Art of Codeartofcode
Lesson 16 of 17

14. Replication

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The picture to keep in your head
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14.1 Why keep copies of the same data?
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14.2 How does leader-follower replication work?
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14.3 Should the leader wait for followers before confirming a write?
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14.4 What is replication lag, and why does it happen?
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14.5 How do you stop lag from confusing users?
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14.6 How do you use read replicas in practice?
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14.7 What happens when the leader fails?
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14.8 What is split brain, and how is a leader safely chosen?
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14.9 When would you have more than one leader?
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14.10 How does leaderless replication work, and what are quorums?
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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
14. Replication

Sources

3 min read+10 XP

Source notes

  • Leader-follower replication, write-ahead log shipping and synchronous standby settings, and pg_stat_replication: PostgreSQL documentation (high availability, replication). MySQL binary log and semi-synchronous replication: MySQL documentation.
  • Leaderless replication, quorums (N, W, R), hinted handoff, read repair and Merkle-tree anti-entropy: Giuseppe DeCandia et al., "Dynamo: Amazon's Highly Available Key-value Store", SOSP 2007; Apache Cassandra documentation for consistency levels.
  • Raft: Diego Ongaro and John Ousterhout, "In Search of an Understandable Consensus Algorithm", USENIX ATC 2014.
  • GitHub split-brain incident: GitHub, "October 21 post-incident analysis" (https://github.blog/news-insights/company-news/oct21-post-incident-analysis/), describing a network partition, Orchestrator promoting primaries in another region, data reconciliation, and the decision to stop cross-region automatic promotion.
  • Patroni (PostgreSQL high availability) and Orchestrator (MySQL topology and failover): project documentation.
  • All bookstore numbers are illustrative assumptions; arithmetic computed 2026-10-08.
  • Level 3 and Level 4 code: tested 2026-10-08 (code/ch14_replication.py, all checks pass).