The summary.
A LOW CACHE HIT RATE MEANS THE DELIVERY NETWORK IS ACHIEVING LITTLE
Query strings treated as distinct, cookies preventing caching and excessive variants are the usual causes. Verify the hit rate rather than assuming.
VERSIONED FILENAMES ARE THE BEST CACHING PATTERN
Content at an address never changes, so it caches indefinitely and changes appear immediately. Purging should be an exception, not a mechanism.
NEVER LET A SHARED CACHE HOLD ANYTHING VARYING BY USER
Test explicitly whether a signed-in user's content can be served to someone else.
CONFIGURE WHICH UPSTREAMS MAY SET CLIENT-ADDRESS HEADERS
Trusting them from anywhere allows spoofing your logs and access rules.
A HEALTH CHECK MUST EXERCISE DEPENDENCIES
A server accepting connections while its database is unreachable is not healthy.
STATELESS BACKENDS BEAT SESSION PERSISTENCE
Persistence causes uneven distribution, lost sessions on failure, and difficulty draining for maintenance.
YOU CANNOT ABSORB VOLUME EXCEEDING YOUR CONNECTION
Mitigation must happen upstream. Establish what your provider covers and how fast it engages, before you need it.
