Borrow double-entry: a ledger that checks itself

Three ways to notice something missing

Double-entry bookkeeping. Every transaction is written twice: a debit in one account and an equal credit in another. Selling a chair adds to cash and takes from stock. If one half is lost, the books no longer balance, and the trial balance shows it.

A balanced chemical equation. Atoms are never created or destroyed in a reaction, so the left and right sides must have the same atoms. An equation that does not balance has a mistake in it, even before you know which.

Cashing up a till. At the end of a shift, the float plus the day's takings must equal what is in the drawer. A missing receipt shows up as a difference, even though nobody saw it go.

Compare before you code

1. What is conserved in each one?
2. Where does "new" value come from in each (the money for a sale, the chemist's ingredients, the float)? Is that source recorded too?
3. Write the shared idea in one sentence without mentioning money, atoms or tills.

The target

A loyalty programme stores its points as rows in an append-only store. Users earn points, redeem them, transfer them to each other, and lose them when they expire. The store is fed by a queue that, very occasionally, loses a single row. Today nobody can tell when that happens.

op = {"id": "t00017", "kind": "transfer", "user": "u004", "to": "u031", "points": 120}
rows_for(op)   # -> the rows to store, each (op_id, account, delta, ...)
audit(rows)    # -> ids of operations that lost a row