The fintech engineering curriculum

Debits, credits, the two-sided story.

• #1 Your first journal entryThe two-sided story behind every transaction.
• #2 Card deposit with a feeWhy fees are not just 'subtracted from the user's balance'.
• #3 P2P transfer between usersWhere the wallet table starts to lie.
• #4 Provider settlement (T+1)The difference between earning money and having money.

Refunds, holds, segregation, reconciliation.

• #5 The refundWhy 'just delete the transaction' is the most expensive mistake.
• #6 Authorization holds and the reservation patternWhy a 'wallet balance' is actually three different numbers.
• #7 Merchant float (holding money you do not own)The pattern that separates licensed fintechs from illegal ones.
• #8 End-of-day reconciliationYour books and the provider's books will drift. That is normal.
• #9 A purchase is three ledger eventsAuth, capture, and settle each touch your books differently.

Recognition over time, releases, dedup keys.

• #10 Deferred revenueWhy annual subscriptions cannot all hit January's revenue.
• #11 The chargeback lifecycleProvisional reserves, the resolution, and the moment money becomes a loss.
• #12 Idempotency at the ledger boundaryHow webhook retries silently double-book your fintech.
• #13 Failed transaction: releasing the holdEvery reservation needs a release path, or pending lives forever.
• #14 Interest accrualEarning interest daily, paying it monthly. The gap matters.
• #15 Period closeHow the P&L resets to zero while the balance sheet carries on.

Where money goes wrong and how to record it.

• #16 Bad debt write-offThe moment a receivable becomes a loss.
• #17 Promo credits and cashbackWhere 'free' money comes from on your books.
• #18 Sanctions and the wallet freezeA regulatory hold is reclassification, not removal.
• #19 Settlement nettingMany entries on your books, one wire on your bank statement.

Multi-currency, audit, statements, reconciliation.

• #20 The trial balance (capstone)Sum all debits, sum all credits, prove they're equal.
• #21 Multi-currency FX swapEach currency is its own balanced book. Conversions take two entries.
• #22 Audit trail with structured metadataWHO did it, WHAT triggered it, WHY, and the dedup key.
• #23 The audit (capstone)Three bugs. Three diagnoses. Three corrective entries.
• #24 Account statementsA statement is a VIEW of the ledger, not stored data.
• #25 Bank reconciliationYour ledger and your bank's ledger will disagree. Reconcile them.

Issuing cards, lending money, and the sponsor-bank patterns underneath everything.

• #26 Interchange revenue (issuing side of cards)When your user swipes your card, YOU earn money.
• #27 BIN sponsor splitsWhen you don't own the BIN, your interchange has TWO cuts taken out.
• #28 Loan disbursement and repaymentPrincipal goes out as a receivable. Interest accrues. Repayment clears both.
• #29 Loan loss provisionRecognize losses BEFORE they happen, not after.
• #30 FBO accounts and sub-accountingOne bulk asset at the sponsor bank, many user liabilities on your books.
• #31 Float incomeYour sponsor pays you interest on user money. That interest is revenue.

KYC tiers, velocity gates, and AML holds, risk controls implemented at the ledger.

• #32 KYC tier limits at the ledgerRisk controls implemented as accounting code, not application logic.
• #33 Velocity limits and review queuesWhen a single transaction is too big or too fast, the ledger holds it for human review.
• #34 AML transaction monitoring and held statesFunds arrive but stay in escrow until compliance clears the transaction.

Outbox, sagas, maker-checker, patterns that keep money movement honest across systems.

• #35 The outbox patternReliably fan out side-effects after a ledger entry commits.
• #36 Sagas and compensating actionsWhen step 2 fails after step 1 committed, you can't rollback, you compensate.
• #37 Maker-checker controlsTwo humans, two entries, one approved transfer. Regulator-mandated for treasury.

Projections, locks, rebuild paths, partitioning. How the ledger stays fast at billions of rows.

• #38 The balance projectionCache the balance as a projection, never as truth. The ledger always wins.
• #39 Double-spend preventionTwo concurrent withdrawals can each pass the same balance check. Lock or constrain.
• #40 Rebuilding from the ledgerWhen the cache corrupts, the ledger lets you start over.
• #41 Partitioning and historical snapshotsBound the hot working set. Snapshot at period close. Archive the rest.

Proration, reserves, multi-entity consolidation, FX revaluation. The reporting-side mechanics every CFO needs.

• #42 Subscription prorationUpgrades mid-period: credit the unused, charge the new prorated, settle the difference.
• #43 Reserve accountsYour provider holds back a percentage of your earnings. Track it as an asset, not as cash.
• #44 Multi-entity consolidationWhen your group has multiple legal entities, inter-company transactions must cancel at the top.
• #45 FX revaluation: marking foreign positions to marketWhen the rate moves, every foreign-currency balance on your books moves with it.

Push vs pull, instant systems, correspondent banking, the Wise model, 3DS step-up. The engineering inside the rails every fintech rides on.

• #46 Push vs pull paymentsWho initiates the money movement decides who eats the chargeback.
• #47 Instant payment railsNIBSS, UPI, Pix, FedNow, RTP, same pattern, different country code.
• #48 Correspondent banking and Nostro/VostroWhy a USD wire from Lagos to Singapore goes through three banks that each take a slice.
• #49 The Wise model: cross-border without correspondent bankingMoney never crosses borders. The accounting just makes it look like it did.
• #50 3D Secure step-up: the ledger pauseWhen the issuer wants extra verification, the payment is in limbo. Your books need a name for that state.

DPD buckets, BNPL, revolving credit, restructuring, IFRS 9 ECL. The accounting that makes a loan book honest.

• #51 DPD buckets and impairment stagesA loan that's overdue isn't impaired yet. But every day matters.
• #52 BNPL: the 4-installment ledgerKlarna's 'pay in 4' is one purchase, five accounting events, and zero interest charged.
• #53 Revolving credit and the statement cycleCredit cards don't have one balance. They have current, statement, and minimum.
• #54 Loan restructuring and modificationExtending tenor or cutting rate is a NEW loan on the books. It just doesn't feel like one.
• #55 IFRS 9 Expected Credit Loss (ECL): the formula behind the provisionProvision = PD × LGD × EAD. Three numbers, one row in your loan book.

Issuer vs acquirer, interchange / MDR breakdown, loyalty, network tokens, chargeback disputes. Where the basis points actually go.

• #56 Issuer vs acquirer: same swipe, different ledgerTwo fintechs touch the same card payment. Their books look nothing alike.
• #57 Interchange, scheme fees, MDR: where the money actually goesA ₦10,000 swipe pays five different actors. Here's the math.
• #58 Loyalty and rewards: a deferred-revenue liability hiding in plain sightEvery point you give a user is debt. Real debt. With an interest expense.
• #59 Network tokens vs PSP tokens: the same card, different replacement chainsWhen a card expires, one of these breaks. The other doesn't.
• #60 Chargeback dispute lifecycle: representment to arbitrationLesson 11 was the chargeback. This is when you fight back.

Liquidity laddering, float optimisation, FX hedging, capital adequacy, settlement netting. The CFO's quiet engine room.

• #61 Liquidity laddering: matching duration to demandYour user money isn't one bucket. It's a curve.
• #62 Float optimization: the money in motionFunds in flight are an asset class with their own optimisation problem.
• #63 FX hedging with forwardsLock today's rate for tomorrow's payment. The ledger marks the deal twice.
• #64 Capital adequacy: the CAR ratio your sponsor bank cares aboutCapital ÷ risk-weighted assets. Cross 15% or your license tightens.
• #65 Settlement netting at scale10,000 transactions, two banks, one wire.

SAR workflow, sanctions / PEP screening, beneficial ownership, Travel Rule, AML monitoring. The regulator-facing ledger.

• #66 SAR workflow: tying suspicious activity to the ledgerFiling a Suspicious Activity Report doesn't move money. It DOES freeze funds. The bookkeeping matters.
• #67 Sanctions and PEP screening cascadesA name hit on a sanctions list blocks a payment at the ledger boundary.
• #68 Beneficial ownership: who actually owns the accountA corporate user is a fiction. Money is owed to humans behind the corporation.
• #69 The Travel Rule at the crypto-fiat boundaryMove >$1,000 of value? Send the originator and beneficiary details with the money.
• #70 AML transaction monitoring: structuring + smurfing detectionWhen you can't prove it's clean, you treat it as suspicious. The ledger reflects the decision.

On-chain reconciliation, stablecoin reserves, hot / cold wallets, tax lots, failed-tx handling. Where chain truth meets ledger truth.

• #71 On-chain ↔ off-chain reconciliationYour ledger says you hold 10 BTC. The blockchain says you hold 9.998. Which is right?
• #72 Stablecoin reserve attestationEvery USDC in circulation must have $1 backing it. The ledger proves it.
• #73 Hot wallet, cold wallet: the custody splitSame BTC. Different keys. Wildly different operational risk.
• #74 Tax-lot tracking: FIFO vs LIFO vs HIFOYou bought BTC at three different prices. Which 'cost basis' did you sell?
• #75 Failed on-chain transactions: gas paid, value not movedEthereum transactions can fail. The gas is gone. The user expects their balance back.

Income statement, balance sheet, cash flow, footnotes, materiality. The financials your CFO actually publishes.

• #76 Constructing the income statementRevenue, cost of revenue, opex, EBITDA, net income, all from your ledger.
• #77 Balance sheet at a point in timeAssets, liabilities, equity. The 'where you stand right now' query.
• #78 The cash flow statementIncome statement shows profit. Balance sheet shows position. Only cash flow shows whether you'll survive next month.
• #79 Footnotes and disclosures: the story the numbers don't tellThree numbers on the income statement. Three pages of footnotes explaining them.
• #80 Materiality: the threshold below which auditors don't sweatMisstating ₦100 isn't a big deal. Misstating ₦100K is.

Event sourcing + CQRS, multi-region replication, sharding, WORM, column encryption. The architecture for ledgers at billions of rows.

• #81 Event sourcing vs CQRS for the ledgerYour journal IS an event log. Don't fight that fact.
• #82 Multi-region replication for ledgersEventual consistency is fine for blog posts. It's not fine for double-spend prevention.
• #83 Sharding strategies for ledger tables100 million users, one Postgres won't hold them. Pick a shard key.
• #84 WORM enforcement: write-once-read-many for the journalAuditors trust your ledger to the degree they trust nobody can edit history.
• #85 Encryption at rest for sensitive ledger fieldsEncrypt at the column level, not just the disk. Your DBA shouldn't see PII.