CDNs and media delivery

Object storage keeps bytes safe; a content delivery network (CDN) caches them near users, reducing round trips and origin traffic. It's the caching module applied to files at global scale.

A hit stays at the edge

Requests reach a nearby CDN edge. A hit returns from cache; a miss fetches from the origin, often an object store, and caches a copy according to policy.

At 100,000 requests a second and a 95% edge hit ratio, roughly 5,000 reach the next layer. Watch byte hit ratio too: a video miss costs more bandwidth than an icon miss.

The cache key decides what's shared

A cache key includes host, path and selected parameters or headers. Include inputs that change the representation, but avoid irrelevant variation that fragments the cache.

• /photo/92?w=200 and /photo/92?w=800 need different entries if they return different sizes.
• Tracking parameters shouldn't create a new cached copy of identical bytes.
• Arbitrary sizes can create unlimited variants. Allow a small set of transformations and normalize their keys.
• Personalized or private responses need an explicit access policy; sharing a key carelessly can leak data.

A Vary header describes representation differences; configure the CDN cache policy to honor the intended inputs too.

TTLs, invalidation and versions

Immutable URLs such as /assets/photo92.v3.webp can use long max-age and immutable. The mutable record pointing to version 3 needs shorter freshness. Keep old objects while clients still reference them.

An origin shield is a shared cache between edges and origin. With request collapsing, it combines simultaneous cold misses into fewer fetches, at the cost of another hop on some misses.

Private content

The API authenticates and authorizes a viewer, then issues a short-lived CDN-signed URL or cookie. The CDN must validate access before serving a cache hit, and the origin must reject direct public access. Otherwise cached bytes or a guessed origin URL can bypass the permission check.

Signed access can still share cached bytes when the CDN safely separates authorization from the object cache key. Only exclude signature parameters under such a policy. A leaked signed URL works until expiry; short lifetimes limit that window, while immediate revocation needs additional enforcement.

Video: cache segments, adapt quality

HLS and DASH split video into short segments and describe available renditions in a manifest. Encode aligned segments at several bitrates, perhaps 1, 3 and 6 Mbps. The player chooses the next segment based on estimated throughput and buffered playback, moving down in quality before the buffer empties.

A 4-second segment at 3 Mbps is roughly 1.5 MB, before overhead. Popular segments benefit from CDN reuse. Live manifests need short freshness; immutable segments can use long TTLs. Encoding costs more, but the origin doesn't stream separately to every viewer.