Caching — Speed Layer | System Design

02 · Performance

Caching — Speed Layer

Store expensive results in fast memory (RAM). Redis can return data in <1ms vs 50ms+ from a database. A cache hit saves an entire DB round trip.

CLIENT REQUEST

REDIS CACHE 🎯 DATA FOUND

✓ CACHE HIT — returns instantly

RESPONSE → CLIENT~0.5ms

DB never touched

CLIENT REQUEST

REDIS CACHE ✗ NOT FOUND

✗ MISS — fetch from DB

DATABASE Query + return data

→ Store result in cache for next time

Cache-Aside (Lazy)

App reads cache first → miss → fetch DB → write to cache. Most common pattern.

Used by: Redis, Memcached

Write-Through

Every write goes to cache AND DB simultaneously. Cache always consistent but slower writes.

Used by: banking, inventory

Write-Back (Write-Behind)

Write to cache first, async sync to DB later. Fastest writes, risk of data loss on crash.

Used by: high-throughput logs