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High Performance Web Sites
book

High Performance Web Sites

by Steve Souders
September 2007
Intermediate to advanced content levelIntermediate to advanced
170 pages
4h 24m
English
O'Reilly Media, Inc.
Content preview from High Performance Web Sites

Max-Age and mod_expires

Before I explain how better caching improves performance, it's important to mention 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 component is cached. It defines the freshness window in seconds. If less than max-age seconds 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, ...

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Publisher Resources

ISBN: 9780596529307Errata Page