Cache Invalidation — Overview#

"There are only two hard things in computer science: cache invalidation and naming things." The reason it's hard: the cache doesn't automatically know when the DB changes. You have to tell it — or wait for time to expire it. Every strategy is a different trade-off between freshness, complexity, and performance.

Files in this folder#

File	Topic
01-TTL-Based.md	Let time expire stale keys — simple, always use as safety net
02-Event-Driven.md	Invalidate the moment the DB changes — no stale window
03-Write-Through.md	Update the cache on every write instead of deleting it
04-Cache-Versioning.md	Change the key itself — old keys expire naturally
05-Stale-While-Revalidate.md	Serve stale immediately, refresh in background
06-Interview-Cheatsheet.md	Which strategy to mention and when

The spectrum#

TTL-based              → simplest, stale window up to TTL duration
Event-driven           → instant, needs infrastructure
Write-through          → no miss after write, slower writes
Cache versioning       → no invalidation needed, two cache lookups per read
Stale-while-revalidate → one stale response, then fresh