26.2 What are edge locations (PoPs)?

In the warehouse: The warehouses sit in big cities, near highways, where most shops can reach one quickly. A shop goes to whichever is nearest and open.

In the system: A CDN runs points of presence (PoPs): data centres in many cities, each with cache servers, connected to local internet providers. Big CDNs run hundreds of them worldwide.

How a reader reaches the nearest one:

• Anycast: many PoPs announce the same IP address; the internet's routing (BGP) delivers each user's packets to a nearby one (Chapter 4 touched on this for DNS).
• DNS-based steering: the CDN's DNS answers with the IP of a good PoP for the user's location and current load (Chapter 4).

Inside a PoP:

• Many cache servers share the work, often using consistent hashing (Chapter 28) so each object lives on one or two servers in the PoP, instead of every server caching everything.
• Hot objects sit in memory, the rest on SSDs.
• A miss goes to the origin, or to a shield first (26.6).

Things to know:

• Hit ratio varies by PoP. A small-city PoP sees less traffic, so more of its requests are first-time misses.
• Caches evict. Rarely requested files (an old book's cover) may be dropped and fetched again later; "cached for a year" means "allowed to be", not "guaranteed to be".

Memory code: "A warehouse in every city; the road takes you to the nearest."

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