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Part 6 of 7 · Parking permit issuer series ~5 min read

What the parking permit issuer costs

There is no model in this system and almost nothing happens on most days: an allocation round once a year and a usage review each quarter. Two hundred permits is a large site. Here is where each cent goes.

Key takeaways

  • About $2 a month at 200 permits. Roughly $2 at 800 permits.
  • One Bedrock read per permit is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • Messaging is annual allocation plus quarterly usage reviews, averaged across the year.
  • 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 parking permit issuer at three volumesA stacked bar chart with three bars, one per volume tier: 60 permits totalling about $1, 200 permits totalling about $2, and 800 permits totalling about $2. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per permit, 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~$1.3360 permits~$1.55200 permits~$2.5800 permitsBedrock — one read per permitSES — 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 permit — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 200 permitsHow it scales
Bedrock read$0.00Linear. One call per permit, roughly 1,800 in and 200 out tokens.
SES$0.11Linear. About 0.6 messages per permit.
DynamoDB + S3$0.31Storage 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: nothing here calls a model. Messaging is the allocation round, waiting list movements and quarterly usage prompts.

The three ways this bill surprises you

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

  • Storing every barrier event indefinitely. Aggregate to a count per permit per quarter and discard the detail; the detail is an attendance record.
  • Running an allocation pass continuously. It is an annual event with a waiting list in between.
  • Building enforcement machinery first. Not a cloud cost, and the largest cost in the whole project. Part 5 covers it.

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. 60 permits, about $1. 200 permits, about $2. 800 permits, 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~$160 permits~$1200 permits~$2800 permits~$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.

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