Problem 4.12 — Notification System (LLD)

Strategy (channels) + Factory (registry) + Observer (event fan-out), the three patterns this problem is built to showcase.

public interface Channel {
  String name();
  void send(String to, String message) throws ChannelException;
}

@Component class EmailChannel implements Channel {
  public String name() { return "EMAIL"; }
  public void send(String to, String msg) { /* SMTP */ }
}
@Component class SmsChannel implements Channel {
  public String name() { return "SMS"; }
  public void send(String to, String msg) { /* SMS gateway */ }
}

@Service
public class NotificationService {
  private final Map<String, Channel> channels;          // built from all Channel beans — Factory/Registry
  private final UserPreferences preferences;

  public NotificationService(List<Channel> all, UserPreferences preferences) {
    this.channels = all.stream().collect(Collectors.toMap(Channel::name, c -> c));
    this.preferences = preferences;
  }

  @Retryable(maxAttempts = 3, backoff = @Backoff(delay = 1000, multiplier = 2))
  public void notify(String userId, String channelName, String template, Map<String, String> vars) {
    if (preferences.isMuted(userId, channelName)) return;      // opt-out checked before dispatch
    Channel channel = channels.get(channelName);
    if (channel == null) throw new IllegalArgumentException("unknown channel: " + channelName);
    String message = render(template, vars);
    try {
      channel.send(preferences.addressFor(userId, channelName), message);
    } catch (ChannelException e) {
      throw e;                                                 // @Retryable retries; after 3 tries → DLQ handler
    }
  }

  private String render(String template, Map<String, String> vars) {
    String out = template;
    for (var e : vars.entrySet()) out = out.replace("{" + e.getKey() + "}", e.getValue());
    return out;
  }
}

// Fan-out (Observer): one event, many notifications, each independently retried and mutable.
@EventListener
public void onOrderPlaced(OrderPlacedEvent e) {
  notificationService.notify(e.customerId(), "SMS", "order-confirmed", Map.of("id", e.orderId()));
  notificationService.notify(e.restaurantId(), "PUSH", "new-order", Map.of("id", e.orderId()));
}