Double-Entry Intro

A customer swears they were charged twice. Support has a screenshot, the payments dashboard shows one charge, and the bank statement shows two. Three systems, three numbers, and nothing in any of them that settles which is wrong.

A double-entry ledger exists to end that argument. Every movement of money is recorded as at least two entries, one debit and one credit, against named accounts, and within a single transaction the debits and the credits sum to zero. Accounts are buckets: this user's balance, the platform's fee income, the settlement account at the bank. Because the record is the entries and nothing else, you can reconcile against the bank without having to trust any one system's opinion of the total.

The second rule is that entries are permanent. You never edit a posted row and you never delete one. To undo $50 you post a reversing entry of $50 in the opposite direction, and both rows stay in the ledger for good. The history is the audit trail, and an audit trail you can edit is not one.

That is also why a balance is derived rather than stored. An account's balance is the sum of its entries as of a moment, which means any figure you show can be recomputed and defended. Cache it for speed if you have to, but the cache is an optimization and the entries stay the source of truth.