Skip to content
allanninal.dev/build
Collaborate

Part 6 of 7 · DNS change auditor series ~5 min read

What the dns change auditor costs

Hourly DNS queries are free and there is almost no model use, so this is a fixed-cost system with a rounding error attached. Seven hundred and twenty check-hours is one zone checked every hour for a month. Here is where each cent goes.

Key takeaways

  • About $5 a month at 3 zones hourly. Roughly $12 at 10 zones hourly.
  • One Bedrock read per check hour is the only line that scales. Everything else is rounding.
  • Nothing is always-on, so a quiet month genuinely costs almost nothing.
  • DNS queries cost nothing and the model runs only on SPF and DMARC changes, which happen a few times a 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 dns change auditor at three volumesA stacked bar chart with three bars, one per volume tier: 720 hours totalling about $2, 3 zones hourly totalling about $5, and 10 zones hourly totalling about $12. Each bar is stacked from bands. The largest and fastest-growing is Bedrock, one read per check hour, 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$5$10$15$20~$2.34720 hours~$4.563 zones hourly~$12.2710 zones hourlyBedrock — one read per check hourSES — 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 check hour — is the only part that grows; the orange fixed band is the same 86 cents at every volume.

Line by line

LineAt 3 zones hourlyHow it scales
Bedrock read$0.04Linear. One call per check hour, roughly 1,800 in and 200 out tokens.
SES$1.09Linear. About 0.03 messages per check hour.
DynamoDB + S3$0.63Storage 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 line is nominal: a model is called only when an SPF or DMARC record changes, which for most businesses is 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.

  • Running every minute instead of every hour. Tempting, and it multiplies the Lambda invocations by sixty for a detection improvement measured against DNS TTLs that are usually an hour anyway.
  • Snapshotting the full response every hour. Store a hash and keep the full response only when it changes. An unchanged zone should cost one small item, not one per hour.
  • Log retention left at never. An hourly job across ten zones produces a steady trickle of log lines that will out-cost every other line within months.

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. 720 hours, about $2. 3 zones hourly, about $5. 10 zones hourly, about $12. 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~$1720 hours~$23 zones hourly~$510 zones hourly~$12One 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