Fintech Payment Flow – Starter

Context: Early payment product. 1k transactions/day. Domestic only.

Starter Design

• Database: PostgreSQL. Tables: accounts, transactions, ledger. ACID for balance updates.
• Flow: Synchronous. Debit A, credit B, record transaction in one transaction.
• Idempotency: Client sends idempotency_key; duplicate requests return same result.

Why This Works

• Strong consistency out of the box
• Simple to audit and debug
• Latency acceptable at low volume

Inline Check 1

Why is idempotency critical for payment systems?

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