Stock and equipment

There is no expired column

/inventory — reagent and control lots, with what is left of each.

Expiry is a comparison between a date and today, so it is asked rather than stored. A column would be a second answer to a question the date already settles, and it would be wrong every day nobody ran the job that updates it.

On hand is the same shape: a SUM over an append-only movement trail, computed and stored nowhere. Receipts, issues and adjustments are appended; the quantity is what they add up to.

A stored quantity plus a movement trail is two writers on one number, and they disagree the first time one of them fails halfway.

An expired lot cannot establish control

This is a patient-safety statement, not a housekeeping one. QC run against an expired lot does not establish that an analyser is in control — so results produced against that lot are not releasable, and the release gate enforces it rather than trusting the page to say it.

An analyser's status is a fold, not a field

/equipment — the instrument register.

Status is derived from the service trail and written by one function through a trigger. Nobody sets it directly, and no client role can:

• calibration_verified returns an analyser to service;
• taken_out_of_service removes it;
• preventive_service moves nothing — a serviced instrument is not thereby a verified one.

That last one is the interesting case, and it is why this value is a cache with rules rather than a plain derivation: the answer depends on the order of what happened, and it is filtered on, so it is stored — but only ever by the fold, never by a caller.

A cache earns its place when the value has rules, not merely when it is derived.

An out-of-service analyser cannot establish control either

The same rule as the expired lot, for the same reason, and it is enforced in the same place: results produced on an instrument that is out of service are not releasable.