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Lesson plan 42: Observability and verification
Connect tracing, resource profiles, latency distributions, and correctness histories to a stated SLO.
Placement: Part 7: Building and operating applications on these engines.
Prerequisites: Lesson 41: Safe evolution, plus the earlier dependency chain.
Learning outcomes
- Connect tracing, resource profiles, latency distributions, and correctness histories to a stated SLO.
- Explain workload, crash, fault-injection, and recovery tests and what each can establish.
- Distinguish a passing example from proof of correctness across all possible executions.
- Separate a general mechanism, a documented implementation choice, and an illustrative numerical assumption.
Teaching sequence
- Establish the motivating situation: A latency dashboard hides the storage queue that is delaying durable acknowledgments.
- Introduce the vocabulary and the correctness obligation before an optimization. Define tracing, profiling, SLOs.
- Develop the mechanism: Connect trace spans to resource metrics and correctness histories; separate measured observations from interpretation.
- Follow the complete worked example: A 99.9% monthly availability objective allows 43.2 min in an illustrative 30-day month. Explain the assumptions, intermediate units, and limiting interpretation.
- Extend the scenario to changed load, skew, or failure, explaining saturation, load tests, crashes, fault injection, histories.
- 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 request trace connects client timing to queue, execution, storage, and delivery stages; a fault-history table separates invocation, outcome, and uncertainty. 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.