CDN Glossary
Comprehensive definitions of CDN terminology, caching concepts, and industry jargon.
A
Access Time
The total time to retrieve data from storage. For HDDs, this includes seek time (moving the read head) and rotational latency (waiting for the platter to spin). For SSDs and RAM, access time is much lower because there are no moving parts.Access time directly impacts cache miss latency—the time u...
Age Header
A response header added by caches indicating how long (in seconds) the response has been stored. The remaining freshness is calculated as: max-age - Age. When Age equals or exceeds max-age, the content is stale.
Autonomous System (AS)
A collection of IP networks under a single administrative domain with a unified routing policy. Each AS has a unique ASN (Autonomous System Number). CDNs, ISPs, and large enterprises each operate their own AS.
C
Cache Fill
The process of populating a cache tier with content from upstream (shield or origin). A "fill" is essentially a cache miss that results in the content being stored. CDNs often track fill rates separately from hit rates to measure origin protection effectiveness.
Cache Key
A unique identifier generated from request attributes that determines whether requests can share a cached response. Typically derived from the URL and Host header, but can include other headers (via Vary), cookies, or query parameters depending on CDN configuration.
Cache Miss Types
Cold miss: First ever request (unavoidable). Capacity miss: Evicted due to full cache. Invalidation miss: Purged or expired. Fragmentation miss: Cached elsewhere, not in this PoP.
CMAF (Common Media Application Format)
A standard that allows the same media segments to be used with both HLS and DASH manifests. Instead of encoding and storing two separate sets of segments (MPEG-TS for HLS, fMP4 for DASH), you encode once in CMAF-compatible fMP4 and create only different manifests. This halves storage costs and si...
Content-Encoding
An HTTP response header indicating the compression algorithm applied to the response body. Common values: gzip, br (Brotli), zstd. The client must decompress using the specified algorithm before processing the content.
CSAI (Client-Side Ad Insertion)
The player fetches ads separately from the main content and stitches them together during playback. The CDN serves the same manifest and segments to all users; ad personalization happens in the player.
I
Internet Exchange Point (IXP)
A physical location where multiple networks connect to exchange traffic via peering. IXPs reduce the need for transit and lower latency between peered networks. Major IXPs handle terabits of traffic per second.
IOps (I/O Operations Per Second)
A measure of how many read/write operations a storage device can handle per second. HDDs typically deliver 100-200 IOps; SSDs can deliver 10,000-100,000+ IOps; RAM is effectively unlimited for practical purposes.High IOps matter when serving many small files. Low IOps can become a bottleneck even...
L
LL-DASH (Low-Latency DASH)
DASH equivalent using chunked transfer encoding to deliver segments as they're encoded. Players can start playback before the segment is complete. Often paired with CMAF low-latency mode.
LL-HLS (Low-Latency HLS)
Apple's low-latency extension to HLS. Uses partial segments (sub-second chunks pushed before the full segment is complete) and preload hints to tell players what's coming next. Requires HTTP/2 push or blocking playlist requests.
M
Manifest File
A text file that describes the video stream structure: available quality levels (bitrates), segment URLs, timing information, and encryption details. The player fetches the manifest first, then requests segments based on its contents. HLS uses .m3u8 files; DASH uses .mpd files.
max-age
Specifies the maximum time in seconds that a response is considered fresh. Applies to all caches (browser, CDN, proxies). After this time expires, the cache must revalidate or fetch a new copy.
must-revalidate
Once content becomes stale, the cache must revalidate with the origin before serving it. The cache cannot serve stale content even if the origin is unreachable. This ensures users never see outdated content, but can cause errors if the origin is down.
O
Origin Health Check
Periodic requests to a specific endpoint (usually /health or /ping) to verify the origin is responding correctly. Health checks typically verify HTTP status code (2xx), response time threshold, and optionally response body content.
Origin Shield
A mid-tier cache layer that sits between edge servers and the origin. Aggregates cache misses from multiple edges, reducing origin load. Also called "mid-tier cache" or "parent cache."
P
P2P Hybrid Streaming
A delivery architecture where viewers share cached segments with nearby peers via WebRTC, reducing CDN bandwidth. The CDN serves as a fallback and source of truth. Viewers become part of a mesh network, potentially offloading 60-90% of CDN traffic during peak events.
Point of Presence (PoP)
A physical location containing CDN edge servers and networking equipment. PoPs are identified by airport codes (e.g., AMS for Amsterdam, FRA for Frankfurt, SIN for Singapore).
proxy-revalidate
Like must-revalidate, but only applies to shared caches (CDNs). Browser caches can still serve stale content if the origin is unreachable. Useful when you trust end-users seeing slightly stale content but need CDN freshness guarantees.
Pull Zone / Push Zone
Pull zone: CDN fetches content from your origin server on cache miss. Reactive, on-demand caching. Requires an origin server.Push zone: You upload content directly to CDN storage. Proactive, pre-populated caching. CDN acts as the origin.
S
Segment
A small chunk of encoded video/audio, typically 2-10 seconds duration. Segments are the actual media files that players download and stitch together for playback. Common formats: .ts (MPEG-TS), .m4s (fragmented MP4), .mp4.
s-maxage
Stands for "shared max-age". Overrides max-age specifically for shared caches (CDNs, reverse proxies). Browser caches ignore this directive and use max-age instead. This allows different TTLs for edge caches vs. end-user browsers.
SSAI (Server-Side Ad Insertion)
Ads are stitched into the stream on the server/origin side. Each viewer may receive a personalized manifest with different ad segment URLs inserted. This defeats ad blockers but creates unique manifests per user.
stale-if-error
Allows caches to serve stale content when the origin returns an error (5xx) or is unreachable. This provides resilience—users see cached content rather than error pages when your origin has problems.
stale-while-revalidate
Allows caches to serve stale content immediately while fetching a fresh copy in the background. The user gets fast response times (cached content), and the next user gets fresh content. Specified as additional seconds beyond max-age during which stale serving is allowed.
Strong ETag
A strong ETag guarantees that two resources with the same ETag are byte-for-byte identical. Any change—even a single byte—requires a new ETag. Format: "abc123"
Surrogate Key / Cache Tag
A label attached to cached responses that allows purging all content with that label, regardless of URL. Set via response header (e.g., Surrogate-Key or Cache-Tag). When you purge the tag, all responses carrying that tag are invalidated.