WebSockets & Realtime Systems Programming · レッスン

パブリッシュ／サブスクライブメッセージングの実装

トピック単位でメッセージを配信するpub/subシステムをWebSocket上に設計・構築します。

「パブリッシュ／サブスクライブメッセージングの実装」はCoddyKit上の無料WebSockets & Realtime Systems Programmingレッスンです。これはレッスン1/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはWebSockets & Realtime Systems Programming学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 WebSockets & Realtime Systems Programmingコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

Intro to Pub/Sub Messaging

Welcome to advanced WebSocket patterns! Today, we'll explore Publish/Subscribe (Pub/Sub) messaging, a powerful way to distribute real-time data efficiently.

Imagine a digital newspaper where you only get articles on topics you care about. That's Pub/Sub in action!

It's ideal for features like live chat rooms, social media feeds, stock tickers, or any application needing dynamic, topic-based updates.

Pub/Sub's Key Players

A Pub/Sub system involves a few core components:

Publishers: These are entities that send messages. They don't know who will receive the messages.
Subscribers: These are entities that receive messages. They express interest in specific topics.
Topics: These are categories or channels for messages. Subscribers 'tune into' topics they want to follow.
Broker: This is the central component (our WebSocket server) that receives messages from publishers and forwards them to relevant subscribers.

WebSocket Server as Broker

In our WebSocket Pub/Sub system, the WebSocket server acts as the Broker.

It will be responsible for:

Accepting client connections.
Managing a list of which clients are subscribed to which topics.
Receiving messages from 'publisher' clients.
Distributing those messages to all 'subscriber' clients for the given topic.

We'll use a Map data structure on the server to store subscriptions, mapping each topic name to a Set of connected WebSocket client objects.

Designing Message Formats

To make our Pub/Sub system work, clients and the server need a clear way to communicate. We'll use simple JSON messages.

1. Subscribe Message (Client to Server):

{ "action": "subscribe", "topic": "chat:general" }

2. Publish Message (Client to Server):

{ "action": "publish", "topic": "news", "payload": "Breaking: New feature released!" }

3. Delivered Message (Server to Client):

{ "type": "message", "topic": "chat:general", "payload": "Hello everyone!" }

Server: Basic Pub/Sub Logic

Here's a complete Node.js WebSocket server implementing basic Pub/Sub. It handles both subscribe and publish messages.

Run this code (node server.js after npm install ws) and then connect clients in your browser to test it!

const { WebSocketServer } = require('ws');
const wss = new WebSocketServer({ port: 8080 });

// Stores active subscriptions: Map<topic, Set<WebSocket>>
const subscriptions = new Map();

wss.on('connection', ws => {
  console.log('Client connected.');

  ws.on('message', message => {
    try {
      const data = JSON.parse(message.toString());
      const { action, topic, payload } = data;

      if (action === 'subscribe' && topic) {
        if (!subscriptions.has(topic)) {
          subscriptions.set(topic, new Set());
        }
        subscriptions.get(topic).add(ws);
        ws.send(JSON.stringify({ status: 'subscribed', topic }));
        console.log(`Client subscribed to '${topic}'.`);
      } else if (action === 'publish' && topic && payload) {
        const topicSubscribers = subscriptions.get(topic);
        if (topicSubscribers) {
          topicSubscribers.forEach(subscriber => {
            if (subscriber.readyState === ws.OPEN) {
              subscriber.send(JSON.stringify({ type: 'message', topic, payload }));
            }
          });
          console.log(`Published to '${topic}': ${payload}.`);
        } else {
          ws.send(JSON.stringify({ error: `No subscribers for '${topic}'.` }));
        }
      } else {
        ws.send(JSON.stringify({ error: 'Invalid message.' }));
      }
    } catch (e) {
      console.error('Message error:', e.message);
      ws.send(JSON.stringify({ error: 'JSON parse error.' }));
    }
  });

  // Connection close and error handling in the next scene
});

console.log('Pub/Sub server running on ws://localhost:8080');

Server: Connection Lifecycle

Managing the connection lifecycle is crucial for a robust Pub/Sub server. The server must handle client disconnections gracefully.

The ws.on('close') event fires when a client disconnects.
Inside this handler, we iterate through all topics and remove the disconnected client from any subscription lists it was part of.
We also clean up any topics that become empty after a client leaves.
The ws.on('error') handler catches any communication errors.

Client: Connecting & Subscribing

Now let's look at the client side, typically running in a web browser. We'll use the native WebSocket API.

This code snippet connects to our server and sends a subscribe message for a specific topic.

Remember to open this HTML file in a browser, and have your Node.js server running!

<!-- index.html -->
<!DOCTYPE html>
<html>
<head><title>Pub/Sub Client</title></head>
<body>
  <h1>Pub/Sub Client</h1>
  <p>Status: <span id="status">Disconnected</span></p>
  <ul id="messages"></ul>

  <script>
    const ws = new WebSocket('ws://localhost:8080');
    const statusElem = document.getElementById('status');
    const messagesElem = document.getElementById('messages');

    ws.onopen = () => {
      statusElem.textContent = 'Connected';
      console.log('Connected to server');
      // Subscribe to a topic
      ws.send(JSON.stringify({ action: 'subscribe', topic: 'chat:general' }));
      ws.send(JSON.stringify({ action: 'subscribe', topic: 'news' }));
    };

    ws.onclose = () => {
      statusElem.textContent = 'Disconnected';
      console.log('Disconnected from server');
    };

    ws.onerror = (error) => {
      console.error('WebSocket Error:', error);
    };

    // Message handling in the next scene
  </script>
</body>
</html>

Client: Receiving & Displaying

Once subscribed, the client needs to listen for incoming messages from the server. The ws.onmessage event is where this happens.

When a message arrives, we parse its JSON content and can display it to the user, perhaps filtering or formatting based on the message's topic.

Try opening multiple browser tabs, subscribing to the same topic, and then publishing a message from one tab (e.g., via the console) to see it appear in others!

<!-- index.html (continued script part) -->
    // ... (ws.onopen, ws.onclose, ws.onerror from previous scene)

    ws.onmessage = event => {
      const data = JSON.parse(event.data);
      console.log('Received:', data);

      if (data.type === 'message') {
        const listItem = document.createElement('li');
        listItem.textContent = `[${data.topic}] ${data.payload}`;
        messagesElem.appendChild(listItem);
      } else if (data.status === 'subscribed') {
        const listItem = document.createElement('li');
        listItem.textContent = `Successfully subscribed to '${data.topic}'.`;
        messagesElem.appendChild(listItem);
      } else if (data.error) {
        console.error('Server error:', data.error);
        const listItem = document.createElement('li');
        listItem.style.color = 'red';
        listItem.textContent = `Error: ${data.error}`; 
        messagesElem.appendChild(listItem);
      }
    };

    // Example of how a client could publish a message (e.g., via a button click)
    // ws.send(JSON.stringify({ action: 'publish', topic: 'chat:general', payload: 'Hello from client!' }));
  </script>
</body>
</html>

Advanced Pub/Sub Patterns

While our basic Pub/Sub system is functional, real-world applications often need more:

Unsubscribe: A client should be able to stop receiving messages for a topic. This would involve a new message action (e.g., { "action": "unsubscribe", "topic": "news" }) and server-side logic to remove the client from the topic's subscription set.
Topic Hierarchies: Allowing subscriptions to patterns like news.* to receive all news sub-topics.
Message Persistence: Storing messages so new subscribers can receive past messages (e.g., chat history).

Quick Check

Consider the Pub/Sub system we just built. Which component is primarily responsible for deciding which subscribers receive a message published to a specific topic?

Recap & Beyond

Congratulations! You've learned how to design and implement a fundamental Publish/Subscribe messaging system using WebSockets.

We covered the core components (Publisher, Subscriber, Topic, Broker), defined message formats, and built both the server-side broker logic and client-side subscription/reception.

Pub/Sub is a foundational pattern for many real-time applications. Experiment with adding unsubscribe functionality, more complex topic management, or integrating with external message queues for larger scale.

よくある質問

「パブリッシュ／サブスクライブメッセージングの実装」レッスンは無料ですか？

はい。「パブリッシュ／サブスクライブメッセージングの実装」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、WebSockets & Realtime Systems Programmingコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 WebSockets & Realtime Systems Programmingコースには全4レッスンが含まれています。

「パブリッシュ／サブスクライブメッセージングの実装」で何を学びますか？

トピック単位でメッセージを配信するpub/subシステムをWebSocket上に設計・構築します。ブラウザで直接実行するハンズオンコードでWebSockets & Realtime Systems Programmingを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

WebSockets & Realtime Systems Programmingを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのWebSockets & Realtime Systems Programmingは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン1/4です。

「パブリッシュ／サブスクライブメッセージングの実装」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このWebSockets & Realtime Systems Programmingレッスンでコードを書いて実行できますか？

はい。すべてのWebSockets & Realtime Systems Programmingレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

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