ETag and Conditional Requests for Efficient Caching

API responses are transferred in full on every request even when the data hasn't changed, wasting bandwidth and server processing.

Implement ETags and If-None-Match headers for conditional GET requests.

import crypto from 'node:crypto';

function generateETag(data: unknown): string {
  return crypto
    .createHash('md5')
    .update(JSON.stringify(data))
    .digest('hex');
}

app.get('/users/:id', async (req, res) => {
  const user = await db.getUser(req.params.id);
  const etag = `"${generateETag(user)}"`;

  // Check if client has current version
  if (req.headers['if-none-match'] === etag) {
    return res.status(304).end(); // Not Modified — no body sent
  }

  res.setHeader('ETag', etag);
  res.setHeader('Cache-Control', 'private, max-age=0, must-revalidate');
  res.json(user);
});

// Conditional PUT to prevent lost updates
app.put('/users/:id', async (req, res) => {
  const ifMatch = req.headers['if-match'];
  const current = await db.getUser(req.params.id);
  const currentETag = `"${generateETag(current)}"`;
  
  if (ifMatch && ifMatch !== currentETag) {
    return res.status(412).json({ error: 'Precondition Failed' });
  }
  // Proceed with update...
});

ETags are opaque identifiers for a specific version of a resource. If-None-Match sends the ETag back; if unchanged the server returns 304 with no body, saving transfer. If-Match prevents overwriting concurrent changes.