How to Fix EventSource Server-Sent Events (SSE) Connection Dropping & Reconnection Loops

1. Root Cause Analysis (Engine-Level Breakdown)

When modern JavaScript engines (V8 in Chrome/Node.js, JavaScriptCore in Safari, and SpiderMonkey in Firefox) encounter this failure condition, execution halts or falls back to degraded behavior due to the following primary triggers:

• NGINX or cloud reverse proxy buffering responses instead of streaming chunks (missing X-Accel-Buffering: no).
• HTTP/1.1 6-connection per domain limit exhausted by persistent SSE streams.
• Missing heartbeat ping comments (`: ping\n\n`) allowing idle network NAT firewalls to tear down connections.

In high-scale production systems, this error rarely occurs during local development because local environments lack network latency, third-party browser extensions, complex caching proxies, and production minification transforms that uncover timing race conditions.

2. Verified Production Solutions

The following code recipes provide immediate and architectural fixes for this error:

// Node.js Express Resilient SSE Stream Handler:
app.get('/api/v1/stream', (req, res) => {
  res.setHeader('Content-Type', 'text/event-stream');
  res.setHeader('Cache-Control', 'no-cache, no-transform');
  res.setHeader('Connection', 'keep-alive');
  res.setHeader('X-Accel-Buffering', 'no'); // Disable NGINX response proxy buffering
  res.flushHeaders();

  // Send initial connection ACK with retry configuration (in ms)
  res.write('retry: 3000\n');
  res.write(`data: ${JSON.stringify({ status: 'connected', ts: Date.now() })}\n\n`);

  // Heartbeat ping every 15s to keep NAT firewalls open
  const heartbeat = setInterval(() => {
    res.write(': ping\n\n');
  }, 15000);

  req.on('close', () => {
    clearInterval(heartbeat);
    res.end();
  });
});

3. Step-by-Step Resolution Workflow

Follow this structured checklist to resolve and prevent this error in your CI/CD pipeline: