Firewalls enforce policy at trust boundaries; caches accelerate content. The two appear together because both intercept traffic - one to block, the other to speed.
3.6 Network Security and Performance - Firewalls and Caching
Firewall types
Type
What it inspects
Stateless
Headers only; no connection memory
Stateful
Remembers conversations; modern default
Application
L7 inspection (HTTP, TLS SNI, SMTP)
WAF
OWASP top 10 protection
Cloud (NSG)
Per-resource; AWS SG, Azure NSG
Next-gen (NGFW)
IPS + decryption + identity-aware
Trust boundary diagram
Caching layers
Layer
Where
Tools
Browser cache
Client
Cache-Control headers
CDN
Edge
CloudFront, Cloudflare, Akamai
Reverse proxy
Front of app
Varnish, nginx
Application
In-process
Redis, Memcached
Database
Inside DB engine
Buffer pools, query plan cache
Page cache
OS file cache
Linux page cache
Cache invalidation strategies
Strategy
Trade-off
TTL
Simple; eventual freshness
Manual purge
Precise; needs control endpoint
Tag-based
Coarse-grained business event
Stale-while-revalidate
Best UX; some complexity
Mentor’s tip: Firewalls draw the trust boundaries; caches make them fast. Both intercept traffic - both need explicit invalidation and revalidation rules. NGFW + WAF + CDN is the modern baseline; document each layer and its bypass.
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