10 parts · 18 chapters

Ledgers, Payments and Reconciliation

The backend mirror of Money, Identity and Trust. A ledger is the one system in a fintech that must never be wrong, and the techniques that keep it right are old: double-entry bookkeeping, append-only journals, idempotency and reconciliation. This course teaches them as code.

Ten parts: double-entry accounting for engineers; ledger schemas, accounts, balances and holds; idempotency end to end; payment state machines; rails, webhooks and callbacks; reconciliation and breaks; settlement, float and FX; fees, interest and rounding; audit, regulators and reporting; and a capstone ledger service with a reconciliation job.

Start reading · Part 0 Present as slides
double entry · ledger schemas · idempotency · payment state machines · rails and webhooks · reconciliation · settlement and FX · fees, interest and rounding · audit and regulators · capstonesenior → staff · engineers building wallets, lending, payments or banking backends
double entryDebits, credits, account types and the invariant that every posting sums to zero.
schemasAccounts, journal entries, postings, balances, holds and versions.
idempotencyKeys, request fingerprints, stored responses, and idempotent consumers.
state machinesTransfers, payouts and card authorisations as explicit states.
reconciliationMatching the ledger with banks and processors, and working breaks.
money mathsMinor units, rounding rules, interest accrual, FX and fees.
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Double-Entry Accounting for Engineers

Debits, credits and the zero-sum invariant · Modelling a business as accounts
2 ch · ~12 min01

Ledger Schemas, Accounts, Balances and Holds

The tables · Posting safely: concurrency, hot accounts and corrections
2 ch · ~12 min02

Idempotency End to End

Keys, fingerprints and stored responses · Idempotent consumers and retention
2 ch · ~12 min03

Payment State Machines

States, transitions and the unknown · Timeouts, enquiries and reversals
2 ch · ~12 min04

Rails, Webhooks and Callbacks

How rails behave · Webhooks, done safely
2 ch · ~12 min05

Reconciliation and Breaks

Matching the ledger with the world · Breaks, ageing and operations
2 ch · ~12 min06

Settlement, Float and FX

Settlement and float · FX: quotes, trades and revaluation
2 ch · ~12 min07

Fees, Interest and Rounding

Fees and taxes · Interest accrual and rounding policy
2 ch · ~12 min08

Audit, Regulators and Reporting

Audit trails and maker-checker
1 ch · ~8 min09

Capstone: A Ledger Service with Reconciliation

The build
1 ch · ~8 min
Built on Trust and ScalingAssumes Money, Identity and Trust (the client side of money flows), SQL and PostgreSQL Internals, and Backend System Design part 3.