How CDNs Work

A cache in every city

A content delivery network (CDN) is a globally distributed set of caching servers in points of presence (PoPs) close to users. Users are routed to a nearby PoP by anycast (the same IP address announced from many locations, with the network choosing the nearest) or DNS-based routing. On a cache hit, the edge responds directly, often in a few milliseconds; on a miss, the edge fetches from your origin (a server, load balancer or storage bucket), stores the response according to its cache rules and headers, and serves it. To protect the origin, CDNs use tiered caching or an origin shield: edge PoPs fetch misses from a regional mid-tier cache rather than all going to the origin, so a popular new object causes one origin request instead of hundreds. CDNs also keep connections to the origin warm, terminate TLS near users, support HTTP/2 and HTTP/3, and compress responses. Examples include Cloudflare, Akamai, Fastly, Amazon CloudFront, Google Cloud CDN and Azure Front Door.

Edges, shield and origin

Users hit nearby edges; misses go through a shield tier before reaching the origin.

Request flow through a tiered CDN

Only one request reaches the origin for a new popular object.

user (Chennai)  -> edge PoP Chennai   MISS -> shield (Mumbai) MISS -> origin   (1 origin fetch)
user (Delhi)    -> edge PoP Delhi     MISS -> shield (Mumbai) HIT
user (Pune)     -> edge PoP Mumbai    MISS -> shield (Mumbai) HIT
user (Chennai)  -> edge PoP Chennai   HIT

without a shield, each PoP's first miss would reach the origin separately

Check the cache status header

Most CDNs add a response header such as CF-Cache-Status, X-Cache or Age that shows hit, miss or expired. Inspect it with browser dev tools or curl -I when debugging why something is not cached.

त्वरित जाँच: What is the purpose of an origin shield (tiered cache)?

• Collapsing misses from many edge PoPs into fewer requests to the origin
• Encrypting data at the origin
• Blocking all bots
• Hosting the database