Caching
The highest-leverage tool for latency and scale — and the source of its hardest problem, invalidation. Where caches live, the patterns for filling them, how they evict, and why keeping them correct is the part that stays hard.
Caching as the highest-leverage latency tool — and its hardest problem: where caches live, the patterns (cache-aside/read-through/write-through/write-behind), eviction, and invalidation including cache stampede, penetration, and hot keys.