How to Fix Deadlock in NestJS

Running into a Deadlock in NestJS? This guide walks you through the root cause and a practical fix.

Why This Happens

A deadlock occurs when two or more operations each hold a resource the other needs, creating a circular wait. Neither operation can proceed, and your application hangs or eventually times out.

How to Fix It

The key is to use SELECT FOR UPDATE with consistent ordering inside a database transaction:

@Injectable()
export class TransferService {
  constructor(private dataSource: DataSource) {}

  async transfer(fromId: string, toId: string, amount: number) {
    const [first, second] = fromId < toId ? [fromId, toId] : [toId, fromId];
    return this.dataSource.transaction(async (manager) => {
      await manager.query("SELECT 1 FROM accounts WHERE id = $1 FOR UPDATE", [first]);
      await manager.query("SELECT 1 FROM accounts WHERE id = $1 FOR UPDATE", [second]);
      await manager.query("UPDATE accounts SET balance = balance - $1 WHERE id = $2", [amount, fromId]);
      await manager.query("UPDATE accounts SET balance = balance + $1 WHERE id = $2", [amount, toId]);
    });
  }
}

Common Pitfall

One pitfall to avoid: applying a quick workaround that disables the underlying safety check. This masks the real problem and will come back to haunt you later. Consider adding a health check endpoint or startup validation that catches this misconfiguration before it reaches users.

Testing Your Changes

Run your test suite to make sure the fix doesn't introduce regressions. If you don't have tests covering this area, now is a good time to add a simple integration test. A quick manual smoke test across different browsers or environments can also catch edge cases your tests might miss.

Monitoring

Consider integrating [Bugsly](https://bugsly.dev) into your NestJS workflow to catch, track, and resolve errors like this automatically.