[jasim]

Ask the same programmer to write an invoicing & payments system, then coupon creation and track its claims, then something where users can recharge their phone, and their usage is tracked and balance is computed -- and each one of them will be a set of ad-hoc tables and custom code.

But most transactional systems where things move from one entity to another and has the notion of "balance value" is best represented by the double-entry system.

When a customer purchases pre-paid balance, the phone company can record it as a "credit", and every call they make becomes a "debit". This is an immutable log - one of the earliest application of immutable data structures in human history.

In this post authors used positive and negative values to represent debit and credit - we could call it yin and yang for all it mattered. The core principle is just that every transaction has a source and destination and the ledger is an append-only table.

[chris_wot]

What it really shows is the flow of money through the system. That’s really the crux of double-entry bookkeeping.