Expiration expiration

Used to manage client and proxy server caching. The server calculates the expiration time/date of the HTTP response data by adding this value to the time/date of transmission.

Browsers manage caches using expiration times of files. Before passing a request to the server, the browser checks its cache to see if the file has already been downloaded. If so, and if the file has not yet expired, the browser sends a conditional GET request (for example with the If-Modified-Since field set in the request header) rather than a normal GET request. The server has the option of responding with a ‘304’ status and not transmitting the image. The browser then loads the file from its cache. This may substantially increase overall performance for frequently accessed data.

Expiration is used for these response types:

Certain types of responses (for example, error responses) are always marked for immediate expiration (or tagged as non-cacheable), while others (for example, property or default image responses) use special expiration settings ( attribute::NonImgExpiration and attribute::DefaultExpiration).

Properties section-7f5173d090cf48df8fa1a2c72b8c8c60

Real number, -2, -1, or 0 or greater. Number of hours until expiration since the response image was generated. Set to 0 to always expire the reply image immediately, which effectively disables client caching. Set to -1 to mark as never expire. In this case the server always returns 304 status (not-modified) in response to conditional GET requests without checking whether the file has actually changed. Set to -2 to use the default provided by attribute::Expiration.

Default section-ec72cc1dfc5e4f278174d37da2e39462

attribute::Expiration is used if the field is not present, if the value is -2, or if the field is empty.

See also section-0e5e8595aad641c689726828712a8902

attribute::Expiration, attribute::DefaultExpiration, attribute::NonImgExpiration, req=