Web Application Performance

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Cache

HTTP headers

  • Cache-Control: max-age
  • ETag: Hash of file, don't download if unchanged (but beware of how this is calculated, e.g. Apache may use inodes by default which differ on load-balanced servers)
  • Expires: Re-download after this
  • Last-Modified

Most headers won't help on first visit.

Request headers (conditional GET request):

  • If-Modified-Since: Date/time
  • If-None-Match: Hash (ETag)

Problems:

  • Can't override the browser behaviour - once it's cached with an expiry time

Fingerprinting / hashing

  • Gets around caching, effectively forces a new request by using a unique filename based on file contents
  • Effectively allows you to set long expiry times but still force updates, e.g. 12 months for site logo

Proxies / intermediates

  • Difficult now that everything is HTTPS
  • No longer transparent for ISPs etc.
  • Can pay a cache provider
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