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Lesson plan 21: Multiversion concurrency control
Determine version visibility from a coherent transaction snapshot.
The guided lesson now introduces one mechanism. This original plan covers the wider topic; follow the deeper-reading links at the end of the guided lesson for the remaining material.
Placement: Part 4: Transactions, concurrency, and recovery.
Prerequisites: Lesson 20: Lock-based concurrency control, plus the earlier dependency chain.
Learning outcomes
- Determine version visibility from a coherent transaction snapshot.
- Construct a write-skew history and explain why snapshot isolation differs from serializability.
- Explain version retention and reclamation costs caused by old readers.
- Separate a general mechanism, a documented implementation choice, and an illustrative numerical assumption.
Teaching sequence
- Establish the motivating situation: Two on-call doctors each observe the other available and both go off call.
- Introduce the vocabulary and the correctness obligation before an optimization. Define versions, snapshots, visibility.
- Develop the mechanism: Trace visible versions and read/write dependencies; explain why disjoint writes can violate a shared invariant.
- Follow the complete worked example: 10000 updates/s times 200 bytes times a 600 s retention horizon is 1.2 GB of version bytes. Explain the assumptions, intermediate units, and limiting interpretation.
- Extend the scenario to changed load, skew, or failure, explaining snapshot isolation, write skew, serializability, garbage collection.
- 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 version chain and two-transaction roster trace show disjoint writes that violate a cross-row invariant despite snapshot-consistent reads. 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.