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Lesson plan 28: Consensus and leadership
Explain replicated-state-machine safety through terms, durable votes, log matching, and commitment.
Placement: Part 5: Correctness across machines.
Prerequisites: Lesson 27: Replication strategies, plus the earlier dependency chain.
Learning outcomes
- Explain replicated-state-machine safety through terms, durable votes, log matching, and commitment.
- Trace a Raft election and distinguish leader selection, replication, commitment, and application.
- Explain fencing and membership changes without treating any reachable node as authoritative.
- Separate a general mechanism, a documented implementation choice, and an illustrative numerical assumption.
Teaching sequence
- Establish the motivating situation: An old leader accepts requests while a majority elects a replacement.
- Introduce the vocabulary and the correctness obligation before an optimization. Define Raft, Paxos, replicated state machines.
- Develop the mechanism: Trace a replicated log through failure, election, commitment, and application; distinguish log replication from consensus.
- Follow the complete worked example: A five-voter majority is three, tolerating two unavailable voters under the protocol assumptions. Explain the assumptions, intermediate units, and limiting interpretation.
- Extend the scenario to changed load, skew, or failure, explaining elections, terms, fencing, split brain, membership.
- 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
A five-replica log diagram traces an uncommitted suffix through leadership change and identifies the majority-backed committed prefix. 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.