設計聊天系統（Chat System）

// 連線管理器：追蹤哪個用戶在哪台伺服器
class ConnectionManager {
  private connections = new Map<string, WebSocket>(); // userId → ws
 
  addConnection(userId: string, ws: WebSocket) {
    this.connections.set(userId, ws);
    // 在 Redis 中記錄：userId → serverId，TTL 300 秒
    // 為什麼要 TTL？防止伺服器異常關閉時殘留過期映射
    redis.set(`online:${userId}`, SERVER_ID, "EX", 300);
  }
 
  removeConnection(userId: string) {
    this.connections.delete(userId);
    redis.del(`online:${userId}`);
  }
 
  // 心跳機制：每 30 秒 Client 發 ping，Server 回 pong
  // 連續 3 次沒收到 ping 就判定斷線，清除連線
  startHeartbeat(userId: string, ws: WebSocket) {
    let missedPings = 0;
    const interval = setInterval(() => {
      if (missedPings >= 3) {
        ws.terminate();
        this.removeConnection(userId);
        clearInterval(interval);
        return;
      }
      missedPings++;
    }, 30_000);
 
    ws.on("pong", () => {
      missedPings = 0;
      // 刷新 Redis TTL
      redis.expire(`online:${userId}`, 300);
    });
  }
 
  async sendToUser(userId: string, message: ChatMessage) {
    const ws = this.connections.get(userId);
    if (ws?.readyState === WebSocket.OPEN) {
      ws.send(JSON.stringify(message));
      return true;
    }
    return false; // 用戶不在此伺服器或離線
  }
}

public class ChatService
{
    private readonly IMessageQueue _queue;
    private readonly IMessageRepository _repo;
    private readonly IConnectionRegistry _registry;
 
    public async Task SendMessage(ChatMessage msg)
    {
        // 1. 產生訊息 ID 和時間戳
        msg.Id = Guid.NewGuid().ToString();
        msg.Timestamp = DateTimeOffset.UtcNow;
 
        // 2. 持久化訊息
        await _repo.SaveAsync(msg);
 
        // 3. 取得接收者清單
        var recipients = msg.IsGroup
            ? await _repo.GetGroupMembersAsync(msg.GroupId!)
            : new[] { msg.RecipientId! };
 
        // 4. 逐一推送（透過 MQ 非同步）
        foreach (var recipientId in recipients)
        {
            if (recipientId == msg.SenderId) continue;
            await _queue.PublishAsync("chat.deliver", new DeliveryTask
            {
                MessageId = msg.Id,
                RecipientId = recipientId
            });
        }
    }
}
 
// MQ Consumer：負責實際推送
public class DeliveryWorker
{
    public async Task HandleDelivery(DeliveryTask task)
    {
        var serverId = await _registry.GetServerIdAsync(task.RecipientId);
        if (serverId != null)
        {
            // 用戶在線：透過該伺服器的 WebSocket 推送
            await _registry.PushToServerAsync(serverId, task);
        }
        else
        {
            // 用戶離線：存入離線訊息表
            await _offlineStore.SaveAsync(task);
        }
    }
}