Implementing Publish/Subscribe Messaging
Design and build a pub/sub system over WebSockets for topic-based message distribution.
Implementing Publish/Subscribe Messaging is a free WebSockets & Realtime Systems Programming lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the WebSockets & Realtime Systems Programming learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
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.
Frequently asked questions
Is the “Implementing Publish/Subscribe Messaging” lesson free?
Yes — the full text of “Implementing Publish/Subscribe Messaging” is free to read here on the web, and the WebSockets & Realtime Systems Programming course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the WebSockets & Realtime Systems Programming course, upgrade to CoddyKit PRO.
What will I learn in “Implementing Publish/Subscribe Messaging”?
Design and build a pub/sub system over WebSockets for topic-based message distribution. You practise WebSockets & Realtime Systems Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start WebSockets & Realtime Systems Programming?
No prior experience is required. WebSockets & Realtime Systems Programming on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Implementing Publish/Subscribe Messaging” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this WebSockets & Realtime Systems Programming lesson?
Yes. Every WebSockets & Realtime Systems Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Implementing Publish/Subscribe Messaging
- Request-Response over WebSockets
- Bidirectional Streaming and Flow Control
- Backpressure and Message Batching