Why should you care?

ShopNest wallet top-ups must not create money from air.

See it live — copy this example

Use a .NET console or API project with SQL Server LocalDB. Run dotnet run after pasting.

await using var tx = (SqlTransaction)await conn.BeginTransactionAsync(ct);
// UPDATE Accounts SET Balance=Balance-@Amt WHERE Id=@From AND Balance>=@Amt
// INSERT Ledger ...
// UPDATE Accounts SET Balance=Balance+@Amt WHERE Id=@To
await tx.CommitAsync(ct);

What happened?

• Single transaction.
• Conditional update rows-affected check.
• Idempotency key on ledger.

Practice next

1. Schema Accounts+Ledger.
2. Transfer method.
3. Assert balances.
4. Overdraft rejection test.
5. Concurrent transfer stress.

Remember

Ledger + tx. Balance guard. Idempotency.

ShopNest wallet transfer

Debit/credit commit atomically.

Outcome: Balances always reconcile.