Course contents
On this page
Lesson plan 23: Concurrent engine internals
Distinguish logical transaction locks from short-lived structural latches.
Placement: Part 4: Transactions, concurrency, and recovery.
Prerequisites: Lesson 22: Alternative execution strategies, plus the earlier dependency chain.
Learning outcomes
- Distinguish logical transaction locks from short-lived structural latches.
- Explain atomic publication, memory ordering, cache-line sharing, and safe reclamation under the stated memory model.
- Identify why lock-free access does not automatically mean bounded latency or safe lifetime management.
- Separate a general mechanism, a documented implementation choice, and an illustrative numerical assumption.
Teaching sequence
- Establish the motivating situation: A reader follows a pointer to a node that another thread has already freed.
- Introduce the vocabulary and the correctness obligation before an optimization. Define locks, latches, atomics.
- Develop the mechanism: Separate logical protection from physical structural safety; show publication and deferred reclamation.
- Follow the complete worked example: Two 8-byte counters can share one illustrative 64-byte cache line despite independent logical ownership. Explain the assumptions, intermediate units, and limiting interpretation.
- Extend the scenario to changed load, skew, or failure, explaining memory ordering, concurrent indexes, false sharing, reclamation, NUMA.
- 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 concurrent-reader timeline separates acquiring a pointer, protecting its lifetime, observing initialized state, and allowing reclamation. 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.