TTL (time to live) is the cache lifetime set on an object - how long an edge may keep serving it before checking with the origin for a newer version. It is configured via cache-control headers and is the lever that balances freshness against offload.
For immutable VOD segments, long TTLs are ideal: the bytes never change, so they should stay cached for a long time, maximizing cache-hit ratio. For live manifests that update every few seconds, TTLs must be very short or the player will see stale content. Getting TTLs right per object type - long for media, short for manifests - is fundamental to both correctness and cost.
