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Caches All the Way Down

Caches all the way down

https://ldnwebperf.org/events/caches-all-the-way-down/

Layers of caching in the browser

(kind of Chromium specific here and there)

1. MemoryCache

  • rendering process specific, short-lived
  • strict matching, same resource type, etc, CSP
  • everything (even no-cache, except for no-store)
  • not shown in dev-tools or in Resource Timing
  • not bound to HTTP semantics

2. ServiceWorker

  • explicit API, controlled by the web developer
  • not bound to HTTP semantics

3. HTTP cache

  • yay, made it to the network stack
  • bound to HTTP semantics

4. Push cache

5. network

  • user -> isp -> edge -> internet -> reverse proxy -> origin
  • once got the response:
    • HTTP cache (if cacheable)
    • ServiceWorker can decide to cache
    • resource timing
    • MemoryCache

HTTP caching semantics

  • the URL is the cache key
  • freshness
  • validators:
    • res: ETag, Last-Modified,
    • req: If-None-Match, If-Modified-Since
    • but! 1 RTT penalty
  • scope matters, too: is it private (browser) or a (shared) public proxy

Directives

old directives

  • Expires: define date
  • Pragma: no-cache nah, old, abused

Cache-Control

  • max-age - in seconds
  • s-max-age - max age for shared proxies,
  • immutable - never revalidate
  • must-revalidate - must revalidate if freshness ran out
  • no-cache - caches but revalidates
  • no-cache=set-cookie - can be cached but ignore header
  • no-store - kind of does what it does
  • proxy-revalidate - must-revalidate for proxies
  • no-transform - e.g image/JS minifying proxies
  • stale-while-revalidate - can serve stale, but revalidate at the same time (fairly new)
  • scope: public/private - user vs public caches
  • by default: can cache (except for URLs with params in old implementations)

Surrogate-Control

  • controls your own public cache, not well-defined, spec not great

Age

  • caching proxies must add this header to cached responses

Vary

  • response depends on headers
  • example: image width, encoding, normalized UA

Key

  • creating complex cache keys, with partitions

patterns

immutable content

  • Cache-Control: max-age=<year> immutable

always revalidate

  • Cache-Control: max-age=0 must-revalidate

everything else sucks

  • and can be a gamble

but, hold till told

  • CDNs usually use this too
  • providing API to purge

but, Service Workers to the rescue

  • can purge from cache with API
  • e.g. purge based on a refresh list served up somewhere

but, use H2 server push to purge

  • just an idea for now
  • no browser support
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