3 / 5 HTTP Caching & Client Layers

Fragmented by Agent

An engineer adds Vary: User-Agent to a marketing page so the origin can tweak markup per browser. CDN hit rate for that page falls from about 92 percent to under 20 percent and origin load spikes. What is happening?

A

User-Agent is a hop-by-hop header, so the edge strips it before matching and has no way to select a stored entry without asking the origin

B

The full User-Agent string is now part of the cache key, and it has thousands of distinct values, so stored entries fragment into near-unique copies

C

Vary: User-Agent marks the response as uncacheable by shared caches, exactly as Cache-Control: private does, so the edge has to forward every request

D

Any Vary header suspends the response's freshness lifetime, so the edge has to revalidate with the origin before serving a stored copy