Issue link: https://read.uberflip.com/i/1146286
Max-Age and mod_expires
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This is a far future Expires header, telling the browser that this response won't be
stale until April 15, 2010. If this header is returned for an image in a page, the
browser uses the cached image on subsequent page views, reducing the number of
HTTP requests by one. See Chapter B for a review of the
Expires header and HTTP.
Max-Age and mod_expires
Before I explain how better caching improves performance, it's important to men-
tion an alternative to the
Expires header. The Cache-Control header was introduced
in HTTP/1.1 to overcome limitations with the
Expires header. Because the Expires
header uses a specific date, it has stricter clock synchronization requirements
between server and client. Also, the expiration dates have to be constantly checked,
and when that future date finally arrives, a new date must be provided in the server's
configuration.
Alternatively,
Cache-Control uses the max-age directive to specify how long a compo-
nent is cached. It defines the freshness window in seconds. If less than
max-age sec-
onds have passed since the component was requested, the browser will use the
cached version, thus avoiding an additional HTTP request. A far future
max-age
header might set the freshness window 10 years in the future.
Cache-Control: max-age=315360000
Using Cache-Control with max-age overcomes the limitations of Expires, but you still
might want an
Expires header for browsers that don't support HTTP/1.1 (even
though this is probably less than 1% of your traffic). You could specify both
response headers,
Expires and Cache-Control max-age. If both are present, the HTTP
specification dictates that the
max-age directive will override the Expires header.
However, if you're conscientious, you'll still worry about the clock synchronization
and configuration maintenance issues with
Expires.
Fortunately, the
mod_expires Apache module (http://httpd.apache.org/docs/2.0/mod/
mod_expires.html) lets you use an
Expires header that sets the date in a relative fash-
ion similar to
max-age. This is done via the ExpiresDefault directive. In this example,
the expiration date for images, scripts, and stylesheets is set 10 years from the time of
the request: