Is server code executed when server returns http status code 304

Question

Does/should server code run when the server return status code 304? I understand that the server should not return anything (client should use the cache), but I cant find any info on whether the server will executed the code in an api endpoint for example.

Answer 1

RFC 2616 section 10.3.5 describes the 304 Not Modified response:

If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server SHOULD respond with this status code. The 304 response MUST NOT contain a message-body

The server will send Date, ETag and/or Content-Location (200 only), or Expires, Cache-Control, and/or Vary if the respond may vary.

What is a Conditional GET?

An cliet application, browser, or proxy with a retained Last-Modified or Etag value will issue a Conditional GET as an initial header only. This allows the client to determine if the resource has been updated.

How does the client know if the resource changed?

Well it depends on how the server is configured.

The Origin Server May:

• Ignore caching, serve every request new.
• Similar to ignoring, you may develop the application to change query strings. This prevents caching at Proxy Servers and invalidates the client cache.
• If configured to do so, issue a Last-Modified or Etag value. Often done for static content. Proxy Servers and Client Caches use to invalidate their version.
• A Web application could issue a Last-Modified far into the future then change the URL to invalidate stale content. This requires the application be developed with this feature in mind.
• Resources may also be issued version numbers. This allows them to preserve Proxy Caches but invalidate Client Caches.

Does server code run when the server return status code 304?

Almost Never. Disregarding that it is technically possible for an incorrectly configured application to respond with a 304 Not Modified code instead of a 200 code.

With a ETag and/or Content-Location value, a server (nginx for example) can confirm nothing has changed without issued a call to the application. This also neatly handles resources with version numbers the same way.

For query strings (image.jpg?version=12), the client cache will invalidate the content. A Proxy Server will also invalidate, and the query will be requested fresh.

I understand that the server should not return anything (client should use the cache), but I cant find any info on whether the server will executed the code in an api endpoint for example.

I'm a fan of nginx, here is a good resource on how caching applies to it.

In short, as much as you can do to support various caches between your client and the application the more requests you can support per day.