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Cachability

The cachability of an object is determined from the following properties:

• URL naming heuristics (e.g. the URL may end in .cgi)
• The Last-modified HTTP header - if this is recent, then the object is likely to be dynamic
• The Cache-control HTTP header - this header can indicate explicitly whether a document is cachable or not and its age
• The Pragma: nocache HTTP header
• The Expires HTTP header

Note that for headers that contain date/time information, the object is considered dynamic if it expires or was modified within five minutes of the retrieval time.

Where there may be cachable and uncachable characteristics of an object, the greater number of characteristics is used to determine cachability. If these cancel out, the cachability algorithm returns a conflict.