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Lesson plan 27: Replication strategies
Trace leader/follower and leaderless read/write acknowledgment paths.
Placement: Part 5: Correctness across machines.
Prerequisites: Lesson 26: Distributed consistency, plus the earlier dependency chain.
Learning outcomes
- Trace leader/follower and leaderless read/write acknowledgment paths.
- Explain the additional ordering, membership, and failure assumptions needed beyond quorum intersection.
- Follow lag, failover, repair, and divergent versions through an explicit example.
- Separate a general mechanism, a documented implementation choice, and an illustrative numerical assumption.
Teaching sequence
- Establish the motivating situation: Acknowledged writes disappear after promotion because the selected follower never received them.
- Introduce the vocabulary and the correctness obligation before an optimization. Define leader/follower, quorums, leaderless.
- Develop the mechanism: Trace write acknowledgment and read paths; state why quorum intersection alone is insufficient for full linearizability.
- Follow the complete worked example: For N=5, W=3, and R=3, read and write sets intersect in at least one replica. Explain the assumptions, intermediate units, and limiting interpretation.
- Extend the scenario to changed load, skew, or failure, explaining sync, async, stale reads, repair, divergence.
- Connect the mechanism to a documented implementation and an alternative with a different workload fit. Close by stating the remaining cost and the next lesson dependency.
Required chapter material
Use a complete execution trace or worked calculation, supported by an annotated static diagram or table. Include language-neutral pseudocode or labeled SQL when it clarifies the mechanism. State preconditions, ownership, visibility, and failure boundaries for algorithms. Describe operational costs as well as the normal path. All questions raised in the narrative must be resolved in the text.
Visual explanation
Replica timelines mark receipt, persistence, acknowledgment, application, and promotion; a quorum-set diagram demonstrates intersection without claiming it alone proves linearizability. The accompanying prose explains the arrows, state changes, or table entries so the lesson remains understandable in a plain Markdown reader.
Editorial checks
Confirm that the numerical example agrees with its units and assumptions. Check that a faster path preserves the stated contract, or explicitly identify the changed guarantee. Keep hypothetical figures separate from product measurements. Explain both an appropriate use and a limitation of the case design. Check terminology against the glossary and links against the course index.