Content Delivery

Push bytes to the network edge so requests terminate close to the user and never reach the origin. A CDN is the outermost cache layer of a system: get it right and most static/media traffic and a chunk of latency vanish before they hit your servers; get it wrong and you serve stale assets, leak origin load, or pay egress twice.

When to reach for this

The same files (images, video, JS/CSS bundles, downloads, fonts) are read repeatedly by a geographically spread audience; the origin or its bandwidth is the bottleneck for static reads; or cross-region latency on first byte hurts (a cross-continent round trip is ~100 ms — see back-of-the-envelope). A CDN buys latency and origin offload at once.

When NOT to

Highly personalized, per-request dynamic responses with no cacheable shape (a CDN adds a hop and caches nothing). Tiny single-region audiences where the origin already serves reads comfortably (YAGNI — a CDN is another vendor, another bill, another invalidation problem). Strictly fresh data that cannot tolerate any staleness window — that belongs at the origin or behind consistency-coordination, not a TTL-based edge. Naming a CDN before a number shows static reads or geography is the problem is a red flag.

Clarify first

• Content mix — what fraction is cacheable static/media vs uncacheable dynamic/personalized? (Only the cacheable part benefits.)
• Update cadence & staleness budget — how often do assets change, and how stale may an edge copy be? (Drives TTL and invalidation strategy.)
• Geography — where are users, and how concentrated? (Decides whether edge PoPs and geo-routing matter at all.)
• Object size & egress volume — average asset size × requests = egress; this sizes the bill and the offload (→ back-of-the-envelope).
• Origin shape — object store (S3/GCS/blob) or dynamic app server? Can it survive a cold-cache stampede if the edge flushes?

The options

Distribution model — how content reaches the edge

• Pull (origin-pull): the edge fetches on first miss, caches per TTL, serves the rest. Use when traffic is high and content is large or churny — the edge holds only what's actually requested. The default for most systems.
• Push: you upload assets to the CDN ahead of demand and rewrite URLs. Use when the catalog is small/static or launch spikes can't tolerate a cold first-miss (you pre-warm); you accept managing storage and uploads yourself.

Caching key & TTL — what the edge keys on and for how long

• Long TTL + fingerprinted URLs (app.4f9a.js, image.png?v=2): immutable assets cached for months; a content change is a new URL, not an invalidation. Use when you control asset URLs — the cleanest model.
• Short TTL / stale-while-revalidate: bound staleness for content that changes on a schedule. Use when URLs are stable but content updates.

Edge proximity & routing — how a user reaches the nearest PoP

• Anycast / DNS geo-routing: route each user to the closest healthy edge. Use when the audience is multi-region (almost always, for a CDN). Shared with load-balancing — see there for the routing mechanics.

Origin protection — shrinking the origin's exposed surface

• Origin shield / mid-tier cache: a single regional cache layer in front of the origin that all edges pull through, collapsing N edge misses into one origin fetch. Use when origin offload or stampede protection matters more than a little extra latency on cold misses.

Trade-offs

Behavior under stress

A CDN usually absorbs load spikes — that's its job — but it has its own failure shapes, and they tend to dump straight onto the origin.

• Cold cache / mass eviction: after a purge, config push, or TTL synchronized expiry, edge hit rate craters and every PoP pulls from the origin at once. This is a caching thundering herd at global scale. Mitigate: origin shield to collapse misses, TTL jitter, stale-while-revalidate so the edge serves stale while it refetches, staged purges.
• Cache busting / low hit rate: unbounded query-string variation or cookies in the cache key explode the keyspace so nothing stays cached — the origin sees full traffic while you still pay the CDN. Mitigate: normalize/whitelist cache-key params; strip cookies on static paths.
• Hot object: one viral file can exceed a single PoP's capacity, but CDNs scale this far better than an origin — the real risk is a hot uncacheable path punching through to the origin.
• CDN outage / partial PoP failure: the edge is now a dependency in front of everything. Plan origin fallback (clients or DNS failover to origin) and accept the origin must briefly take full load, or use a second CDN (multi-CDN).
• Egress surprise: a misconfigured no-cache or a hot uncacheable asset can 10× the origin egress bill silently.

Monitor: edge hit ratio (cache hit rate), origin offload %, origin request rate (the number that spikes when the edge fails), p95 edge latency by region, egress bytes, and 4xx/5xx at the edge vs origin.

How to apply

1. Clarify the inputs — content mix (cacheable fraction), staleness budget, geography, object size × volume, and origin shape (see Clarify first). If the cacheable fraction is near zero or the audience is single-region, stop here.
2. Pick the distribution model and cache key from the trade-off table: default to pull; switch to push/pre-warm only when cold-miss latency or launch spikes hurt. Prefer long TTL + fingerprinted URLs when you control URLs, else short TTL / stale-while-revalidate.
3. Set the knobs — Cache-Control (max-age, immutable, stale-while-revalidate), the cache key (URL path + whitelisted params; strip cookies on static paths), Vary only where you truly differ, and add an origin shield if offload or stampede protection matters.
4. Stress-test the design — walk a global purge, a config push, and a CDN/PoP outage. Confirm TTL jitter + stale-while-revalidate + shield keep the origin survivable, and that a client/DNS fallback to origin exists.
5. Size it with numbers — estimate hit ratio (target 90%+), origin offload %, and egress (requests × avg object size) via back-of-the-envelope. If egress or origin request rate is alarming, revisit the cache key and TTL.
6. Pick a provider — default to the generic recipe; if a cloud is named, read its provider file for the service mapping and limits (see Choosing a provider).
7. Show the trade-off in the answer — give a Decision | Solves | Worsens | Change it when… table with one row per choice you made here (e.g. the option, its store, its failure policy); the last column is a concrete trigger (a number or observable signal). Then name what breaks first.

Dos and don'ts

Do

• Fingerprint immutable assets and cache them for months — turn updates into new URLs, not invalidations.
• Whitelist cache-key params and strip cookies on static paths to keep hit ratio high.
• Add stale-while-revalidate and TTL jitter so synchronized expiry can't stampede the origin.
• Add an origin shield when many edges would otherwise miss to the origin at once.
• Plan an origin/DNS fallback (or multi-CDN) for a CDN or PoP outage.
• Monitor edge hit ratio and origin request rate — the number that spikes when the edge fails.

Don't

• Reach for a CDN before a number shows static reads or geography is the bottleneck.
• Let unbounded query strings or Vary: Cookie explode the keyspace and gut caching.
• Treat a single-region origin shield as free — it is a new chokepoint/SPOF.
• Cache strictly-fresh data on a TTL when zero staleness is required.
• Ship a careless no-cache on a hot asset — it can silently 10× origin egress.

Numbers that matter

The decisive quantities are hit ratio (90%+ is the goal; below ~80% question whether content is cacheable), origin offload % (1 − origin-requests/total), and egress (requests × avg object size). Edge-vs-origin latency is the payoff: an edge hit is a same-region round trip (~ms to tens of ms) instead of a cross-continent one (~100 ms). Do the egress and offload math with back-of-the-envelope — don't restate its tables here; egress is the line item that usually dominates a CDN bill.

Interface sketch

The contract is mostly HTTP cache headers the origin sets and the edge obeys:

• Cache-Control: public, max-age=31536000, immutable for fingerprinted static.
• Cache-Control: public, max-age=60, stale-while-revalidate=600 for stable URLs with periodic updates.
• ETag / Last-Modified to enable cheap revalidation (304 Not Modified).
• Vary only on headers you truly serve differently on (a careless Vary: Cookie destroys hit rate).
• The cache key: URL path + an explicit whitelist of query params; decide which cookies/headers (if any) are part of it. Invalidation is a PURGE/invalidation API call or (preferably) a URL version bump. Versioned URLs sidestep the purge-propagation race entirely.

Choosing a provider

Default to the generic recipe above. If the user names a cloud, read references/providers/<provider>.md for the managed-service mapping, quotas/limits, and provider-specific trade-offs. If no file exists for that provider, the generic recipe is the answer.

Diagram

To visualize the edge → shield → origin pull path (and the dashed cold-miss arrow, plus geo-routing from clients to the nearest PoP), use the in-plugin architecture-diagram skill. Sketch the edge nodes in the cache color and the origin in its store color; do not embed Mermaid here.

Related building blocks

• caching — owned-concept lives in: invalidation, eviction, TTL, and thundering-herd theory live there; the CDN is the edge tier above the app/distributed cache and alternative to origin reads for static/media.
• load-balancing — owned-concept lives in: the geo/anycast routing and origin health checks that send users to the nearest edge.
• back-of-the-envelope — feeds into this: supplies the egress, offload %, and latency-payoff numbers that justify a CDN.
• data-storage — depends on: the object store that is usually the CDN's origin.
• consistency-coordination — alternative to this for data that cannot tolerate any staleness window (serve from origin, not a TTL-based edge).
• system-design — pairs with (back-link): the orchestrator that routes here when a design serves static/media at geographic scale.

References

• references/deep-dive.md — cache-key normalization, Cache-Control directive semantics, push vs pull mechanics, origin shield / tiered topology, invalidation vs versioning races, multi-CDN, and media-specific delivery (segmented HLS/DASH, range requests, signed URLs). Read when designing the edge layer in detail.
• references/providers/{generic,aws,azure,gcp}.md — service mappings, limits, and pitfalls per environment.