1−// 1. Begin a transaction2−// 2. Check the reference3−// 3. Lock both balances4−// 4. Insert DR/CR entries5−// 5. Update both balances6−// 6. Record the transaction7−// 7. Commit8− 9−func Deposit(db *sql.DB, ref string, amount int64, src, dst string) error {10−  tx, err := db.Begin()11−  if err != nil {12−    return err13−  }14−  defer tx.Rollback()15− 16−  var taken int17−  err = tx.QueryRow(`SELECT 1 FROM txns18−    WHERE reference = $1`, ref).Scan(&taken)19−  if err == nil {20−    return ErrDuplicateReference21−  }22−  if err != sql.ErrNoRows {23−    return err24−  }25− 26−  err = tx.QueryRow(`SELECT 1 FROM balances27−    WHERE id = $1 FOR UPDATE`, src).Scan(&taken)28−  if err != nil {29−    return err30−  }31−  err = tx.QueryRow(`SELECT 1 FROM balances32−    WHERE id = $1 FOR UPDATE`, dst).Scan(&taken)33−  if err != nil {34−    return err35−  }36− 37−  _, err = tx.Exec(`INSERT INTO entries38−    (txn_ref, account, side, amount, ccy)39−    VALUES ($1, $2, 'DR', $3, 'USD'),40−           ($1, $4, 'CR', $3, 'USD')`,41−    ref, src, amount, dst)42−  if err != nil {43−    return err44−  }45− 46−  _, err = tx.Exec(`UPDATE balances47−    SET amount = amount - $1 WHERE id = $2`, amount, src)48−  if err != nil {49−    return err50−  }51−  _, err = tx.Exec(`UPDATE balances52−    SET amount = amount + $1 WHERE id = $2`, amount, dst)53−  if err != nil {54−    return err55−  }56− 57−  _, err = tx.Exec(`INSERT INTO txns58−    (reference, amount, status)59−    VALUES ($1, $2, 'APPLIED')`, ref, amount)60−  if err != nil {61−    return err62−  }63− 64−  return tx.Commit()65−}