BUILD STATUS: V1 TYPESCRIPT CORE & SOLANA ANCHOR SPECS • monsoonbank.com

Building a Pure Small Savings Bank in Public.

Engineering notes, architectural trade-offs, and progress logs as we build The Monsoon Bank: a strictly retail small-savings bank ($1,000/mo KYCed exchange inflow cap, unrestricted outgoing spending) powered by ring-fenced 14%–16% APR proprietary trading margin facilities.

Small Saver Inflow Cap $1,000 / mo KYCed CEX Inflow • Anti-SVB Moat
Outgoing Transfers Unrestricted Any Wallet / Card / Merchant + 50ms OFAC
Prop Margin Revenue 14% – 16% APR 20–30% First-Loss • 105% Auto-Liq
Saver Yield Architecture 5.25% – 8.85% US 2Y+1% Base + Founding Booster
Architecture Reference v1.0

End-to-End System Blueprint

How our off-chain TypeScript monorepo orchestrates the 5 on-chain Solana Anchor programs and institutional prime brokerage sub-accounts.

LAYER 01 • ENTRY & CO-OWNERSHIP

Asymmetric 2-of-2 Transit Wallet

Every saver gets a deterministic Solana PDA co-owned by Key 1 (User WebAuthn Passkey) and Key 2 (Bank HSM Policy Daemon).

  • Strict Inflow: Only accepts transfers from the customer's verified KYCed exchange up to a hard $1,000/month Small Savings cap.
  • Unrestricted Outflow: Customers can send or spend interest and principal to any wallet, card, or merchant with zero SOL gas fees (screened in <50ms against OFAC blacklists).
LAYER 02 • CORE LEDGER & VAULT

TypeScript BIGINT Double-Entry Ledger

Built on Node.js 22 LTS + TypeScript + PostgreSQL 16 (Drizzle ORM) with SERIALIZABLE transaction isolation.

  • Zero Floating-Point Math: All balances use 6-decimal atomic bigint units (1_000_000n = $1.00).
  • Daily Midnight Snapshot: Calculates daily interest accrual and executes batched top-level SPL payouts on the 1st of every month.
LAYER 03 • REVENUE & RISK ENGINE

14%–16% Prop Margin Sub-Accounts

Fixed-rate institutional margin facilities deployed inside ring-fenced sub-accounts (Binance Institutional, Coinbase Prime, OKX, Copper ClearLoop).

  • Zero Borrower Withdrawal Custody: Borrowers receive trade-only API keys on whitelisted liquid pairs (BTC, ETH, SOL).
  • 20%–30% First-Loss Equity: Prop firm posts 20–30% cash margin first.
  • 115% / 110% / 105% Auto-Liquidation: 250ms WebSocket loop revokes API keys and closes positions at 105% principal coverage.
Interactive Financial Model

Net Interest Margin & "Yield-as-CAC" Sandbox

Test how The Monsoon Bank generates a 3%–4% net profit spread while paying early small savers up to 8.85% APY (SBI MCLR parity)—and remains cash-flow positive even if prop borrowing drops in a bear market.

$100,000,000
80% Prop / 20% T-Bills
15.50% APR
1.20%
Blended Gross Asset Yield (Prop + 4.35% T-Bills) 13.27%
Depositor Interest Payout -8.85%
Operating & Gas Relayer Cost -1.20%
NET BANK PROFIT SPREAD (AFTER OPEX)
+3.22%
ANNUAL NET BANK PROFIT
$3,220,000 / yr

At $1,000/mo max deposit, $100M AUM represents ~20,000–40,000 granular retail savers with zero whale bank-run concentration risk.