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Lesson plan 22: Alternative execution strategies
Compare optimistic validation, timestamp ordering, deterministic scheduling, and single-writer execution.
Placement: Part 4: Transactions, concurrency, and recovery.
Prerequisites: Lesson 21: Multiversion concurrency control, plus the earlier dependency chain.
Learning outcomes
- Compare optimistic validation, timestamp ordering, deterministic scheduling, and single-writer execution.
- Identify read/write conflicts and distinguish ordered execution from replicated durable agreement.
- Estimate wasted retry work and explain how contention changes a strategy’s useful operating range.
- Separate a general mechanism, a documented implementation choice, and an illustrative numerical assumption.
Teaching sequence
- Establish the motivating situation: A flash sale creates retries under optimistic execution while independent tenants have abundant parallelism.
- Introduce the vocabulary and the correctness obligation before an optimization. Define optimistic control, timestamp ordering, deterministic execution.
- Develop the mechanism: Compare validation sets, timestamp rules, ordered scheduling, and serialized execution under the same workload.
- Follow the complete worked example: With independent success probability 0.2, expected attempts per success are 5. Explain the assumptions, intermediate units, and limiting interpretation.
- Extend the scenario to changed load, skew, or failure, explaining single-writer, contention.
- 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
Competing timelines show optimistic aborts versus ordered execution for the same hot resource; the illustration states where validation or ownership is required. 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.