HTTP Caching

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HTTP Caching

Cache-Control directives

• If-Modified-Since The If-Modified-Since request-header field is used with a method to make it conditional: if the requested variant has not been modified since the time specified in this field, an entity will not be returned from the server; instead, a 304 (not modified) response will be returned without any message-body.

• If-None-Match A client that has one or more entities previously obtained from the resource can verify that none of those entities is current by including a list of their associated entity tags in the If-None-Match header field. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.

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Understanding Conditional Requests and Refresh Web browsers make two types of requests over HTTP and HTTPS—conditional requests and unconditional requests. There are a number of reasons why IE might make a conditional request for an item that is already in the cache:

• The cached item is no longer fresh according to Cache-Control or Expires
• The cached item was delivered with a VARY header
• The containing page was navigated to via META REFRESH
• JavaScript in the page called reload on the location object, passing TRUE for bReloadSource
• The request was for a cross-host HTTPS resource on browser startup
• The user refreshed the page

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Calculating Heuristic Freshness

Since origin servers do not always provide explicit expiration times, a cache may assign a heuristic expiration time when an explicit time is not specified, employing algorithms that use other header field values (such as the Last-Modified time) to estimate a plausible expiration time.