이벤트 소싱

Java code

import java.util.*;
import java.time.Instant;

// 이벤트 정의
sealed interface AccountEvent {
    String accountId();
    Instant timestamp();
}
record AccountCreated(String accountId, String owner, Instant timestamp)
    implements AccountEvent {}
record MoneyDeposited(String accountId, long amount, Instant timestamp)
    implements AccountEvent {}
record MoneyWithdrawn(String accountId, long amount, Instant timestamp)
    implements AccountEvent {}

// 이벤트 스토어
class EventStore {
    private final List<AccountEvent> events = new ArrayList<>();
    void append(AccountEvent event) { events.add(event); }
    List<AccountEvent> getEvents(String accountId) {
        return events.stream()
            .filter(e -> e.accountId().equals(accountId))
            .toList();
    }
}

// 상태 재구성
record AccountState(String id, String owner, long balance) {
    static AccountState replay(List<AccountEvent> events) {
        String id = "", owner = "";
        long balance = 0;
        for (AccountEvent e : events) {
            switch (e) {
                case AccountCreated c -> { id = c.accountId(); owner = c.owner(); }
                case MoneyDeposited d -> balance += d.amount();
                case MoneyWithdrawn w -> balance -= w.amount();
            }
        }
        return new AccountState(id, owner, balance);
    }
}

class Main {
    public static void main(String[] args) {
        EventStore store = new EventStore();
        store.append(new AccountCreated("A1", "홍길동", Instant.now()));
        store.append(new MoneyDeposited("A1", 50000, Instant.now()));
        store.append(new MoneyWithdrawn("A1", 20000, Instant.now()));

        AccountState state = AccountState.replay(store.getEvents("A1"));
        System.out.printf("%s 잔액: %d원%n", state.owner(), state.balance());
    }
}