Caches All the Way Down

caches-all-the-way-down.md

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

• same rendering process, 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

• unclaimed push stream container
• only on same H2 connection
• https://jakearchibald.com/2017/h2-push-tougher-than-i-thought/
• goes into HTTP cache once 'claimed' by a request
• lost after 5 mins unless claimed (in Chromium)

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