A check belongs where the behavior or evidence it tests is visible. In Philip Shaw’s September 21, 2026 Sentinel dev diary, a mismatch between a driver specification and its implementation exposes the cost of getting that placement wrong: a module-level test cannot prove host behavior, and a register’s integrity test cannot prove what is in an external repository.
How a specification gap became an implementation decision
The driver contract described a module that emitted observations, with MQTT callbacks versus polling treated as an internal choice. But a later interface block specified a host-invoked read(points) → observations method. Those instructions implied different control flows: the module pushes observations, or the host asks for them. The host implemented pull because the signature appeared explicitly in a code block, leaving the conflict unresolved in the specification.
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The diary’s compact lesson is that “A signature beats a sentence, every time, and it beats it silently.” The problem was not simply that one sentence was overlooked: the build-step exit criteria did not require anyone to record which interpretation had been selected. Shaw argues that a specification gap, an interim implementation decision, and the work to amend the specification should be tracked as separate project records.
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In the implementation described, a push adapter retained only the latest frame for each point, and the host sampled that value at the point’s assigned interval. When no interval was declared, the host used 30 seconds. This can be adequate for a sustained reading, such as a level that remains stable long enough to be sampled. It is not equivalent to a queue of events.
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A button press that begins and ends between host ticks can be overwritten or cleared before the next sample. The host may never observe it, even though the latch itself reports no drop. That matters because a separate buffering statement described the bounded disk queue as the only intentional drop point. As Shaw recounts it, transient loss at the latch did not fit that stated contract.
Choose a test location by observability and ownership
A useful placement test is to ask whether the component under test can see the state or behavior named by the requirement, and whether that component owns it. The diary describes later documents adding epoch rejection, publish scope, and buffer bounds to the conformance harness, then correcting the allocation: only publish scope belonged in the module harness. Buffering and epoch fencing were host responsibilities and needed host-side checks.
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| Claim or behavior | What the check needs to observe | Appropriate check location |
|---|---|---|
| Publish scope | Whether the module publishes only within its allowed scope | Module conformance harness |
| Buffer bounds | The host-owned buffer and its behavior | Host test suite |
| Epoch rejection | The host’s handling of epochs | Host test suite |
| Register structure and references into the register | The register’s own structure and internal links | Register integrity checks |
| Which repository file contains a test | The external repository and its files | A repository-aware check or human audit |
This distinction is about what a check can establish, not how sophisticated the test is. A module harness cannot prove behavior that only exists in the host. As Shaw puts it, “A test that proves the machinery works is not a test that proves anything runs it.”
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Why register integrity tests cannot validate repository claims
The diary reports 57 register-integrity tests at the time of the audit. They could identify structural problems in the register and references pointing into it. They could not establish a claim about the location of a test in another repository, because that repository was outside the checks’ scope.
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One audit found stale cross-reference anchors. A test that followed those anchors could still pass by returning to the register itself; it did not thereby verify the external artifact the reference was meant to identify. The same limitation applied to roadmap and driver-host docstring statements that described publish scope and buffer bounds as module checks in the harness from the beginning: the integrity tests did not verify those claims about test history or placement. Shaw says buffer bounds had never been in that harness.
For claims about external files, the check must inspect the repository or a person must audit it. A self-check that validates only a document’s own shape cannot certify the truth of every statement the document makes about the outside world.
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Definition of done should follow the owning document
In the diary’s build-step example, status was tied to the exit criteria in the step’s owning document. When that document gained additional criteria, the step turned amber until the newly required work was complete. Shaw presents this as status reflecting the current definition of done, rather than preserving a green status against an older checklist.
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For a specification gap, that means the record should distinguish the unresolved contract, the choice made to keep implementation moving, and the amendment or verification work still required. If the owning document changes what completion means, the status should reflect the changed criteria.
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Built and tested is not the same as called
The entry also describes availability and quarantine message paths that had been constructed and tested against a broker but were not called by the driver host. A passing test of a constructed path establishes that the path can work under the test conditions; it does not establish that production code invokes it.
Shaw reports that availability was subsequently connected at the core and driver lifecycle edges. Quarantine remained unresolved because its producer was not buildable. This makes execution a separate placement question: find where the real application calls the path, then check the behavior at that boundary, rather than treating construction or isolated testing as proof of integration.
Two timing choices in the project
Fixed command deduplication window
The project used a fixed five-minute command deduplication window rather than deriving the window from the command deadline. The stated reason was to avoid driver-side timing behavior that could let a redelivered command pulse a relay twice. This is a project-specific design choice described by Shaw, not an externally established recommendation for all command systems.
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For a point with no declared interval, the host’s sampling interval was 30 seconds. That value describes the implementation in this diary; it should not be read as a general interval suitable for other systems, particularly where transient events must not be missed.
A practical placement check
- Observability: Can the component running the check see the relevant state or event?
- Ownership: Does the module own the timer or state, or does the host/core?
- Scope: Is the claim about the register itself, or an external artifact such as a repository file?
- Execution: Is the path merely constructible, or does production code actually call it?
- Definition of done: Does the project status reflect the current exit criteria in the owning document?
Shaw’s September 21, 2026 diary is a project case study and process argument, not an independently evaluated study of how often these failures occur or how much a particular fix improves reliability. Its central test is still practical: place each requirement in the contract that governs it, and place each check where the relevant behavior can actually be observed.
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