Petty cash tracker
A small system that keeps a petty cash float honest without turning it into paperwork. Somebody photographs a receipt as they put it in the tin, the system reads it, updates the balance, and says nothing. When the count does not match, it says so on the day rather than four weeks later. Seven posts on the same system — one diagram at a time — with a cost breakdown and an engineering reference at the end.
- 01
A petty cash tracker on AWS for a few dollars a month
The whole system on one page — a reader, a ledger and a reconciler — and the constraint that makes it survivable: it must be faster than not using it.
- 02
How a petty cash receipt gets captured
Reading a creased thermal receipt photographed in bad light, what the reader refuses to guess, and how the right float gets picked without anybody having to choose one.
- 03
How the float balance stays right
Two people spending at once, the conditional increment that prevents them both computing from the same starting figure, and why the balance is both stored and derivable.
- 04
How a petty cash count works
Counting a tin in under a minute, why the count is itself a movement, what a mismatch message contains, and the cadence that keeps differences resolvable.
- 05
How the petty cash month closes
What the bookkeeper receives, why categorising happens once at month end rather than at every spend, and the export that turns a tin into a single journal line.
- 06
What the petty cash tracker costs
About $2 a month. One Bedrock read per receipt is the only line that grows; the queue, the table, the mail and the storage are rounding errors. Plus the three ways the bill could surprise you.
- 07
Engineering reference: the petty cash tracker architecture
Same system, drawn purely for engineers. Service names, region, Lambda inventory, IAM scopes, the schemas and the exact model id.
Frequently asked questions
- What is a petty cash tracker?
- A small serverless system that keeps a running balance for a cash float. Each spend is a photographed receipt, each top-up is a recorded line, and a periodic count is compared against the expected balance. It never moves money and never accuses anybody; it tells you on the day that a count did not match.
- Why photograph receipts rather than just record amounts?
- Because the receipt is the accounting record and the amount is only half of it. The photo gives you the vendor, the date, the tax treatment and the evidence, all of which your bookkeeper needs and none of which survives being typed as a number.
- What happens when the count is short?
- The system says so on the day it is counted, with the transactions since the last count listed. Most shortfalls are a receipt that was never photographed, which is a two-minute fix while it is still findable in a pocket.
- Does it work with more than one tin?
- Yes, and it is designed for it. Each float is separate with its own balance, its own custodian and its own count cadence, because a site tin and an office tin behave nothing alike.
- What does it cost to run?
- A couple of dollars a month. Petty cash volume is low and nothing is always-on. See part six.