Cold chain monitor
A small system that records temperature properly, distinguishes an excursion from a breach, catches the sensor that has quietly stopped working, and produces a tamper-evident record. It deliberately does not decide whether stock is safe, because that decision belongs to a person and needs to be signed. Seven posts on the same system, one diagram at a time, with a cost breakdown and an engineering reference at the end.
- 01
A cold chain monitor on AWS for a few dollars a month
The whole system on one page — record, judge, escalate — and the fail-closed rule that separates a useful monitor from a decorative one.
- 02
How a reading becomes a record
How often to sample, why the sensor’s own clock matters, how missing readings are recorded as events, and why the store is append-only.
- 03
How an excursion is told from a breach
Why a single reading out of range means nothing, how duration rules are set, why cumulative time matters as well as consecutive, and the case for a small number of loud alarms.
- 04
How a broken sensor is caught
Why a suspiciously stable reading is a failure signal, how drift is detected without a reference, the calibration schedule, and comparing units against their own history.
- 05
Who decides what happens to the stock
Why the disposal decision stays with a named person, what evidence the decision needs, how the decision is recorded, and what a good inspection export looks like.
- 06
What the cold chain monitor costs
About $1 a month. One Bedrock read per sensor is the only line that grows; the queue, the table, the mail and the storage are rounding errors. Plus the three ways the bill could surprise you.
- 07
Engineering reference: the cold chain monitor architecture
Same system, drawn purely for engineers. Service names, region, Lambda inventory, IAM scopes, the schemas and the exact model id.
Frequently asked questions
- What is a cold chain monitor?
- A small serverless system that records temperature readings from chilled and frozen storage, detects genuine breaches, catches failing sensors, and keeps a tamper-evident record for inspection.
- What is the difference between an excursion and a breach?
- An excursion is a temperature going out of range, which happens whenever a door opens. A breach is an excursion that lasted long enough to matter, and the post on this covers how that line is drawn.
- What happens if the sensor stops sending data?
- It is treated as a breach, not as everything being fine. Missing data is the most common failure and the most commonly mishandled.
- Does it decide whether stock is safe?
- No. It presents the evidence and a person decides and signs. That separation is deliberate and is the subject of part five.
- What does it cost to run?
- A couple of dollars a month. See part six.