The Thundering Origin
With many edge POPs, a popular but uncached object can cause every POP to fetch from origin at once. Tiered caching adds an intermediate layer so misses funnel through fewer servers.
Origin Shield
An origin shield is a chosen cache tier that all misses pass through before reaching origin.
- Edge POPs that miss ask the shield, not origin directly.
- The shield holds one copy and serves it to every POP.
- Only the shield ever talks to origin for a given object.
This collapses many origin requests into one.
Why It Matters
- Offload: origin sees a single fetch instead of one per POP.
- Higher hit rate: the shield aggregates traffic from all edges, so even moderately popular content stays warm there.
- Stable origin: traffic spikes are absorbed before reaching it.
Placing The Shield
The shield is usually placed near the origin in network terms, so the shield to origin path is short and reliable. Edges may be far from origin but close to the shield through the CDN backbone.
The trade off is an extra hop for true misses. That cost is small compared with the protection it gives origin during surges.
Key idea
Tiered caching routes all edge misses through a single origin shield tier, so origin sees one fetch per object instead of one per POP, raising hit rate and absorbing spikes.