Don't Panic — Content Delivery Networks

A CDN is a fleet of caches standing between your users and your server, each one holding a copy of a response so the server does not have to. Before these existed, every request crossed the network to one room where one machine answered everybody. That works until a popular page meets a distant continent, at which point the speed of light starts sending invoices.

The whole arrangement runs on five words: route, key, freshness, fill, observe. Routing picks which edge answers. The key decides which stored response counts as the one being asked for. Freshness says how long that response may be reused. Fill is what happens on a miss — the edge fetches from an upper tier or the origin. Observation is everything that tells you the other four are working.

Here is the part that surprises people: the geographically nearest edge is not necessarily the one chosen. Routing weighs network latency, reachability, and capacity, so the right answer is a suitable delivery point, not the closest dot on the map. Map distance and network distance agree about as often as maps and territory generally do.

The surprise that bites, though, is that a long TTL is a promise, and not the one you think. It permits reuse for that period; it does not guarantee any cache keeps the object for that long. Eviction can remove a perfectly fresh response to make room for something else. Fresh and stored are two different questions, and conflating them is how "but I set it to a year" ends up in an incident channel.

The cache key deserves equal suspicion. Leave out something that changes the response — a language header, say — and one stored answer serves the wrong people. Put in something unique per request, like a tracking value, and one object becomes thousands of near-identical entries while the origin does all the work anyway. The key is a correctness boundary first and an optimization second, which is a polite way of saying it is a correctness boundary.

Updates have three exits: short lifetimes and more origin traffic, a purge when content changes, or a new versioned URL whose bytes never change so it can carry a long lifetime. Broad purges are dramatic in the wrong direction — they empty caches everywhere and send a wave of misses at the origin. Purge narrowly.

And no, a CDN will not fix a slow origin, wrong headers, or an unreachable server. It distributes copies, not competence.

The Intro carries the full architecture, the Cheatsheet holds the directives and decision rules, the Practice tab turns headers into evidence, and the Quiz checks you can tell a key problem from a freshness problem — the two that look identical from a dashboard.

Don't Panic — Content Delivery Networks

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