Repair HTTP Cache Revalidation Bugs

I have analyzed your ETag revalidation flow. The issue stems from a mismatch between the quoted ETag string expected by HTTP semantics and the raw version string being compared in your middleware, coupled with a missing 304 status assignment.

Cache Revalidation Diagnosis & Fix

Proposed Repair (Node.js/Express)

app.use((req, res, next) => {
  const resourceVersion = getResourceVersion(); // e.g., "v123"
  const etag = `"${resourceVersion}"`; // ETags must be wrapped in quotes
  
  res.setHeader('ETag', etag);
  res.setHeader('Cache-Control', 'public, no-cache'); // Force revalidation

  const ifNoneMatch = req.headers['if-none-match'];

  // Repair: Handle weak ETags and quoted string comparison
  if (ifNoneMatch === etag || ifNoneMatch === `W/${etag}`) {
    // Critical: You must set the status AND end the response
    return res.status(304).end();
  }

  next();
});

Next steps

• Verify if your CDN (e.g., Cloudflare) is stripping the ETag header due to specific "Email Obfuscation" or "Rocket Loader" settings.
• Run a curl -I -H 'If-None-Match: "v123"' https://api.example.com/resource test to confirm the 304 response.
• Ensure the Vary header includes Accept-Encoding if your ETag changes based on compression.