Cache in HTTP

Caching is a technique that stores a copy of a given resource and serves it back when requested

Terminologies

Removing from Cache is called Eviction
Cache before expiration time is called Fresh
Cache after expiry is called Stale

Advantages - reduces latency & network traffic

Summary about Caches

Types of caches - private (browser), shared/proxy caches (ISP level), gateway (CDN)
HTTPS won't be cached by shared caches
Cache is fresh (server without checking server) if expiry time set, and is still within the fresh period. A request for a stale resource will be forwarded with If-None-Match to check if it is in fact still fresh. If so, the server returns a 304 (Not Modified)
If stale, the origin server will be asked to validate it. It won't be evicted or ignored
Scenarios like disconnected from a network — a cache can serve stale responses without checking with the origin server

Cache-Control

The Cache-control header for defining cache policy

Cache Control Header	Usage
no-store	Don't Cache
no-cache	Cache but revalidate
private	can be stored in browser
public	can be stored anywhere
max-age=seconds	max time a resource will be considered fresh
s-maxage	similar to max-age but only for shared
must-revalidate	expired ones should not be used
proxy-revalidate	similar to must revalidate but only for shared
stale-while-revalidat	can use the current (stale) version while it re-fetches the latest version in the background
stale-if-error	use stale is refetch fails

*If both Cache-Control and Expires present, Cache-Control takes precedence

Cache validation

If Cache-control: must-revalidate present or If the Cache is expired, Cache will be validated with server

ETag - opaque-to-the-useragent - strong validator If ETag present, the client can issue an If-None-Match in the header of future requests to validate the cached resource

Last-Modified - a weak validator If present client can issue an If-Modified-Since request header to validate the cached document

On validation, server can response with a normal 200 OK, or it can return 304 Not Modified (with an empty body)

Best practices

If Cache implemented, page visit stats may get affected. To solve it use 1x1 transparent uncacheable image for every page
- Perfect for Cache - 200 (OK), 301 (Moved Permanently), 404 (Not Found)
For frequently changing content like train live status, use no-cache
For contents that are static, use cache with long life time

Cache Explained in one Image - Source: Google