When a popular object expires, every edge can miss at once. Without a shield, the origin sees a fan-out of identical fetches — the thundering herd that looks like an attack but is only your own TTL.
Hits stay local, misses get a single path
Browsers still hit the nearest PoP. On a miss, Cloud CDN Origin Shield consolidates eligible fetches so the origin is not opened 210+ times for the same URL.
That is the difference between a cache and a well-behaved cache.
When it matters most
After a purge, after a deploy, at the start of a live event, and whenever HTML TTLs are short. APIs with a 30-second TTL are a classic shield workload.
Large files and video segments also benefit: first-byte to origin should not multiply by PoP count.
Origin groups
Shield sits with origin groups: more than one origin on a distribution for failover or path-based routing. Visitors still never aim at those origins if you restrict access.
Serve-stale is the other half
If origin is down, eligible stale objects can still be served. Shield reduces how often you need that; serve-stale is the belt.