SSE Debugging

Handle failures in realtime streams for robust social apps and notifications.

Common Issues & Fixes

Issue	Cause	Fix
All data buffers until stream closes	Vercel delays streaming until response is complete	Use TransformStream instead of ReadableStream; ensure response returns immediately
Client disconnects after 60s	Vercel Pro timeout	Add heartbeat (send `:\n\n` every 15-30s) or upgrade to Vercel Fluid Compute
No auto-reconnect	EventSource closed due to error	Browser retries automatically after error; set custom `retry: 5000` in message
Connection drops silently	No heartbeat keeps connection alive	Server sends heartbeat comment lines; client logs onerror for debugging
Client stuck in reconnect loop	Server returns error status	Verify endpoint returns 200 OK with proper headers

Debugging Strategy

1. Check Server Response Headers

Ensure route handler returns correct headers:

typescript

const headers = {
  'Content-Type': 'text/event-stream',
  'Cache-Control': 'no-cache',
  'Connection': 'keep-alive',
};

2. Add Server-Side Logging

Log when client connects and disconnects:

typescript

const stream = new ReadableStream({
  start(controller) {
    console.log('SSE client connected');

    request.signal.addEventListener('abort', () => {
      console.log('SSE client disconnected');
      controller.close();
    });
  },
});

3. Monitor Network Traffic

Use browser DevTools Network tab:

•Check that response type is "text/event-stream"
•Verify messages end with \n\n
•Look for timing of messages (should be immediate, not buffered)

4. Test with curl

bash

curl -v http://localhost:3000/api/notifications

Watch for chunked transfer encoding and message timing.

Heartbeat Implementation

Keep SSE connection alive by sending comment lines every 15-30 seconds:

typescript

// Server: Send heartbeat
const heartbeat = setInterval(() => {
  controller.enqueue(encoder.encode(':\n\n')); // Comment line
}, 30000);

// Cleanup on disconnect
request.signal.addEventListener('abort', () => {
  clearInterval(heartbeat);
  controller.close();
});

Client Error Handling

Implement robust error handling with exponential backoff:

typescript

es.onerror = (error) => {
  console.error('SSE connection error:', error);
  es.close();

  // Manual reconnection with backoff
  setTimeout(() => {
    // Reconnect (component remounts or creates new EventSource)
  }, 5000 * Math.pow(2, retryCount)); // Exponential backoff
};

Vercel-Specific Issues

Issue: Response buffers all data

Cause: Vercel waits for response to complete before flushing to client.

Solution: Return response immediately, keep stream open in background.

typescript

// Return response right away (don't await async work)
return new NextResponse(stream, { headers });

// Async work happens in background
(async () => {
  // Send events
})();

Issue: 60-second timeout on Vercel Pro

Cause: Default timeout for streaming responses.

Solution:

•Add heartbeat (comment lines every 15-30s prevent timeout)
•Or upgrade to Vercel Fluid Compute for unlimited duration

Testing SSE Connections

Local Testing

bash

# Terminal 1: Start dev server
npm run dev

# Terminal 2: Connect to SSE endpoint
curl -v http://localhost:3000/api/notifications

Watch for incoming data messages.

Automated Testing with Jest

typescript

test('SSE sends data before connection timeout', async () => {
  const response = await fetch('/api/notifications');
  const reader = response.body?.getReader();

  const { value } = await reader!.read();
  const text = new TextDecoder().decode(value);

  expect(text).toContain('data:');
});

AI Debugging Prompts

Prompt: "Debug why SSE buffers all data until the stream ends in Next.js on Vercel."

Prompt: "Write client-side error handling for SSE with exponential backoff reconnection."

Prompt: "Add heartbeat to my SSE endpoint to prevent timeout disconnects."

Related Concepts

•Retry: Browser automatically reconnects after retry: <ms> delay
•Event IDs: Use id: <value> to track message sequence for deduplication
•Named events: Use event: <name> to distinguish message types on client