Vary = cache miss | Proposal = /.well-known/accept-language

[dtruffaut]

Problem

Given

• Developers need to send a Vary header on Accept-Language
• Accept Language has lots of variations (language, script, region, subtags...)

This solution could lead to multiply each cache entry (example.com has many urls) by an order of thousand (all accept-language variations), leading to cache exclusions (LRU), thus tons of cache misses on edge servers. This would hurt performance a lot.

References on Vary and locales:

• https://www.fastly.com/blog/best-practices-using-vary-header#bad-to-the-bone
• https://www.rfc-editor.org/rfc/rfc4647.txt (search for "goethe")
• https://www.soluling.com/Help/MultiScriptLanguages.htm

Solution

Expose accepted languages on a well-known url

/.well-known/accept-language
es fr en en-US en-GB pt pt-BR zh zh-Hant

1. When user goes to example.com, the static page /.well-known/accept-language is queryied in parallel, and cached accordingly.
2. While the page is loading, the Browser checks /.well-known/accept-language fast response and determines if there is a better language for the user among those proposed by example.com
3. If so, Browser stops navigation and send Accept-Language: es
4. Browser receives es content and displays it.
5. On further navigations,

• If the cache of the /.well-known/accept-language is valid, Browser queries this local cached file, select the best language and send Accept-Language: es
• If the cache of the /.well-known/accept-language file is expired, Browser goes to step 1)

References on well-known urls:

• https://web.dev/change-password-url/

Other possibilities:

• robots.txt (easy, mastered by webdevs, but some noise involved by other information already present in that file)
• dedicated sitemap (a little bit overwhelming)