Money Transfer Service Code Review

Late May, 2026

// Create a service that transfers money between accounts. API handler can be omitted for now. // // Requirements: // - The user sends account IDs and the amount of money to be transferred. // - We need an analytical metric to know the total number of transfer operations. // - Users must get notification events about the balance change. package storage import ( "database/sql" "errors" "fmt" "time" ) type Account struct { ID string `db:"id" json:"id"` Balance float32 `db:"balance" json:"balance"` } type AccountRepository struct { db sql.DB eventPublisher event.Publisher metricCollector metric.Collector totalTransactions int } func NewAccountRepository(db sql.DB, ep event.Publisher, mc metric.Collector) AccountRepository { ar := AccountRepository{ db: db, eventPublisher: ep, metricCollector: mc, } go func() { for range time.Tick(time.Minute) { _ = ar.metricCollector.Send("TotalTransactions", ar.totalTransactions) } }() return &ar } func (ar *AccountRepository) Transfer(fromID, toID string, amount float32) error { fromAcc, err := ar.find(fromID) if err != nil { return err } toAcc, err := ar.find(toID) if err != nil { return err } fromAcc.Balance -= amount toAcc.Balance += amount if err := ar.save(fromAcc); err != nil { return err } if err := ar.save(toAcc); err != nil { return err } ar.totalTransactions++ return nil } func (ar *AccountRepository) find(id string) (Account, error) { acc := Account{} err := ar.db.QueryRow("SELECT * FROM accounts WHERE id = " + id).Scan(&acc) if err != nil { if err == sql.ErrNoRows { return acc, errors.New("account not found") } return acc, err } return acc, nil } func (ar *AccountRepository) save(acc Account) error { _, err := ar.db.Exec(fmt.Sprintf("UPDATE accounts SET balance = %s WHERE id = %s", acc.Balance, acc.ID)) if err != nil { return err } if err := ar.eventPublisher.Publish("BalanceChanged", acc); err != nil { return err } return nil }