Skip to content
allanninal.dev/build
Collaborate

Part 6 of 7 · Training reminder bot series ~5 min read

What the training reminder bot costs

This system barely uses a model at all — everything in it is set arithmetic over a few hundred rows, run nightly. The bill is dominated by email and by the fixed band. Here is where each cent goes.

Key takeaways

  • About $1 a month at 100 assignments. Roughly $2 at 400 assignments.
  • One Bedrock read per assignment is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • There is almost no model use here. Deriving a required-training list is set arithmetic, and set arithmetic should be code.
  • 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 training reminder bot at three volumesA stacked bar chart with three bars, one per volume tier: 30 assignments totalling about $1, 100 assignments totalling about $1, and 400 assignments totalling about $2. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per assignment, 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$1$2$3$4~$1.330 assignments~$1.45100 assignments~$2.07400 assignmentsBedrock — one read per assignmentSES — 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 assignment — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 100 assignmentsHow it scales
Bedrock read$0.04Linear. One call per assignment, roughly 1,800 in and 200 out tokens.
SES$0.07Linear. About 1.8 messages per assignment.
DynamoDB + S3$0.30Storage 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.

The read cost here is nominal: a model is used only to match an externally worded qualification name against your requirement list, which happens a handful of times a year.

The three ways this bill surprises you

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

  • Recomputing everything on every change. A nightly full recompute over a few hundred people is fine; the same recompute fired on every attribute edit during a busy HR afternoon is not, and it will also send duplicate assignments.
  • Assignment duplicates from a bad completion check. Checking the completion date instead of the validity end creates a fresh assignment for training somebody did last month, which is the fastest way to lose people’s trust in the system.
  • Log retention left at never. A nightly job producing nothing most nights will still fill a log group forever without a retention setting.

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. 30 assignments, about $1. 100 assignments, about $1. 400 assignments, about $2. 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~$130 assignments~$1100 assignments~$1400 assignments~$2One 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.

All posts