The Vary header has long been a vital tool for website owners to make content cacheable even if it changes based on inputs other than the URL. The problem is this tool is typically either severely underused, usually limited to Accept-Encoding variation, or is used very badly, resulting in a complete destruction of the cache potential of the page.
Now the IETF is bringing forward a new standard, Key, which aims to provide a more powerful replacement for Vary. Andrew Betts examines the common misconceptions about Vary, the resulting common misconfigurations and how to fix them, advanced use cases for Vary that vastly increase the efficiency of content distribution, and the changes Key will bring about. Along the way, Andrew also considers whether Key actually fails to cater to some of the current use cases for Vary.
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