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Part 6 of 7 · Shift swap broker series ~5 min read

What the shift swap broker costs

This system barely touches a model at all — eligibility is set arithmetic over a rota. What it does do is send a lot of messages, and if those messages are texts rather than push notifications they are the entire bill. Here is where each cent goes.

Key takeaways

  • About $2 a month at 40 swaps. Roughly $4 at 150 swaps.
  • One Bedrock read per swap is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • SMS is the whole variable cost. Push notifications through a web app make this system essentially free to run.
  • 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 shift swap broker at three volumesA stacked bar chart with three bars, one per volume tier: 15 swaps totalling about $2, 40 swaps totalling about $2, and 150 swaps totalling about $4. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per swap, 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$2$4$6$8~$1.5315 swaps~$2.0540 swaps~$4.26150 swapsBedrock — one read per swapSES — asks, results, receiptsSNS SMS — offers to phonesS3 + 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 swap — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 40 swapsHow it scales
Bedrock read$0.01Linear. One call per swap, roughly 1,800 in and 200 out tokens.
SNS SMS$0.72Linear at $0.0180 per swap.
SES$0.04Linear. About 4.5 messages per swap.
DynamoDB + S3$0.29Storage 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 SMS band assumes texts to every offer recipient. A business whose staff use a saved home-screen link with web push pays essentially nothing for messaging, which takes the whole system to about a dollar a month.

The three ways this bill surprises you

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

  • SMS for everything. An offer loop that texts three people per batch across two batches is six messages per swap, and at a hundred and fifty swaps a month that is the dominant line. A web push, or email for anything more than a day out, removes most of it.
  • Re-running eligibility on every batch. Compute the ordered list once when the shift is offered up, not per batch. Recomputing also risks the list changing mid-loop, which produces somebody being offered a shift and then silently dropped.
  • Log retention left at never. A busy rota produces a lot of small events, and without a retention setting the logs will out-cost every other line.

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. 15 swaps, about $2. 40 swaps, about $2. 150 swaps, about $4. 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~$115 swaps~$240 swaps~$2150 swaps~$4One 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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