WebSockets vs. SSE vs. Long Polling: Which Should You Use?

Real-time communication has become essential for modern web applications. Users expect instant updates in chat apps, live notifications, and collaborative tools. But choosing the right technology for real-time data transfer can be challenging. Let’s compare the three main approaches: WebSockets, Server-Sent Events (SSE), and Long Polling.

Why Real-Time Communication Matters

Traditional HTTP follows a request-response pattern where clients must ask for updates. This works poorly for applications needing instant data delivery. Real-time communication solves this by enabling servers to push updates as they happen, creating responsive user experiences that feel immediate and interactive.

Long Polling: The HTTP Workaround

Long polling extends traditional HTTP requests to simulate real-time updates. The client sends a request, but instead of responding immediately, the server holds the connection open until new data arrives or a timeout occurs.

// Client-side long polling
async function poll() {
  try {
    const response = await fetch('/poll');
    const data = await response.json();
    handleUpdate(data);
    poll(); // Immediately reconnect
  } catch (error) {
    setTimeout(poll, 5000); // Retry after error
  }
}

poll(); // Start polling

Pros:

• Works everywhere (uses standard HTTP)
• Simple to implement
• No special server requirements

Cons:

• High overhead from repeated connections
• Increased latency between updates
• Resource-intensive at scale

Best for: Legacy systems, simple notifications, or when other methods aren’t available.

Server-Sent Events: One-Way Streaming

SSE provides a standardized way for servers to push updates to clients over a single HTTP connection. It’s built into browsers through the EventSource API.

// Client-side SSE
const eventSource = new EventSource('/events');

eventSource.onmessage = (event) => {
  const data = JSON.parse(event.data);
  handleUpdate(data);
};

eventSource.onerror = () => {
  console.log('Connection lost, auto-reconnecting...');
};

Pros:

• Automatic reconnection built-in
• Simple API and implementation
• Works over standard HTTP
• Efficient for server-to-client updates

Cons:

• One-way communication only
• Limited to UTF-8 text data
• Browser connection limits (typically 6 per domain)

Best for: Live feeds, progress updates, server notifications, real-time dashboards.

WebSockets: Full-Duplex Communication

WebSockets establish a persistent, bidirectional connection between client and server. After an initial HTTP handshake, the protocol upgrades to WebSocket, enabling more efficient communication.

// Client-side WebSocket
const ws = new WebSocket('wss://example.com/socket');

ws.onopen = () => {
  ws.send(JSON.stringify({ type: 'subscribe' }));
};

ws.onmessage = (event) => {
  const data = JSON.parse(event.data);
  handleUpdate(data);
};

ws.onclose = () => {
  setTimeout(() => {
    // Reconnect logic here
    console.log('Reconnecting...');
  }, 1000);
};

Pros:

• True bidirectional communication
• Low latency and overhead
• Supports binary data
• Excellent for interactive applications

Cons:

• More complex implementation
• Requires manual reconnection logic
• Some proxies/firewalls may block connections
• Stateful connections complicate scaling

Best for: Chat applications, collaborative editing, multiplayer games, trading platforms.

Making the Right Choice: WebSockets vs SSE vs Long Polling

Quick Decision Guide:

Choose SSE when:

• You only need server-to-client updates
• Automatic reconnection is important
• You’re building dashboards or notification systems

Choose WebSockets when:

• You need bidirectional communication
• Low latency is critical
• Building interactive, real-time features

Choose Long Polling when:

• Working with legacy systems
• Firewall restrictions block persistent connections
• Updates are infrequent

Performance and Scaling Considerations

Each approach has different scaling characteristics:

• Long Polling: Stateless but creates many connections
• SSE: Holds connections open but uses standard HTTP infrastructure
• WebSockets: Most efficient but requires sticky sessions or message brokers

For production systems, consider using established libraries like Socket.IO or Pusher that handle reconnection, fallbacks, and scaling challenges automatically.

Conclusion

Start with SSE for simple one-way push notifications—it’s the easiest to implement and maintain. Upgrade to WebSockets when you need interactive features or bidirectional communication. Keep Long Polling as a fallback option for compatibility with older systems or restrictive network environments. The best choice depends on your specific use case, but understanding these trade-offs ensures you’ll build efficient real-time features that scale.

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