I Built a NestJS Banking Ledger That Cannot Be Overdrawn, Even Under Concurrent Requests

Short summary

A developer built a NestJS banking ledger that prevents overdrafts under concurrent requests by using PostgreSQL pessimistic write locks (SELECT FOR UPDATE) inside transactions. The article explains the lost update anomaly, why application-level checks fail, and how forcing the database to serialize row access makes the system self-correcting. Honest mistakes encountered during implementation are also shared.

• •Lost update anomaly causes silent overdrafts when concurrent requests read stale balances
• •Fix uses PostgreSQL SELECT FOR UPDATE pessimistic locks inside TypeORM transactions
• •NestJS exception handling pitfalls and practical debugging lessons included

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