HTTP 304 Not Modified

What It Means

HTTP 304 Not Modified indicates that the requested resource has not been modified since the version specified by the If-Modified-Since or If-None-Match request headers. The server does not return a body — the client should use its cached copy of the resource.

This is a critical part of HTTP caching and helps reduce bandwidth and improve page load performance.

Common Causes

• Browser sends a conditional request with If-None-Match (ETag) and the resource hasn’t changed
• Browser sends If-Modified-Since and the resource hasn’t been modified since that date
• CDN caching layers validating that cached content is still fresh
• API clients using conditional requests to poll for changes efficiently
• Static assets (CSS, JS, images) that haven’t changed between requests

How to Fix

HTTP 304 is a normal caching response. Here’s how to implement it properly:

Express.js with ETag

const express = require('express');
const app = express();

// Express sends ETags and handles 304 automatically for static files
app.use(express.static('public'));

// For dynamic content, set ETag manually
app.get('/api/data', (req, res) => {
  const data = getData();
  const etag = generateETag(data);

  // Check if client has current version
  if (req.headers['if-none-match'] === etag) {
    return res.status(304).end();
  }

  res.set('ETag', etag);
  res.json(data);
});

Nginx caching headers

# Enable ETag (on by default)
etag on;

# Set Cache-Control for static assets
location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg)$ {
    expires 30d;
    add_header Cache-Control "public, no-transform";
}

Client-side conditional requests

// Using fetch with conditional headers
let cachedETag = null;
let cachedData = null;

async function fetchData() {
  const headers = {};
  if (cachedETag) {
    headers['If-None-Match'] = cachedETag;
  }

  const response = await fetch('/api/data', { headers });

  if (response.status === 304) {
    // Use cached data
    return cachedData;
  }

  cachedETag = response.headers.get('ETag');
  cachedData = await response.json();
  return cachedData;
}

Python requests

import requests

# First request
response = requests.get('https://api.example.com/data')
etag = response.headers.get('ETag')

# Subsequent requests with conditional header
response = requests.get(
    'https://api.example.com/data',
    headers={'If-None-Match': etag}
)
if response.status_code == 304:
    print('Resource not modified, use cached version')

Related Errors

• HTTP 200 - OK: Returned when the resource has been modified and the full response is sent.
• HTTP 301 - Moved Permanently: Returned when the resource has moved to a new URL.
• HTTP 404 - Not Found: Returned when the resource no longer exists at the URL.