 30 minDesign: 60 min
Core Concepts (5)
- 1.At-least-once + idempotency = effectively-once (the only practical guarantee)
- 2.Idempotency keys: client-generated, server-validated, deduplication window
- 3.Exactly-once delivery is impossible in async systems — don't promise it
- 4.Outbox pattern: write to DB + message atomically, relay publishes
- 5.Duplicate handling: detect, deduplicate, or make operations commutative
Map to Your Experience
Where in your system do you rely on "exactly-once"? Replace with idempotency keys + at-least-once. What changes?
Design Brief: Idempotency Design for a Critical Operation
Requirements
- •Design idempotency key scheme
- •Specify storage and TTL for deduplication
- •Handle retry storms and client crashes
- •Document failure modes and recovery
Constraints
- •No distributed transactions
- •At-least-once transport assumed
Must Cover
idempotency-key-designdeduplication-windowretry-behaviorfailure-modes
Trade-off Axis
idempotency complexity vs duplicate risk
Referenced Patterns
Competencies Advanced
data state → L3reliability ops → L3
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