How to Use 304 Not Modified in REST APIs

Use the 304 Not Modified status code in your REST API to tell clients that the requested resource has not changed since their last request. This avoids sending the full response body again, saving bandwidth and improving performance. Implement it by checking request headers like If-Modified-Since or If-None-Match and responding with 304 when appropriate.

Syntax

The 304 Not Modified response is sent without a message body. It relies on conditional request headers from the client:

• If-Modified-Since: Date/time of the last known modification.
• If-None-Match: ETag value representing the resource version.

If the resource has not changed since the date or ETag provided, the server responds with 304 Not Modified and no body.

http

HTTP/1.1 304 Not Modified
Date: Wed, 21 Oct 2020 07:28:00 GMT
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"

Example

This example shows a simple Node.js Express server that uses 304 Not Modified to avoid sending unchanged data.

javascript

import express from 'express';
const app = express();

const resource = {
  data: 'Hello, world!',
  lastModified: new Date('2024-01-01T12:00:00Z'),
  etag: '12345abcde'
};

app.get('/resource', (req, res) => {
  const ifModifiedSince = req.headers['if-modified-since'];
  const ifNoneMatch = req.headers['if-none-match'];

  if ((ifModifiedSince && new Date(ifModifiedSince) >= resource.lastModified) ||
      (ifNoneMatch && ifNoneMatch === resource.etag)) {
    res.status(304).end();
    return;
  }

  res.setHeader('Last-Modified', resource.lastModified.toUTCString());
  res.setHeader('ETag', resource.etag);
  res.send(resource.data);
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Output

Server running on http://localhost:3000

Common Pitfalls

• Not setting Last-Modified or ETag headers properly, which makes conditional requests fail.
• Sending a response body with 304, which is against the HTTP specification.
• Ignoring client headers If-Modified-Since and If-None-Match, causing unnecessary data transfer.
• Using inaccurate timestamps or ETag values, leading to wrong cache validation.

javascript

/* Wrong: Sending body with 304 */
res.status(304).send('This should not be sent');

/* Right: No body with 304 */
res.status(304).end();

Quick Reference

Header	Purpose
If-Modified-Since	Client asks if resource changed since this date
If-None-Match	Client asks if resource ETag matches
Last-Modified	Server tells when resource was last changed
ETag	Server provides a unique version identifier
304 Not Modified	Server says resource unchanged, no body sent

Key Takeaways

Use 304 Not Modified to save bandwidth by not resending unchanged resources.

Check client headers If-Modified-Since and If-None-Match to decide when to send 304.

Always set Last-Modified and ETag headers for proper cache validation.

Never send a response body with a 304 status code.

Accurate timestamps and ETags ensure correct cache behavior.