The edge cache is the CDN server nearest the viewer that stores popular segments so they can be served locally instead of fetched from origin. It is the workhorse of video delivery: when a viewer requests a segment, the edge checks its cache, serves a hit instantly, and only on a miss reaches back toward the origin (often via an origin shield).
A high cache-hit ratio is what keeps delivery fast and cheap. It depends on cache key design (identical requests must map to the same cached object), segment and rendition popularity (a handful of titles and rungs dominate requests), and tuning of cache TTLs. Tokenized URLs and personalization can fragment the cache if done carelessly — append a per-user token to the cache key and suddenly every viewer's segment is a unique, uncacheable object.
Live premieres stress the edge differently than VOD: a synchronized audience requests the same new segment within seconds, so the edge must coalesce those requests rather than send thousands of identical misses to the origin.
