Event-Based Architecture

Dalam real-time systems, everything is an event. User mengetik → event. Pesan terkirim → event. User join → event. Arsitektur event-based memisahkan producer dari consumer.

Event Pattern

// Event format
interface AppEvent {
  type: string;        // "user:joined", "message:sent"
  payload: unknown;    // event-specific data
  metadata: {
    timestamp: number;
    userId: string;
    correlationId: string;
  };
}

EventEmitter Pattern

import { EventEmitter } from "events";

class ChatService extends EventEmitter {
  async sendMessage(userId, channelId, text) {
    const message = await db.messages.create({
      data: { userId, channelId, text },
    });

    // Emit event — decoupled dari delivery method
    this.emit("message:sent", {
      message,
      channelId,
      userId,
    });

    return message;
  }
}

const chat = new ChatService();

// Different listeners for different concerns
chat.on("message:sent", (event) => {
  // 1. Push via WebSocket
  io.to(event.channelId).emit("new-message", event.message);
});

chat.on("message:sent", (event) => {
  // 2. Send push notification to offline users
  notificationService.notifyOffline(event.channelId, event.message);
});

chat.on("message:sent", (event) => {
  // 3. Update unread count
  unreadService.increment(event.channelId, event.userId);
});

Benefits

• Decoupling — Producer tidak tahu siapa consumer
• Extensibility — Tambah listener tanpa ubah producer
• Testability — Test producer dan consumer independently