if-modified-since vs if-none-match

http caching request response
Tower picture Tower · Jun 16, 2009 · Viewed 51.2k times · Source

What could be the difference between if-modified-since and if-none-match? I have a feeling that if-none-match is used for files whereas if-modified-since is used for pages?

Answer

trendels picture trendels · Jun 17, 2009

Regarding the differences between Last-Modified/If-Modified-Since and ETag/If-None-Match:

Both can be used interchangeably. However depending on the type of resource, and how it is generated on the server, one or the other question ("has this been modified since ...?" / "does this still match this ETag?") may be easier to answer.

Examples:

  • If you're serving files, using the file's mtime as the Last-Modified date is the simplest solution.
  • If you're serving a dynamic web page built from a number of SQL queries, checking whether the data returned by any of those queries has changed may be impractical (unless all of them have some sort of "last modified" column). In this case, using e.g. an md5 hash of the page content as the ETag will be a lot easier.
    OTOH, this means that you still have to generate the whole page on the server, even for a conditional GET. Figuring out what exactly has to go into the ETag (primary keys, revision numbers, ... etc.) can save you a lot of time here.

See these links for more details on the topic:

Related questions

How do we control web page caching, across all browsers?

Our investigations have shown us that not all browsers respect the HTTP cache directives in a uniform manner. For security reasons we do not want certain pages in our application to be cached, ever, by the web browser. This must …

Read More
What requests do browsers' "F5" and "Ctrl + F5" refreshes generate?

Is there a standard for what actions F5 and Ctrl + F5 trigger in web browsers? I once did experiment in IE6 and Firefox 2.x. The F5 refresh would trigger a HTTP request sent to the server with an If-Modified-Since header, …

Read More
What's the difference between Cache-Control: max-age=0 and no-cache?

The header Cache-Control: max-age=0 implies that the content is considered stale (and must be re-fetched) immediately, which is in effect the same thing as Cache-Control: no-cache.

Read More