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Part 6 of 7 · Cold chain monitor series ~5 min read

What the cold chain monitor costs

There is no model in this system and almost nothing is sent. The volume is readings: a sensor at five-minute intervals produces about nine thousand a month. Twenty sensors is a restaurant group or a small distribution site. Here is where each cent goes.

Key takeaways

  • About $1 a month at 20 sensors. Roughly $1 at 80 sensors.
  • One Bedrock read per sensor is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • Readings are the entire variable cost, and they are cheap even at eighty sensors.
  • The duplicate test runs before the read, so resends are free.
  • The three real risks: a retry loop, storage nobody expires, and a bigger model than the job needs.

The bill at three volumes

These are US East prices at the time of writing, at three volumes that bracket most small businesses. Find the bar closest to your own and read across.

Monthly cost of the cold chain monitor at three volumesA stacked bar chart with three bars, one per volume tier: 6 sensors totalling about $1, 20 sensors totalling about $1, and 80 sensors totalling about $1. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per sensor, in teal. Then SES for the messages, in pink. Then S3 and DynamoDB storage in green. Then a fixed orange band for Secrets Manager and AWS Budgets, which is eighty-six cents at every volume. Then a grey band for Lambda, SQS and CloudWatch. A note says the read is the only bar that grows with the business and the orange band never moves.$0$0.5$1$1.5$2~$1.256 sensors~$1.2720 sensors~$1.3680 sensorsBedrock — one read per sensorSES — asks, results, receiptsS3 + DynamoDBFixed — Secrets Manager, BudgetsLambda, SQS, CloudWatchThe read is the only bar that grows with the business. The orange fixed band never moves.
Fig 1. The monthly bill at three volumes. The teal band — one model read per sensor — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 20 sensorsHow it scales
Bedrock read$0.00Linear. One call per sensor, roughly 1,800 in and 200 out tokens.
SES$0.01Linear. About 0 messages per sensor.
DynamoDB + S3$0.28Storage grows with what you retain, not with throughput.
Lambda + SQS$0.12Linear, and effectively free at this scale.
CloudWatch$0.16Flat, if you set retention. Unbounded if you do not.
Secrets Manager$0.40Flat. One secret, $0.40 a month.
AWS Budgets$0.46Flat. Two actions, so you find out before the bill does.

There is no read line and effectively no messaging line: alarms are rare by design. Reading writes and their storage are the whole bill.

The three ways this bill surprises you

Every one of these has happened to somebody, and all three are cheap to prevent.

  • One write per reading, forever. Batch readings per sensor per hour into a single item and the write count drops by an order of magnitude.
  • Never expiring at full resolution. Keep full resolution for two years, then roll up to per-hour minimum, maximum and mean. Do not delete breach windows.
  • Alarm delivery over a single channel. Not a cost problem, but an SMS bill is cheaper than a freezer of stock, and email alone will not wake anybody.

What it costs when nothing happens

This matters more than the headline number for a seasonal business. In a month with nothing to process the bill is the fixed band: Secrets Manager at forty cents, AWS Budgets at forty-six, and a few cents of storage. Call it a dollar. There is no instance to stop and nothing to remember to turn off.

The monthly bill at four volumes plus one failure modeA horizontal row of five boxes. Quiet month, about one dollar. 6 sensors, about $1. 20 sensors, about $1. 80 sensors, about $1. And one bad retry loop, about two hundred dollars. A note says four of these are the design working and the fifth is a missing dead-letter queue.THE BILL, AT A GLANCEQuiet month~$16 sensors~$120 sensors~$180 sensors~$1One bad loop~$200Four of these are the design working. The fifth is a missing dead-letter queue.
Fig 2. The bill at a glance, including the one that is not a volume at all. A retry loop with no dead-letter queue costs more than every legitimate use of the system put together.
  • Management
  • Analytics
  • Front-end & mobile

Set these on day one

  • A dead-letter queue on every SQS queue, with a maximum receive count of three.
  • Thirty-day retention on every CloudWatch log group. There is no default that is safe.
  • An S3 lifecycle rule on the object prefix, tiering at 90 days and expiring at your actual record-keeping horizon.
  • Two AWS Budgets actions — one that emails at half your expected spend, one at double it. The second is how you find out about a loop in an hour instead of a month.
  • Provisioned concurrency: none. Nothing here is latency-sensitive enough to justify paying for a warm function.

Next: the same system drawn for engineers — service names, resource identifiers, IAM scopes, table schemas and the model id.

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