Art of Codeartofcode
Lesson 17 of 18

22. Message Queues

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The picture to keep in your head
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22.1 What are producers and consumers?
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22.2 Why go async?
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22.3 What are the delivery guarantees?
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22.4 How do acks and visibility timeouts work?
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22.5 What are retries and dead-letter queues?
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22.6 What is an idempotent consumer?
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22.7 How does ordering work?
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22.8 What is backpressure?
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22.9 How do you scale consumers?
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22.10 Log (Kafka) vs queue (RabbitMQ, SQS): what's the difference?
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22.11 How do delayed and scheduled jobs work?
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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
18
Code lab
22. Message Queues

Sources

3 min read+10 XP

Source notes

  • Amazon SQS: visibility timeout default 30 seconds, maximum 12 hours; message retention default 4 days, maximum 14 days; delivery delay up to 15 minutes; standard queues at-least-once with best-effort ordering; FIFO queues with a 5-minute deduplication interval and message group IDs; dead-letter queues via redrive policy. Amazon SQS Developer Guide, checked 2026-10-08.
  • Apache Kafka: partitions, per-partition ordering, consumer groups, offsets, default log retention of 168 hours (7 days), idempotent producers and transactions. Apache Kafka documentation.
  • RabbitMQ: consumer acknowledgements, prefetch, requeueing and dead-letter exchanges. RabbitMQ documentation.
  • Little's law (L = λW): John D. C. Little, "A Proof for the Queuing Formula: L = λW", Operations Research, 1961.
  • Order rates, processing times, gateway limits and worker counts are illustrative assumptions; arithmetic computed 2026-10-08.
  • Level 3 and Level 4 code: tested 2026-10-08 (code/ch22_queues.py, all checks pass).