304 Not Modified

What is a 304 Not Modified Response?

The 304 Not Modified HTTP server response code indicates that the requested resource has not been modified since the last time it was loaded, meaning there’s no need to transfer it again. For browsers, this means the cached version of the resource can be shown to the user. For crawlers, such as Googlebot, it means there’s no need to recrawl the page because nothing has changed on it.

How It Works

Here's a simple explanation of how the 304 Not Modified response works:

1. Initial Request When the client (browser or crawler) requests a resource from the web server for the first time, the server sends the requested resource with a 200 OK HTTP code, along with its hash code, called an ETag. The client also records the time when it requested the page/resource.

2. Subsequent Requests When the client requests the resource again, the server checks the If-None-Match and/or If-Modified-Since headers from the client. This is known as a conditional HTTP request.

   • If-None-Match contains the ETag (content hash code). If it matches the value on the server, this indicates that the content has not changed, and there’s no need to load it again (when the content changes, so does its hash code).
   • If-Modified-Since contains the date and time when the client last requested the content. If the server sees that content has not changed since this date, there’s no need to send the resource to the client.

3. Server Response In both cases, if the content hasn't changed, the server responds with the 304 HTTP code.

When both If-None-Match and If-Modified-Since are used, If-None-Match takes precedence over If-Modified-Since.

Browser and Crawler Behavior

When the browser receives the 304 Not Modified HTTP code from the server, it will show the cached version to the user, making it one of the client-side redirection codes. For crawlers, receiving a 304 response means they can avoid downloading the same content again, conserving bandwidth and resources.

Why Is the 304 Response Code Important?

For small websites, the caching benefits provided by the 304 HTTP code may not be as significant. However, for large websites, the 304 response code offers substantial advantages:

• Performance Optimization By leveraging cached content, browsers can load pages faster for users, improving the overall user experience.
• Crawl Budget Management For large sites, managing the crawl budget is crucial. Google’s crawler won’t recrawl pages that have not changed, allowing it to focus on new and updated content. This helps ensure that fresh content is indexed more efficiently.

Best Practices for Using 304 Not Modified

• Set Proper Cache Headers Ensure your server is configured to send appropriate ETag and Last-Modified headers.
• Monitor Cache Efficiency Regularly check your server logs to see how often 304 responses are being issued and adjust caching policies as needed.
• Optimize Content Delivery Use content delivery networks (CDNs) that support efficient caching mechanisms to reduce latency and improve load times.

For more detailed information on HTTP status codes and best practices for web performance and SEO, refer to authoritative sources such as Mozilla Developer Network (MDN) and Google's Web Fundamentals.