Polling vs Long Polling vs SSE vs WebSockets: Real-Time Data Strategies

Your app needs real-time updates. Stock prices, chat messages, live scores, notification counts. How do you get data from server to client instantly?

You have four main options. Each has tradeoffs.

1. Polling (Regular/Short Polling)

The simplest approach. Client asks server for updates on a regular schedule.

// Check for updates every 5 seconds
setInterval(() => {
  fetch('/api/messages')
    .then((response) => response.json())
    .then((data) => updateUI(data));
}, 5000);

How it works:

1. Client makes HTTP request
2. Server responds immediately with current data
3. Connection closes
4. Wait X seconds, repeat

Pros:

• Simple to implement
• Works with any HTTP setup
• Easy to debug and monitor
• Stateless - works with load balancers

Cons:

• Wasted requests when nothing changes
• Delayed updates (up to polling interval)
• Increased server load and bandwidth usage
• Not truly real-time

When to use: Infrequent updates, simple requirements, when other methods aren’t feasible.

2. Long Polling

More sophisticated polling. Client asks for updates and server waits to respond until there’s new data.

async function longPoll() {
  try {
    const response = await fetch('/api/messages/poll');
    const data = await response.json();
    updateUI(data);
  } catch (error) {
    console.error('Polling error:', error);
  } finally {
    // Immediately start next poll
    setTimeout(longPoll, 1000); // Small delay to prevent tight loops
  }
}

longPoll();

How it works:

1. Client makes HTTP request
2. Server holds connection open (30-60 seconds typically)
3. When new data arrives OR timeout occurs, server responds
4. Connection closes
5. Client immediately starts new request

Pros:

• Near real-time updates
• Fewer wasted requests than regular polling
• Works with existing HTTP infrastructure
• Fallback compatibility

Cons:

• Ties up server connections (resource intensive)
• Complex timeout handling
• Can be tricky with proxies and firewalls
• Still request overhead

When to use: Moderate real-time needs, when WebSockets aren’t available, behind corporate firewalls.

3. Server-Sent Events (SSE)

Server pushes data to client over a persistent HTTP connection.

const eventSource = new EventSource('/api/messages/stream');

eventSource.onmessage = function (event) {
  const data = JSON.parse(event.data);
  updateUI(data);
};

eventSource.onerror = function (event) {
  console.error('SSE error:', event);
};

Server side (Node.js example):

app.get('/api/messages/stream', (req, res) => {
  res.writeHead(200, {
    'Content-Type': 'text/event-stream',
    'Cache-Control': 'no-cache',
    Connection: 'keep-alive',
  });

  // Send data
  const sendUpdate = (data) => {
    res.write(`data: ${JSON.stringify(data)}\n\n`);
  };

  // Subscribe to updates and send them
  messageService.onUpdate(sendUpdate);
});

How it works:

1. Client opens persistent HTTP connection
2. Server sends data as it becomes available
3. Connection stays open indefinitely
4. Built-in reconnection on connection loss

Pros:

• True real-time, server-initiated updates
• Automatic reconnection
• Simple client-side API
• Works over standard HTTP
• Built-in event types and IDs

Cons:

• One-way communication only (server → client)
• Limited browser connection pool (6 per domain)
• Can be blocked by some corporate firewalls
• No binary data support

When to use: Real-time dashboards, live feeds, notifications, when you only need server-to-client communication.

4. WebSockets

Full bidirectional communication channel.

const socket = new WebSocket('ws://localhost:8080');

socket.onopen = () => {
  console.log('Connected');
  socket.send(JSON.stringify({ type: 'subscribe', channel: 'messages' }));
};

socket.onmessage = (event) => {
  const data = JSON.parse(event.data);
  updateUI(data);
};

socket.onclose = () => {
  console.log('Disconnected');
  // Implement reconnection logic
};

How it works:

1. HTTP upgrade handshake establishes WebSocket connection
2. Persistent TCP connection maintained
3. Both client and server can send messages anytime
4. Very low overhead per message

Pros:

• True real-time, bidirectional communication
• Very low latency
• Minimal overhead per message
• Binary data support
• Custom protocols possible

Cons:

• More complex to implement correctly
• Doesn’t work with standard HTTP caching/proxies
• Stateful connections complicate load balancing
• Manual reconnection handling required
• Can be blocked by corporate firewalls

When to use: Interactive applications, gaming, collaborative editing, trading platforms, chat applications.

Quick Comparison

Choosing the Right Approach

Use Polling when:

• Updates are infrequent (every few minutes)
• Simple requirements
• Maximum compatibility needed

Use Long Polling when:

• Need better responsiveness than polling
• WebSockets blocked by infrastructure
• Moderate real-time requirements

Use SSE when:

• Need real-time server-to-client updates
• Don’t need client-to-server real-time messaging
• Building dashboards, feeds, notifications

Use WebSockets when:

• Need true bidirectional real-time communication
• Building interactive applications
• Latency is critical
• Willing to handle complexity

Implementation Considerations

Fallback Strategy

Many applications use progressive enhancement:

// Try WebSocket first, fallback to SSE, then long polling
if (window.WebSocket) {
  useWebSocket();
} else if (window.EventSource) {
  useSSE();
} else {
  useLongPolling();
}

Connection Management

All persistent connections need handling for:

• Reconnection on network issues
• Heartbeat/keepalive messages
• Graceful degradation
• Connection pooling limits

Scaling Considerations

• Polling: Scales well with stateless servers
• Long Polling: Limited by concurrent connection capacity
• SSE: Limited by concurrent connections
• WebSockets: Requires sticky sessions or message broadcasting

Real-World Examples

Polling: Weather widgets, system dashboards with infrequent updates

Long Polling: Facebook chat (historically), some REST APIs with real-time needs

SSE: Live sports scores, stock tickers, system monitoring dashboards

WebSockets: Slack/Discord chat, collaborative editors (Google Docs), online games, trading platforms

Bottom Line

Start simple and evolve based on needs:

1. Begin with polling if updates are infrequent
2. Move to SSE when you need server-initiated real-time updates
3. Upgrade to WebSockets when you need bidirectional real-time communication
4. Always plan for fallbacks and connection management

The “best” choice depends on your specific requirements for latency, complexity, infrastructure, and user experience. Don’t over-engineer—many applications work perfectly fine with simple polling.