Art of Codeartofcode
Lesson 13 of 14

23. Pub/Sub and Fan-Out

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The picture to keep in your head
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23.1 What's the difference between a queue and pub/sub?
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23.2 What are topics and subscribers?
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23.3 What are consumer groups?
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23.4 What are the fan-out strategies?
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23.5 What is event-driven architecture?
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23.6 How do you handle duplicates and ordering?
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23.7 How do you publish events reliably (the outbox pattern)?
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Memory card and where the analogy breaks
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Practice: five levels
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Interview questions
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Solutions
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Sources
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Code lab
23. Pub/Sub and Fan-Out

Sources

3 min read+10 XP

Source notes

  • Pub/sub, topics, subscriptions and consumer groups: Apache Kafka documentation; Google Cloud Pub/Sub documentation; Amazon SNS documentation (SNS to SQS fan-out).
  • Fan-out on write with special handling for high-follower accounts in social timelines: Raffi Krikorian (Twitter), "Timelines at Scale", QCon San Francisco 2012 (InfoQ).
  • Transactional outbox pattern: Chris Richardson, microservices.io, "Pattern: Transactional outbox"; Debezium documentation, outbox event router.
  • Event notification vs event-carried state transfer: Martin Fowler, "What do you mean by 'Event-Driven'?", 2017.
  • Follower counts, release volumes, write rates and reader numbers are illustrative assumptions; arithmetic computed 2026-10-08.
  • Level 3 and Level 4 code: tested 2026-10-08 (code/ch23_pubsub.py, all checks pass).